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AsyncStateMachineAttribute Class

.NET Framework 4.6 and 4.5

Indicates whether a method is marked with either the Async (Visual Basic) or async (C# Reference) modifier.


Namespace:  System.Runtime.CompilerServices
Assembly:  mscorlib (in mscorlib.dll)

[AttributeUsageAttribute(AttributeTargets.Method, Inherited = false, AllowMultiple = false)]
public sealed class AsyncStateMachineAttribute : StateMachineAttribute

The AsyncStateMachineAttribute type exposes the following members.

Public methodAsyncStateMachineAttributeInitializes a new instance of the AsyncStateMachineAttribute class.

Public propertyStateMachineTypeReturns the type object for the underlying state machine type that was generated by the compiler to implement the state machine method. (Inherited from StateMachineAttribute.)
Public propertyTypeIdWhen implemented in a derived class, gets a unique identifier for this Attribute. (Inherited from Attribute.)

Public methodEqualsInfrastructure. Returns a value that indicates whether this instance is equal to a specified object. (Inherited from Attribute.)
Public methodGetHashCodeReturns the hash code for this instance. (Inherited from Attribute.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodIsDefaultAttributeWhen overridden in a derived class, indicates whether the value of this instance is the default value for the derived class. (Inherited from Attribute.)
Public methodMatchWhen overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from Attribute.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)

Explicit interface implemetationPrivate method_Attribute.GetIDsOfNamesMaps a set of names to a corresponding set of dispatch identifiers. (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.GetTypeInfoRetrieves the type information for an object, which can be used to get the type information for an interface. (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.GetTypeInfoCountRetrieves the number of type information interfaces that an object provides (either 0 or 1). (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.InvokeProvides access to properties and methods exposed by an object. (Inherited from Attribute.)

You shouldn't apply the AsyncStateMachine attribute to methods in your code. For methods that have the async modifier, the compiler will apply the AsyncStateMachine attribute in the IL that the compiler emits.

When a method (MethodName) has the Async or async modifier, the compiler emits IL that includes a state machine structure. This structure contains the code in the method. That IL also contains a stub method (MethodName) that calls into the state machine. The compiler adds the AsyncStateMachine attribute to the stub method so that tools can identify the corresponding state machine. Details of the emitted IL might change in future releases of the compilers.

For information about the Async feature, see Asynchronous Programming with Async and Await (C# and Visual Basic).

As the following example shows, you can determine whether a method is marked with Async (Visual Basic) or async (C# Reference) modifier. In the example, IsAsyncMethod performs the following steps:

using System;
using System.Collections.Generic;
using System.Reflection;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;

namespace ConsoleApplication1

    // This class is used by the code below to discover method attributes. 
    public class TheClass
        public async Task<int> AsyncMethod()
            await Task.Delay(5);
            return 1;

        public int RegularMethod()
            return 0;

    class Program
        private static bool IsAsyncMethod(Type classType, string methodName)
            // Obtain the method with the specified name.
            MethodInfo method = classType.GetMethod(methodName);

            Type attType = typeof(AsyncStateMachineAttribute);

            // Obtain the custom attribute for the method. 
            // The value returned contains the StateMachineType property. 
            // Null is returned if the attribute isn't present for the method. 
            var attrib = (AsyncStateMachineAttribute)method.GetCustomAttribute(attType);

            return (attrib != null);

        private static void ShowResult(Type classType, string methodName)
            Console.Write((methodName + ": ").PadRight(16));

            if (IsAsyncMethod(classType, methodName))
                Console.WriteLine("Async method");
                Console.WriteLine("Regular method");

        static void Main(string[] args)
            ShowResult(classType: typeof(TheClass), methodName: "AsyncMethod");
            ShowResult(classType: typeof(TheClass), methodName: "RegularMethod");

            // Note: The IteratorStateMachineAttribute applies to Visual Basic methods 
            // but not C# methods.


        // Output: 
        //   AsyncMethod:    Async method 
        //   RegularMethod:  Regular method

.NET Framework

Supported in: 4.6, 4.5

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
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