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Using type dynamic

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The dynamic type is a static type, but an object of type dynamic bypasses static type checking. In most cases, it functions like it has type object. The compiler assumes a dynamic element supports any operation. Therefore, you don't have to determine whether the object gets its value from a COM API, from a dynamic language such as IronPython, from the HTML Document Object Model (DOM), from reflection, or from somewhere else in the program. However, if the code isn't valid, errors surface at run time.

For example, if instance method exampleMethod1 in the following code has only one parameter, the compiler recognizes that the first call to the method, ec.exampleMethod1(10, 4), isn't valid because it contains two arguments. The call causes a compiler error. The compiler doesn't check the second call to the method, dynamic_ec.exampleMethod1(10, 4), because the type of dynamic_ec is dynamic. Therefore, no compiler error is reported. However, the error doesn't escape notice indefinitely. It appears at run time and causes a run-time exception.

static void Main(string[] args)
{
    ExampleClass ec = new ExampleClass();
    // The following call to exampleMethod1 causes a compiler error
    // if exampleMethod1 has only one parameter. Uncomment the line
    // to see the error.
    //ec.exampleMethod1(10, 4);

    dynamic dynamic_ec = new ExampleClass();
    // The following line is not identified as an error by the
    // compiler, but it causes a run-time exception.
    dynamic_ec.exampleMethod1(10, 4);

    // The following calls also do not cause compiler errors, whether
    // appropriate methods exist or not.
    dynamic_ec.someMethod("some argument", 7, null);
    dynamic_ec.nonexistentMethod();
}

class ExampleClass
{
    public ExampleClass() { }
    public ExampleClass(int v) { }

    public void exampleMethod1(int i) { }

    public void exampleMethod2(string str) { }
}

The role of the compiler in these examples is to package together information about what each statement is proposing to do to the dynamic object or expression. The runtime examines the stored information and any statement that isn't valid causes a run-time exception.

The result of most dynamic operations is itself dynamic. For example, if you rest the mouse pointer over the use of testSum in the following example, IntelliSense displays the type (local variable) dynamic testSum.

dynamic d = 1;
var testSum = d + 3;
// Rest the mouse pointer over testSum in the following statement.
System.Console.WriteLine(testSum);

Operations in which the result isn't dynamic include:

  • Conversions from dynamic to another type.
  • Constructor calls that include arguments of type dynamic.

For example, the type of testInstance in the following declaration is ExampleClass, not dynamic:

var testInstance = new ExampleClass(d);

Conversions

Conversions between dynamic objects and other types are easy. Conversions enable the developer to switch between dynamic and non-dynamic behavior.

You can convert any to dynamic implicitly, as shown in the following examples.

dynamic d1 = 7;
dynamic d2 = "a string";
dynamic d3 = System.DateTime.Today;
dynamic d4 = System.Diagnostics.Process.GetProcesses();

Conversely, you can dynamically apply any implicit conversion to any expression of type dynamic.

int i = d1;
string str = d2;
DateTime dt = d3;
System.Diagnostics.Process[] procs = d4;

Overload resolution with arguments of type dynamic

Overload resolution occurs at run time instead of at compile time if one or more of the arguments in a method call have the type dynamic, or if the receiver of the method call is of type dynamic. In the following example, if the only accessible exampleMethod2 method takes a string argument, sending d1 as the argument doesn't cause a compiler error, but it does cause a run-time exception. Overload resolution fails at run time because the run-time type of d1 is int, and exampleMethod2 requires a string.

// Valid.
ec.exampleMethod2("a string");

// The following statement does not cause a compiler error, even though ec is not
// dynamic. A run-time exception is raised because the run-time type of d1 is int.
ec.exampleMethod2(d1);
// The following statement does cause a compiler error.
//ec.exampleMethod2(7);

Dynamic language runtime

The dynamic language runtime (DLR) provides the infrastructure that supports the dynamic type in C#, and also the implementation of dynamic programming languages such as IronPython and IronRuby. For more information about the DLR, see Dynamic Language Runtime Overview.

COM interop

Many COM methods allow for variation in argument types and return type by designating the types as object. COM interop necessitates explicit casting of the values to coordinate with strongly typed variables in C#. If you compile by using the EmbedInteropTypes (C# Compiler Options) option, the introduction of the dynamic type enables you to treat the occurrences of object in COM signatures as if they were of type dynamic, and thereby to avoid much of the casting. For more information on using the dynamic type with COM objects, see the article on How to access Office interop objects by using C# features.

Title Description
dynamic Describes the usage of the dynamic keyword.
Dynamic Language Runtime Overview Provides an overview of the DLR, which is a runtime environment that adds a set of services for dynamic languages to the common language runtime (CLR).
Walkthrough: Creating and Using Dynamic Objects Provides step-by-step instructions for creating a custom dynamic object and for creating a project that accesses an IronPython library.