为什么Type.IsGenericType为Task返回TRUE，而没有通过方法的反射获得返回类型，但是typeof（Task）.IsGenericTyp返回FALSE

Question

Can someone explain this? Per documentation IsGenericType

indicatesing whether the current Type represents a type parameter in the definition of a generic type or method.

So this (LINQPad) code:

bool bStraight = typeof(Task).IsGenericType;
bStraight.Dump("typeof(Task).IsGenericType");

works as expected and yields the output:

typeof(Task).IsGenericType
False

But when I retrieve it from a method by reflection:

public class MyClass
{
    public async Task Method()
    {
        await Task.Run(() =>
        {
            Thread.Sleep(3000);
        });
    }
}

public async Task TEST()
{
    MyClass theObject = new MyClass();

    Task task = (Task)typeof(MyClass).GetTypeInfo()
                            .GetDeclaredMethod("Method")
                            .Invoke(theObject, null);

    bool b = task.GetType().IsGenericType;  
    bool b2 = task.GetType().GetGenericTypeDefinition() == typeof(Task<>);
    b.Dump("IsGenericType");
    b2.Dump("GetGenericTypeDefinition");

    bool bStraight = typeof(Task).IsGenericType;
    bStraight.Dump("typeof(Task).IsGenericType");
}

I get the unexpected output of :

IsGenericType
True

GetGenericTypeDefinition
True

Answer 1

In some situations, the framework returns a Task<VoidTaskResult> disguised as a Task . Imagine you have some logic relying on TaskCompletionSource<T> . If you don't actually intend to return a result, you still have to fill the T generic parameter. You could use TaskCompletionSource<object> , but you would be wasting a pointer worth of memory (4 bytes in 32 bit, 8 bytes in 64 bit). To avoid that, the framework uses an empty struct: VoidTaskResult .