Dynamic member lookup

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A new pseudo-type dynamic is introduced into the C# type system. It is treated as System.Object, but in addition, any member access (method call, field, property, or indexer access, or a delegate invocation) or application of an operator on a value of such type is permitted without any type checking, and its resolution is postponed until run-time. This is known as duck typing or late binding. For example:

// Returns the value of Length property or field of any object
int GetLength(dynamic obj)
    return obj.Length;
GetLength("Hello, world");        // a string has a Length property,
GetLength(new int[] { 1, 2, 3 }); // and so does an array,
GetLength(42);                    // but not an integer - an exception will be thrown
                                  // in GetLength method at run-time

In this case, dynamic type is used to avoid more verbose Reflection. It still uses Reflection under the hood, but it’s usually faster thanks to caching.

This feature is primarily targeted at interoperability with dynamic languages.

// Initialize the engine and execute a file
var runtime = ScriptRuntime.CreateFromConfiguration();
dynamic globals = runtime.Globals;

// Use Calc type from Ruby
dynamic calc = globals.Calc.@new();
calc.valueA = 1337;
calc.valueB = 666;
dynamic answer = calc.Calculate();

Dynamic type has applications even in mostly statically typed code, for example it makes double dispatch posible without implementing Visitor pattern.

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