Making a return inside callback

问题

public static string GetFoo() {

        string source = GameInfoUtil.GetSource(repairRequest, () => {
            return "0"; // this line gives error
        });
        .
        .
        MORE WORK, BUT WANT TO SKIP IT
    }


public static string GetSource(WebRequest request, Action failureCallback) {
        // DOING WORK HERE WITH REQUEST
        if(WORK IS SUCCESSFULL) RETURN CORRECT STRING ELSE CALL ->
        failureCallback();
        return "";
    }

I want to do smthing like this, but it gives me error:

Error   2   Cannot convert lambda expression to delegate type 'System.Action' because some of the return types in the block are not implicitly convertible to the delegate return type.
Error   1   Since 'System.Action' returns void, a return keyword must not be followed by an object expression   C:\Users\Jaanus\Documents\Visual Studio 2012\Projects\Bot\Bot\Utils\GameInfoUtil.cs 58  5   Bot

What I want to do, is when something happens in GameInfoUtil.GetSource, it will call out my delegate, and the GetFoo method will return and not continue work.

回答1:

The Action delegate returns void. You are trying to return the string "0".

If you change Action to Func<string> and return that value.

public static string GetSource(WebRequest request, Func<string> failureCallback) {
    // DOING WORK HERE WITH REQUEST
    if(!(WORK IS SUCCESSFULL))
    {
        return failureCallback();
    }
    return "";
}

your code will work.

The code within the lambda can't return from the outer function. Internally the lambda is converted to a regular method (with an unspeakable name).

public static string GetFoo() {
    string source = GameInfoUtil.GetSource(repairRequest, () => {
        return "0"; // this line gives error
    });
}

is equivalent to

public static string GetFoo() {
    string source = GameInfoUtil.GetSource(repairRequest, XXXYYYZZZ);
}

public static string XXXYYYZZZ()
{
    return "0";
}

now you can easily understand why return "0" can't return from GetFoo.

回答2:

An Action delegate should return void. You cannot return a string. You can change it to Func<string>:

string source = GameInfoUtil.GetSource(repairRequest, () => {
        return "0";
    });

public static string GetSource(WebRequest request, Func<string> failureCallback)
{
    if( <some condition> )
        return failureCallback(); // return the return value of callback
    return "";
}

来源：https://stackoverflow.com/questions/15447218/making-a-return-inside-callback

标签

.net

delegates

callback