.NET Framework 4.6 and 4.5
When implemented in a derived class, gets or sets a value that indicates whether asynchronous operations are allowed during parts of ASP.NET request processing when they are not expected.
Assembly: System.Web (in System.Web.dll)
abstract AllowAsyncDuringSyncStages : bool with get, set override AllowAsyncDuringSyncStages : bool with get, set
Property ValueType: System.Boolean
false if ASP.NET will throw an exception when the asynchronous API is used at a time when it is not expected; otherwise, true. The default value is false.