System.Runtime.CompilerServices.AsyncMethodBuilderAttribute sealed class AsyncMethodBuilderAttribute : Attribute Indicates the type of the async method builder that should be used by a language compiler to build the attributed async method or to build the attributed type when used as the return type of an async method. Documentation Code namespace System.Runtime.CompilerServices { internal sealed class AsyncMethodBuilderAttribute : Attribute { public Type BuilderType { get; } public AsyncMethodBuilderAttribute(Type builderType); } }