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This chapter is from the book

This chapter is from the book

C# Programming Language (Covering C# 4.0), The, 4th EditionC# Programming Language (Covering C# 4.0), The, 4th Edition

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This chapter is from the book

This chapter is from the book

C# Programming Language (Covering C# 4.0), The, 4th EditionC# Programming Language (Covering C# 4.0), The, 4th Edition

Learn More Buy

4.7 The dynamic Type

The type dynamic has special meaning in C#. Its purpose is to allow dynamic binding, which is described in detail in §7.2.2.

The dynamic type is considered identical to the object type except in the following respects:

  • Operations on expressions of type dynamic can be dynamically bound (§7.2.2).
  • Type inference (§7.5.2) will prefer dynamic over object if both are candidates.

Because of this equivalence, the following statements hold:

  • There is an implicit identity conversion between object and dynamic, and between constructed types that are the same when replacing dynamic with object.
  • Implicit and explicit conversions to and from object also apply to and from dynamic.
  • Method signatures that are the same when replacing dynamic with object are considered the same signature.

The type dynamic is indistinguishable from object at runtime.

An expression of the type dynamic is referred to as a dynamic expression.

Eric Lippert

The type dynamic is a bizarre type, but it is important to note that, from the compiler’s perspective, it is a type. Unlike with var, you can use it in most of the situations that call for a type: return types, parameter types, type arguments, and so on.

Peter Sestoft

Actually, var is a reserved word, not a compile-time type, whereas dynamic is a compile-time type. The var keyword tells the compiler, “Please infer the compile-time type of this variable from its initializer expression.” The dynamic type essentially tells the compiler, “Do not worry about compile-time type checking of expressions in which this variable appears; the runtime system will do the right thing based on the runtime type of the value of the variable (or throw an exception, where the compiler would have reported a type error).” The type dynamic cannot be used as receiver (this type) of an extension method, as base type of a class, or as type bound for a generic type parameter, but otherwise it can be used pretty much like any other type.

Marek Safar

Method signatures are considered to be same when using the dynamic and object types. This allows use of a nice trick: The interface method declared using type object can be directly implemented using a method with type dynamic.

Chris Sells

I begin to wonder about any language where the following string of characters is both valid and meaningful:

class Foo {
  public static dynamic DoFoo() {...}
}

Of course, this means that the DoFoo method is a type method (as opposed to an instance method) and that the type of the return value is unknown until runtime, but it’s hard not to read DoFoo as both static and dynamic at the same time and worry about an occurrence of a singularity.

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