netcore/System.Private.CoreLib/shared/System/ParamsArray.cs

   1 // Licensed to the .NET Foundation under one or more agreements.
   2 // The .NET Foundation licenses this file to you under the MIT license.
   3 // See the LICENSE file in the project root for more information.
   4
   5 namespace System
   6 {
   7     internal readonly struct ParamsArray
   8     {
   9         // Sentinel fixed-length arrays eliminate the need for a "count" field keeping this
  10         // struct down to just 4 fields. These are only used for their "Length" property,
  11         // that is, their elements are never set or referenced.
  12         private static readonly object?[] s_oneArgArray = new object?[1];
  13         private static readonly object?[] s_twoArgArray = new object?[2];
  14         private static readonly object?[] s_threeArgArray = new object?[3];
  15
  16         private readonly object? _arg0;
  17         private readonly object? _arg1;
  18         private readonly object? _arg2;
  19
  20         // After construction, the first three elements of this array will never be accessed
  21         // because the indexer will retrieve those values from arg0, arg1, and arg2.
  22         private readonly object?[] _args;
  23
  24         public ParamsArray(object? arg0)
  25         {
  26             _arg0 = arg0;
  27             _arg1 = null;
  28             _arg2 = null;
  29
  30             // Always assign this.args to make use of its "Length" property
  31             _args = s_oneArgArray;
  32         }
  33
  34         public ParamsArray(object? arg0, object? arg1)
  35         {
  36             _arg0 = arg0;
  37             _arg1 = arg1;
  38             _arg2 = null;
  39
  40             // Always assign this.args to make use of its "Length" property
  41             _args = s_twoArgArray;
  42         }
  43
  44         public ParamsArray(object? arg0, object? arg1, object? arg2)
  45         {
  46             _arg0 = arg0;
  47             _arg1 = arg1;
  48             _arg2 = arg2;
  49
  50             // Always assign this.args to make use of its "Length" property
  51             _args = s_threeArgArray;
  52         }
  53
  54         public ParamsArray(object?[] args)
  55         {
  56             int len = args.Length;
  57             _arg0 = len > 0 ? args[0] : null;
  58             _arg1 = len > 1 ? args[1] : null;
  59             _arg2 = len > 2 ? args[2] : null;
  60             _args = args;
  61         }
  62
  63         public int Length => _args.Length;
  64
  65         public object? this[int index] => index == 0 ? _arg0 : GetAtSlow(index);
  66
  67         private object? GetAtSlow(int index)
  68         {
  69             if (index == 1)
  70                 return _arg1;
  71             if (index == 2)
  72                 return _arg2;
  73             return _args[index];
  74         }
  75     }
  76 }