可以将委托分配给匿名方法或lambda，但不分配给方法

Question

In the following code snippet,

class Program
{
    static async Task Work()
    {
        await Task.Delay(1000);
    }

    static async Task Main()
    {
        // Action a =  Work;
        Action b = async () => await Task.Delay(1000);
        Action c =  async delegate () { await Task.Delay(1000);  };
    }
}

delegates of type Action cannot be assigned to a method Work of type Task but weirdly can be assigned to an anonymous method or a lambda of type Task .

In my understanding

• Work
• async () => await Task.Delay(1000)
• async delegate () { await Task.Delay(1000); }

have the same signature that are not compatible to Action . What causes this inconsistency?

Edit:

The following lambda and anonymous method

• async () => await Task.Delay(1000)
• async delegate () { await Task.Delay(1000); }

can represent both Action or Func<Task> depending on the left hand side of assignment. More precisely,

Converted to Action implicitly,

• Action a = async () => await Task.Delay(1000)
• Action b = async delegate () { await Task.Delay(1000); }

Converted to Func<Task> implicitly,

• Func<Task> a = async () => await Task.Delay(1000)
• Func<Task> b = async delegate () { await Task.Delay(1000); }

Is it @JSteward's comment below about?

Answer 1

First of all, Action is a delegate of return type void :

public delegate void Action()

and your method has the return type Task .

Secondly, the return type of the anonymous method that is created by the lambda expression is determined by the type of the delegate ( Action ). Since the delegate is void , you are creating a void anonymous method. The return value of the Task.Delay call is discarded.

Further, your anonymous function from the delegate() expression is not of type Task , as you assume. It is of type void , since it does not have a return statement. It would not even compile if you added one, since you are trying to assign it to a delegate of type void :

Action c =  async delegate () { return await Task.Delay(1000);  };

gives the error message

Anonymous function converted to a void returning delegate cannot return a value.

Answer 2

Work is a function with the return type of Task, thats why you can't "put" it into an Action.

The only reason for your exemples working, is that the lambda function

() => Task.Delay(1000);

creates an Action that calls Task.Delay(1000).

void Function()
{
    Task.Delay(1000);
}

And when you create an async lambda

async () => await Task.Delay(1000);

creates an Action that awaits Task.Delay(1000).

async void Function()
{
    await Task.Delay(1000);
}

same applies to the anonymous function.

PS

When you await an async function, the Task output is changed to its "type" (in this case void). Thats why if you await Task.Delay(1000), you get void for the return type.