C# - Dynamic Types

C# 4.0 (.NET 4.5) introduced a new type called dynamic that avoids compile-time type checking. A dynamic type escapes type checking at compile-time; instead, it resolves type at run time.

A dynamic type variables are defined using the dynamic keyword.

Example: dynamic Variable
dynamic MyDynamicVar = 1;

The compiler compiles dynamic types into object types in most cases. However, the actual type of a dynamic type variable would be resolved at run-time.

Example: dynamic Type at run-time
dynamic MyDynamicVar = 1;

Console.WriteLine(MyDynamicVar.GetType());
Output:
System.Int32

Dynamic types change types at run-time based on the assigned value. The following example shows how a dynamic variable changes type based on assigned value.

Example: dynamic
static void Main(string[] args)
{
    dynamic MyDynamicVar = 100;
    Console.WriteLine("Value: {0}, Type: {1}", MyDynamicVar, MyDynamicVar.GetType());

    MyDynamicVar = "Hello World!!";
    Console.WriteLine("Value: {0}, Type: {1}", MyDynamicVar, MyDynamicVar.GetType());

    MyDynamicVar = true;
    Console.WriteLine("Value: {0}, Type: {1}", MyDynamicVar, MyDynamicVar.GetType());

    MyDynamicVar = DateTime.Now;
    Console.WriteLine("Value: {0}, Type: {1}", MyDynamicVar, MyDynamicVar.GetType());
}
Output:
Value: 100, Type: System.Int32
Value: Hello World!!, Type: System.String
Value: True, Type: System.Boolean
Value: 01-01-2014, Type: System.DateTime

The dynamic type variables is converted to other types implicitly.

Example: dynamic Type Conversion
dynamic d1 = 100;
int i = d1;
		
d1 = "Hello";
string greet = d1;
		
d1 = DateTime.Now;
DateTime dt = d1;

Methods and Parameters

If you assign a class object to the dynamic type, then the compiler would not check for correct methods and properties name of a dynamic type that holds the custom class object. Consider the following example.

Example: Calling Methods
class Program
{
    static void Main(string[] args)
    {
        dynamic stud = new Student();

        stud.DisplayStudentInfo(1, "Bill");// run-time error, no compile-time error
        stud.DisplayStudentInfo("1");// run-time error, no compile-time error
        stud.FakeMethod();// run-time error, no compile-time error
    }
}

public class Student
{
    public void DisplayStudentInfo(int id)
    {
    }
}

In the above example, the C# compiler does not check for the number of parameters, parameters type, or non-existent. It validates these things at run-time, and if it is not valid, then throws a run-time exception. Note that Visual Studio IntelliSense is not supported for the dynamic types. Note that Visual Studio IntelliSense is not supported for the dynamic types.

The dynamic language runtime (DLR) API provides the infrastructure that supports the dynamic type in C#. For more information about the DLR, visit Dynamic Language Runtime Overview.

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