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[mcs.git] / mcs / eval.cs

//
// eval.cs: Evaluation and Hosting API for the C# compiler
//
// Authors:
//   Miguel de Icaza (miguel@gnome.org)
//
// Dual licensed under the terms of the MIT X11 or GNU GPL
//
// Copyright 2001, 2002, 2003 Ximian, Inc (http://www.ximian.com)
// Copyright 2004, 2005, 2006, 2007, 2008 Novell, Inc
//
using System;
using System.Threading;
using System.Collections;
using System.Reflection;
using System.Reflection.Emit;
using System.IO;
using System.Globalization;
using System.Text;

namespace Mono.CSharp {

        /// <summary>
        ///   Evaluator: provides an API to evaluate C# statements and
        ///   expressions dynamically.
        /// </summary>
        /// <remarks>
        ///   This class exposes static methods to evaluate expressions in the
        ///   current program.
        ///
        ///   To initialize the evaluator with a number of compiler
        ///   options call the Init(string[]args) method with a set of
        ///   command line options that the compiler recognizes.
        ///
        ///   To interrupt execution of a statement, you can invoke the
        ///   Evaluator.Interrupt method.
        /// </remarks>
        public class Evaluator {

                static object evaluator_lock = new object ();
                
                static string current_debug_name;
                static int count;
                static Thread invoke_thread;
                
                static ArrayList using_alias_list = new ArrayList ();
                static ArrayList using_list = new ArrayList ();
                static Hashtable fields = new Hashtable ();

                static Type   interactive_base_class = typeof (InteractiveBase);
                static Driver driver;
                static bool inited;

                /// <summary>
                ///   Optional initialization for the Evaluator.
                /// </summary>
                /// <remarks>
                ///  Initializes the Evaluator with the command line options
                ///  that would be processed by the command line compiler.  Only
                ///  the first call to Init will work, any future invocations are
                ///  ignored.
                ///
                ///  You can safely avoid calling this method if your application
                ///  does not need any of the features exposed by the command line
                ///  interface.
                /// </remarks>
                public static void Init (string [] args)
                {
                        lock (evaluator_lock){
                                if (inited)
                                        return;
                                
                                RootContext.Version = LanguageVersion.Default;
                                driver = Driver.Create (args, false);
                                if (driver == null)
                                        throw new Exception ("Failed to create compiler driver with the given arguments");
                                
                                driver.ProcessDefaultConfig ();
                                CompilerCallableEntryPoint.Reset ();
                                Driver.LoadReferences ();
                                RootContext.EvalMode = true;
                                inited = true;
                        }
                }

                static void Init ()
                {
                        Init (new string [0]);
                }
                
                static void Reset ()
                {
                        CompilerCallableEntryPoint.PartialReset ();

                        //
                        // PartialReset should not reset the core types, this is very redundant.
                        //
                        if (!TypeManager.InitCoreTypes ())
                                throw new Exception ("Failed to InitCoreTypes");
                        TypeManager.InitOptionalCoreTypes ();
                        
                        Location.AddFile ("{interactive}");
                        Location.Initialize ();

                        current_debug_name = "interactive" + (count++) + ".dll";
                        if (Environment.GetEnvironmentVariable ("SAVE") != null){
                                CodeGen.Init (current_debug_name, current_debug_name, false);
                        } else
                                CodeGen.InitDynamic (current_debug_name);
                }

                /// <summary>
                ///   The base class for the classes that host the user generated code
                /// </summary>
                /// <remarks>
                ///
                ///   This is the base class that will host the code
                ///   executed by the Evaluator.  By default
                ///   this is the Mono.CSharp.InteractiveBase class
                ///   which is useful for interactive use.
                ///
                ///   By changing this property you can control the
                ///   base class and the static members that are
                ///   available to your evaluated code.
                /// </remarks>
                static public Type InteractiveBaseClass {
                        get {
                                return interactive_base_class;
                        }

                        set {
                                if (value == null)
                                        throw new ArgumentNullException ();

                                lock (evaluator_lock)
                                        interactive_base_class = value;
                        }
                }

                /// <summary>
                ///   Interrupts the evaluation of an expression executing in Evaluate.
                /// </summary>
                /// <remarks>
                ///   Use this method to interrupt long-running invocations.
                /// </remarks>
                public static void Interrupt ()
                {
                        if (!inited || !invoking)
                                return;
                        
                        if (invoke_thread != null)
                                invoke_thread.Abort ();
                }

                /// <summary>
                ///   Compiles the input string and returns a delegate that represents the compiled code.
                /// </summary>
                /// <remarks>
                ///
                ///   Compiles the input string as a C# expression or
                ///   statement, unlike the Evaluate method, the
                ///   resulting delegate can be invoked multiple times
                ///   without incurring in the compilation overhead.
                ///
                ///   If the return value of this function is null,
                ///   this indicates that the parsing was complete.
                ///   If the return value is a string it indicates
                ///   that the input string was partial and that the
                ///   invoking code should provide more code before
                ///   the code can be successfully compiled.
                ///
                ///   If you know that you will always get full expressions or
                ///   statements and do not care about partial input, you can use
                ///   the other Compile overload. 
                ///
                ///   On success, in addition to returning null, the
                ///   compiled parameter will be set to the delegate
                ///   that can be invoked to execute the code.
                ///
                /// </remarks>
                static public string Compile (string input, out CompiledMethod compiled)
                {
                        if (input == null || input.Length == 0){
                                compiled = null;
                                return null;
                        }
                        
                        lock (evaluator_lock){
                                if (!inited)
                                        Init ();
                                
                                bool partial_input;
                                CSharpParser parser = ParseString (true, input, out partial_input);
                                if (parser == null){
                                        compiled = null;
                                        if (partial_input)
                                                return input;
                                        
                                        ParseString (false, input, out partial_input);
                                        return null;
                                }
                                
                                object parser_result = parser.InteractiveResult;
                                
                                if (!(parser_result is Class)){
                                        int errors = Report.Errors;
                                        
                                        NamespaceEntry.VerifyAllUsing ();
                                        if (errors == Report.Errors)
                                                parser.CurrentNamespace.Extract (using_alias_list, using_list);
                                }

                                compiled = CompileBlock (parser_result as Class, parser.undo);
                        }
                        
                        return null;
                }

                /// <summary>
                ///   Compiles the input string and returns a delegate that represents the compiled code.
                /// </summary>
                /// <remarks>
                ///
                ///   Compiles the input string as a C# expression or
                ///   statement, unlike the Evaluate method, the
                ///   resulting delegate can be invoked multiple times
                ///   without incurring in the compilation overhead.
                ///
                ///   This method can only deal with fully formed input
                ///   strings and does not provide a completion mechanism.
                ///   If you must deal with partial input (for example for
                ///   interactive use) use the other overload. 
                ///
                ///   On success, a delegate is returned that can be used
                ///   to invoke the method.
                ///
                /// </remarks>
                static public CompiledMethod Compile (string input)
                {
                        CompiledMethod compiled;

                        // Ignore partial inputs
                        if (Compile (input, out compiled) != null){
                                // Error, the input was partial.
                                return null;
                        }

                        // Either null (on error) or the compiled method.
                        return compiled;
                }
                
                //
                // Todo: Should we handle errors, or expect the calling code to setup
                // the recording themselves?
                //

                /// <summary>
                ///   Evaluates and expression or statement and returns any result values.
                /// </summary>
                /// <remarks>
                ///   Evaluates the input string as a C# expression or
                ///   statement.  If the input string is an expression
                ///   the result will be stored in the result variable
                ///   and the result_set variable will be set to true.
                ///
                ///   It is necessary to use the result/result_set
                ///   pair to identify when a result was set (for
                ///   example, execution of user-provided input can be
                ///   an expression, a statement or others, and
                ///   result_set would only be set if the input was an
                ///   expression.
                ///
                ///   If the return value of this function is null,
                ///   this indicates that the parsing was complete.
                ///   If the return value is a string, it indicates
                ///   that the input is partial and that the user
                ///   should provide an updated string.
                /// </remarks>
                public static string Evaluate (string input, out object result, out bool result_set)
                {
                        CompiledMethod compiled;

                        result_set = false;
                        result = null;

                        input = Compile (input, out compiled);
                        if (input != null)
                                return input;
                        
                        if (compiled == null)
                                return null;
                                
                        //
                        // The code execution does not need to keep the compiler lock
                        //
                        object retval = typeof (NoValueSet);
                        
                        try {
                                invoke_thread = System.Threading.Thread.CurrentThread;
                                invoking = true;
                                compiled (ref retval);
                        } catch (ThreadAbortException e){
                                Thread.ResetAbort ();
                                Console.WriteLine ("Interrupted!\n{0}", e);
                        } finally {
                                invoking = false;
                        }

                        //
                        // We use a reference to a compiler type, in this case
                        // Driver as a flag to indicate that this was a statement
                        //
                        if (retval != typeof (NoValueSet)){
                                result_set = true;
                                result = retval; 
                        }

                        return null;
                }
                        
                /// <summary>
                ///   Executes the given expression or statement.
                /// </summary>
                /// <remarks>
                ///    Executes the provided statement, returns true
                ///    on success, false on parsing errors.  Exceptions
                ///    might be thrown by the called code.
                /// </remarks>
                public static bool Run (string statement)
                {
                        if (!inited)
                                Init ();

                        object result;
                        bool result_set;

                        bool ok = Evaluate (statement, out result, out result_set) == null;
                        
                        return ok;
                }

                /// <summary>
                ///   Evaluates and expression or statement and returns the result.
                /// </summary>
                /// <remarks>
                ///   Evaluates the input string as a C# expression or
                ///   statement and returns the value.   
                ///
                ///   This method will throw an exception if there is a syntax error,
                ///   of if the provided input is not an expression but a statement.
                /// </remarks>
                public static object Evaluate (string input)
                {
                        object result;
                        bool result_set;
                        
                        string r = Evaluate (input, out result, out result_set);

                        if (r != null)
                                throw new ArgumentException ("Syntax error on input: partial input");
                        
                        if (result_set == false)
                                throw new ArgumentException ("The expression did not set a result");

                        return result;
                }
                
                enum InputKind {
                        EOF,
                        StatementOrExpression,
                        CompilationUnit,
                        Error
                }

                //
                // Deambiguates the input string to determine if we
                // want to process a statement or if we want to
                // process a compilation unit.
                //
                // This is done using a top-down predictive parser,
                // since the yacc/jay parser can not deambiguage this
                // without more than one lookahead token.   There are very
                // few ambiguities.
                //
                static InputKind ToplevelOrStatement (SeekableStreamReader seekable)
                {
                        Tokenizer tokenizer = new Tokenizer (seekable, Location.SourceFiles [0]);
                        
                        int t = tokenizer.token ();
                        switch (t){
                        case Token.EOF:
                                return InputKind.EOF;
                                
                        // These are toplevels
                        case Token.EXTERN:
                        case Token.OPEN_BRACKET:
                        case Token.ABSTRACT:
                        case Token.CLASS:
                        case Token.ENUM:
                        case Token.INTERFACE:
                        case Token.INTERNAL:
                        case Token.NAMESPACE:
                        case Token.PRIVATE:
                        case Token.PROTECTED:
                        case Token.PUBLIC:
                        case Token.SEALED:
                        case Token.STATIC:
                        case Token.STRUCT:
                                return InputKind.CompilationUnit;
                                
                        // Definitely expression
                        case Token.FIXED:
                        case Token.BOOL:
                        case Token.BYTE:
                        case Token.CHAR:
                        case Token.DECIMAL:
                        case Token.DOUBLE:
                        case Token.FLOAT:
                        case Token.INT:
                        case Token.LONG:
                        case Token.NEW:
                        case Token.OBJECT:
                        case Token.SBYTE:
                        case Token.SHORT:
                        case Token.STRING:
                        case Token.UINT:
                        case Token.ULONG:
                                return InputKind.StatementOrExpression;

                        // These need deambiguation help
                        case Token.USING:
                                t = tokenizer.token ();
                                if (t == Token.EOF)
                                        return InputKind.EOF;

                                if (t == Token.IDENTIFIER)
                                        return InputKind.CompilationUnit;
                                return InputKind.StatementOrExpression;


                        // Distinguish between:
                        //    delegate opt_anonymous_method_signature block
                        //    delegate type 
                        case Token.DELEGATE:
                                t = tokenizer.token ();
                                if (t == Token.EOF)
                                        return InputKind.EOF;
                                if (t == Token.OPEN_PARENS || t == Token.OPEN_BRACE)
                                        return InputKind.StatementOrExpression;
                                return InputKind.CompilationUnit;

                        // Distinguih between:
                        //    unsafe block
                        //    unsafe as modifier of a type declaration
                        case Token.UNSAFE:
                                t = tokenizer.token ();
                                if (t == Token.EOF)
                                        return InputKind.EOF;
                                if (t == Token.OPEN_PARENS)
                                        return InputKind.StatementOrExpression;
                                return InputKind.CompilationUnit;
                                
                        // These are errors: we list explicitly what we had
                        // from the grammar, ERROR and then everything else

                        case Token.READONLY:
                        case Token.OVERRIDE:
                        case Token.ERROR:
                                return InputKind.Error;

                        // This catches everything else allowed by
                        // expressions.  We could add one-by-one use cases
                        // if needed.
                        default:
                                return InputKind.StatementOrExpression;
                        }
                }
                
                //
                // Parses the string @input and returns a CSharpParser if succeeful.
                //
                // if @silent is set to true then no errors are
                // reported to the user.  This is used to do various calls to the
                // parser and check if the expression is parsable.
                //
                // @partial_input: if @silent is true, then it returns whether the
                // parsed expression was partial, and more data is needed
                //
                static CSharpParser ParseString (bool silent, string input, out bool partial_input)
                {
                        partial_input = false;
                        Reset ();
                        queued_fields.Clear ();

                        Stream s = new MemoryStream (Encoding.Default.GetBytes (input));
                        SeekableStreamReader seekable = new SeekableStreamReader (s, Encoding.Default);

                        InputKind kind = ToplevelOrStatement (seekable);
                        if (kind == InputKind.Error){
                                if (!silent)
                                        Report.Error (-25, "Detection Parsing Error");
                                partial_input = false;
                                return null;
                        }

                        if (kind == InputKind.EOF){
                                if (silent == false)
                                        Console.Error.WriteLine ("Internal error: EOF condition should have been detected in a previous call with silent=true");
                                partial_input = true;
                                return null;
                                
                        }
                        seekable.Position = 0;

                        CSharpParser parser = new CSharpParser (seekable, Location.SourceFiles [0]);
                        parser.ErrorOutput = Report.Stderr;

                        if (kind == InputKind.StatementOrExpression){
                                parser.Lexer.putback_char = Tokenizer.EvalStatementParserCharacter;
                                RootContext.StatementMode = true;
                        } else {
                                //
                                // Do not activate EvalCompilationUnitParserCharacter until
                                // I have figured out all the limitations to invoke methods
                                // in the generated classes.  See repl.txt
                                //
                                parser.Lexer.putback_char = Tokenizer.EvalUsingDeclarationsParserCharacter;
                                //parser.Lexer.putback_char = Tokenizer.EvalCompilationUnitParserCharacter;
                                RootContext.StatementMode = false;
                        }

                        if (silent)
                                Report.DisableReporting ();
                        try {
                                parser.parse ();
                        } finally {
                                if (Report.Errors != 0){
                                        if (silent && parser.UnexpectedEOF)
                                                partial_input = true;

                                        parser.undo.ExecuteUndo ();
                                        parser = null;
                                }

                                if (silent)
                                        Report.EnableReporting ();
                        }
                        return parser;
                }

                //
                // Queue all the fields that we use, as we need to then go from FieldBuilder to FieldInfo
                // or reflection gets confused (it basically gets confused, and variables override each
                // other).
                //
                static ArrayList queued_fields = new ArrayList ();
                
                //static ArrayList types = new ArrayList ();

                static volatile bool invoking;
                
                static CompiledMethod CompileBlock (Class host, Undo undo)
                {
                        RootContext.ResolveTree ();
                        if (Report.Errors != 0){
                                undo.ExecuteUndo ();
                                return null;
                        }
                        
                        RootContext.PopulateTypes ();

                        if (Report.Errors != 0){
                                undo.ExecuteUndo ();
                                return null;
                        }

                        TypeBuilder tb = null;
                        MethodBuilder mb = null;
                                
                        if (host != null){
                                tb = host.TypeBuilder;
                                mb = null;
                                foreach (MemberCore member in host.Methods){
                                        if (member.Name != "Host")
                                                continue;
                                        
                                        MethodOrOperator method = (MethodOrOperator) member;
                                        mb = method.MethodBuilder;
                                        break;
                                }

                                if (mb == null)
                                        throw new Exception ("Internal error: did not find the method builder for the generated method");
                        }
                        
                        RootContext.EmitCode ();
                        if (Report.Errors != 0)
                                return null;
                        
                        RootContext.CloseTypes ();

                        if (Environment.GetEnvironmentVariable ("SAVE") != null)
                                CodeGen.Save (current_debug_name, false);

                        if (host == null)
                                return null;
                        
                        //
                        // Unlike Mono, .NET requires that the MethodInfo is fetched, it cant
                        // work from MethodBuilders.   Retarded, I know.
                        //
                        Type tt = CodeGen.Assembly.Builder.GetType (tb.Name);
                        MethodInfo mi = tt.GetMethod (mb.Name);
                        
                        // Pull the FieldInfos from the type, and keep track of them
                        foreach (Field field in queued_fields){
                                FieldInfo fi = tt.GetField (field.Name);
                                
                                FieldInfo old = (FieldInfo) fields [field.Name];
                                
                                // If a previous value was set, nullify it, so that we do
                                // not leak memory
                                if (old != null){
                                        if (old.FieldType.IsValueType){
                                                //
                                                // TODO: Clear fields for structs
                                                //
                                        } else {
                                                try {
                                                        old.SetValue (null, null);
                                                } catch {
                                                }
                                        }
                                }
                                
                                fields [field.Name] = fi;
                        }
                        //types.Add (tb);

                        queued_fields.Clear ();
                        
                        return (CompiledMethod) System.Delegate.CreateDelegate (typeof (CompiledMethod), mi);
                }
                
                static internal void LoadAliases (NamespaceEntry ns)
                {
                        ns.Populate (using_alias_list, using_list);
                }
                
                /// <summary>
                ///   A sentinel value used to indicate that no value was
                ///   was set by the compiled function.   This is used to
                ///   differentiate between a function not returning a
                ///   value and null.
                /// </summary>
                public class NoValueSet {
                }

                static internal FieldInfo LookupField (string name)
                {
                        FieldInfo fi =  (FieldInfo) fields [name];

                        return fi;
                }

                //
                // Puts the FieldBuilder into a queue of names that will be
                // registered.   We can not register FieldBuilders directly
                // we need to fetch the FieldInfo after Reflection cooks the
                // types, or bad things happen (bad means: FieldBuilders behave
                // incorrectly across multiple assemblies, causing assignments to
                // invalid areas
                //
                // This also serves for the parser to register Field classes
                // that should be exposed as global variables
                //
                static internal void QueueField (Field f)
                {
                        queued_fields.Add (f);
                }

                static string Quote (string s)
                {
                        if (s.IndexOf ('"') != -1)
                                s = s.Replace ("\"", "\\\"");
                        
                        return "\"" + s + "\"";
                }

                static public string GetUsing ()
                {
                        lock (evaluator_lock){
                                StringBuilder sb = new StringBuilder ();
                                
                                foreach (object x in using_alias_list)
                                        sb.Append (String.Format ("using {0};\n", x));
                                
                                foreach (object x in using_list)
                                        sb.Append (String.Format ("using {0};\n", x));
                                
                                return sb.ToString ();
                        }
                }

                static public string GetVars ()
                {
                        lock (evaluator_lock){
                                StringBuilder sb = new StringBuilder ();
                                
                                foreach (DictionaryEntry de in fields){
                                        FieldInfo fi = LookupField ((string) de.Key);
                                        object value = null;
                                        bool error = false;
                                        
                                        try {
                                                if (value == null)
                                                        value = "null";
                                                value = fi.GetValue (null);
                                                if (value is string)
                                                        value = Quote ((string)value);
                                        } catch {
                                                error = true;
                                        }
                                        
                                        if (error)
                                                sb.Append (String.Format ("{0} {1} <error reading value>", TypeManager.CSharpName(fi.FieldType), de.Key));
                                        else
                                                sb.Append (String.Format ("{0} {1} = {2}", TypeManager.CSharpName(fi.FieldType), de.Key, value));
                                }
                                
                                return sb.ToString ();
                        }
                }

                /// <summary>
                ///    Loads the given assembly and exposes the API to the user.
                /// </summary>
                static public void LoadAssembly (string file)
                {
                        lock (evaluator_lock){
                                Driver.LoadAssembly (file, false);
                                RootNamespace.ComputeNamespaces ();
                        }
                }

                /// <summary>
                ///    Exposes the API of the given assembly to the Evaluator
                /// </summary>
                static public void ReferenceAssembly (Assembly a)
                {
                        lock (evaluator_lock){
                                RootNamespace.Global.AddAssemblyReference (a);
                                RootNamespace.ComputeNamespaces ();
                        }
                }
                
        }

        
        /// <summary>
        ///   A delegate that can be used to invoke the
        ///   compiled expression or statement.
        /// </summary>
        /// <remarks>
        ///   Since the Compile methods will compile
        ///   statements and expressions into the same
        ///   delegate, you can tell if a value was returned
        ///   by checking whether the returned value is of type
        ///   NoValueSet.   
        /// </remarks>
        
        public delegate void CompiledMethod (ref object retvalue);

        /// <summary>
        ///   The default base class for every interaction line
        /// </summary>
        /// <remarks>
        ///   The expressions and statements behave as if they were
        ///   a static method of this class.   The InteractiveBase class
        ///   contains a number of useful methods, but can be overwritten
        ///   by setting the InteractiveBaseType property in the Evaluator
        /// </remarks>
        public class InteractiveBase {
                /// <summary>
                ///   Determines where the standard output of methods in this class will go. 
                /// </summary>
                public static TextWriter Output = Console.Out;

                /// <summary>
                ///   Determines where the standard error of methods in this class will go. 
                /// </summary>
                public static TextWriter Error = Console.Error;

                /// <summary>
                ///   The primary prompt used for interactive use.
                /// </summary>
                public static string Prompt             = "csharp> ";

                /// <summary>
                ///   The secondary prompt used for interactive use (used when
                ///   an expression is incomplete).
                /// </summary>
                public static string ContinuationPrompt = "      > ";

                /// <summary>
                ///   Used to signal that the user has invoked the  `quit' statement.
                /// </summary>
                public static bool QuitRequested;
                
                /// <summary>
                ///   Shows all the variables defined so far.
                /// </summary>
                static public void ShowVars ()
                {
                        Output.Write (Evaluator.GetVars ());
                        Output.Flush ();
                }

                /// <summary>
                ///   Displays the using statements in effect at this point. 
                /// </summary>
                static public void ShowUsing ()
                {
                        Output.Write (Evaluator.GetUsing ());
                        Output.Flush ();
                }

                public delegate void Simple ();
                
                /// <summary>
                ///   Times the execution of the given delegate
                /// </summary>
                static public TimeSpan Time (Simple a)
                {
                        DateTime start = DateTime.Now;
                        a ();
                        return DateTime.Now - start;
                }
                
#if !SMCS_SOURCE
                /// <summary>
                ///   Loads the assemblies from a package
                /// </summary>
                /// <remarks>
                ///   Loads the assemblies from a package.   This is equivalent
                ///   to passing the -pkg: command line flag to the C# compiler
                ///   on the command line. 
                /// </remarks>
                static public void LoadPackage (string pkg)
                {
                        if (pkg == null){
                                Error.WriteLine ("Invalid package specified");
                                return;
                        }

                        string pkgout = Driver.GetPackageFlags (pkg, false);
                        if (pkgout == null)
                                return;

                        string [] xargs = pkgout.Trim (new Char [] {' ', '\n', '\r', '\t'}).
                                Split (new Char [] { ' ', '\t'});

                        foreach (string s in xargs){
                                if (s.StartsWith ("-r:") || s.StartsWith ("/r:") || s.StartsWith ("/reference:")){
                                        string lib = s.Substring (s.IndexOf (':')+1);

                                        Evaluator.LoadAssembly (lib);
                                        continue;
                                }
                        }
                }
#endif

                /// <summary>
                ///   Loads the assembly
                /// </summary>
                /// <remarks>
                ///   Loads the specified assembly and makes its types
                ///   available to the evaluator.  This is equivalent
                ///   to passing the -pkg: command line flag to the C#
                ///   compiler on the command line.
                /// </remarks>
                static public void LoadAssembly (string assembly)
                {
                        Evaluator.LoadAssembly (assembly);
                }
                
                /// <summary>
                ///   Returns a list of available static methods. 
                /// </summary>
                static public string help {
                        get {
                                return  "Static methods:\n"+
                                        "  Describe(obj)      - Describes the object's type\n" + 
                                        "  LoadPackage (pkg); - Loads the given Package (like -pkg:FILE)\n" +
                                        "  LoadAssembly (ass) - Loads the given assembly (like -r:ASS)\n" + 
                                        "  ShowVars ();       - Shows defined local variables.\n" +
                                        "  ShowUsing ();      - Show active using decltions.\n" +
                                        "  Prompt             - The prompt used by the C# shell\n" +
                                        "  ContinuationPrompt - The prompt for partial input\n" +
                                        "  Time(() -> { })    - Times the specified code\n" +
                                        "  quit;\n" +
                                        "  help;\n";
                        }
                }

                /// <summary>
                ///   Indicates to the read-eval-print-loop that the interaction should be finished. 
                /// </summary>
                static public object quit {
                        get {
                                QuitRequested = true;
                                return null;
                        }
                }

#if !NET_2_1
                /// <summary>
                ///   Describes an object or a type.
                /// </summary>
                /// <remarks>
                ///   This method will show a textual representation
                ///   of the object's type.  If the object is a
                ///   System.Type it renders the type directly,
                ///   otherwise it renders the type returned by
                ///   invoking GetType on the object.
                /// </remarks>
                static public string Describe (object x)
                {
                        if (x == null)
                                return "";
                        
                        Type t = x as Type;
                        if (t == null)
                                t = x.GetType ();

                        StringWriter sw = new StringWriter ();
                        new Outline (t, sw, true, false, false).OutlineType ();
                        return sw.ToString ();
                }
#endif
        }

        //
        // A local variable reference that will create a Field in a
        // Class with the resolved type.  This is necessary so we can
        // support "var" as a field type in a class declaration.
        //
        // We allow LocalVariableReferece to do the heavy lifting, and
        // then we insert the field with the resolved type
        //
        public class LocalVariableReferenceWithClassSideEffect : LocalVariableReference {
                TypeContainer container;
                string name;
                
                public LocalVariableReferenceWithClassSideEffect (TypeContainer container, string name, Block current_block, string local_variable_id, Location loc)
                        : base (current_block, local_variable_id, loc)
                {
                        this.container = container;
                        this.name = name;
                }

                public override bool Equals (object obj)
                {
                        LocalVariableReferenceWithClassSideEffect lvr = obj as LocalVariableReferenceWithClassSideEffect;
                        if (lvr == null)
                                return false;

                        if (lvr.name != name || lvr.container != container)
                                return false;

                        return base.Equals (obj);
                }

                public override int GetHashCode ()
                {
                        return name.GetHashCode ();
                }
                
                override public Expression DoResolveLValue (EmitContext ec, Expression right_side)
                {
                        Expression ret = base.DoResolveLValue (ec, right_side);
                        if (ret == null)
                                return null;

                        Field f = new Field (container, new TypeExpression (ret.Type, Location),
                                             Modifiers.PUBLIC | Modifiers.STATIC,
                                             new MemberName (name, Location), null);
                        container.AddField (f);
                        if (f.Define ())
                                Evaluator.QueueField (f);
                        
                        return ret;
                }
        }

        /// <summary>
        ///    A class used to assign values if the source expression is not void
        ///
        ///    Used by the interactive shell to allow it to call this code to set
        ///    the return value for an invocation.
        /// </summary>
        class OptionalAssign : SimpleAssign {
                public OptionalAssign (Expression t, Expression s, Location loc)
                        : base (t, s, loc)
                {
                }

                public override Expression DoResolve (EmitContext ec)
                {
                        CloneContext cc = new CloneContext ();
                        Expression clone = source.Clone (cc);

                        clone = clone.Resolve (ec);
                        if (clone == null)
                                return null;

                        // This means its really a statement.
                        if (clone.Type == TypeManager.void_type){
                                source = source.Resolve (ec);
                                target = null;
                                type = TypeManager.void_type;
                                eclass = ExprClass.Value;
                                return this;
                        }

                        return base.DoResolve (ec);
                }

                public override void Emit (EmitContext ec)
                {
                        if (target == null)
                                source.Emit (ec);
                        else
                                base.Emit (ec);
                }

                public override void EmitStatement (EmitContext ec)
                {
                        if (target == null)
                                source.Emit (ec);
                        else
                                base.EmitStatement (ec);
                }
        }

        public class Undo {
                ArrayList undo_types;
                
                public Undo ()
                {
                        undo_types = new ArrayList ();
                }

                public void AddTypeContainer (TypeContainer current_container, TypeContainer tc)
                {
                        if (current_container == tc){
                                Console.Error.WriteLine ("Internal error: inserting container into itself");
                                return;
                        }
                        
                        if (undo_types == null)
                                undo_types = new ArrayList ();
                        undo_types.Add (new Pair (current_container, tc));
                }

                public void ExecuteUndo ()
                {
                        if (undo_types == null)
                                return;

                        foreach (Pair p in undo_types){
                                TypeContainer current_container = (TypeContainer) p.First;

                                current_container.RemoveTypeContainer ((TypeContainer) p.Second);
                        }
                        undo_types = null;
                }
        }
        
}