Multicast delegate of type Func (with return value)?

Question

I have the following code:

Func<string, string> func1 = (param) =>
{
    Console.WriteLine("Func 1 executing");
    return "Hello" + param;
};
Func<string, string> func2 = (param) =>
{
    Console.WriteLine("Func 2 executing");
    return "World" + param;
};
Func<string, string> funcSum = func1 + func2;
string funcResult = funcSum("!");
Console.WriteLine(funcResult);

The output is:

Func 1 executing
Func 2 executing
World!

Inverting the sum:

Func<string, string> funcSum = func2 + func1;

Gives the output:

Func 2 executing
Func 1 executing
Hello!

My initial test was done with a boolean return type, and the returned value was also always determined by the last function. Is it working as intended? Aren't we losing the return value of the other functions? If so, is there a use case in real world of those multicast delegate of functions?

Answer 1

Is it working as intended?

It's working as specified, at least. Whether that's what you intended or not is a different matter :) From section 15.4 of the C# 5 specification - emphasis mine:

Invocation of a delegate instance whose invocation list contains multiple entries proceeds by invoking each of the methods in the invocation list, synchronously, in order. Each method so called is passed the same set of arguments as was given to the delegate instance. If such a delegate invocation includes reference parameters (§10.6.1.2), each method invocation will occur with a reference to the same variable; changes to that variable by one method in the invocation list will be visible to methods further down the invocation list. If the delegate invocation includes output parameters or a return value, their final value will come from the invocation of the last delegate in the list.

Next:

Aren't we losing the return value of the other functions?

Yes, at the moment.

If so, is there a use case in real world of those multicast delegate of functions?

Very rarely, to be honest. However, you can split a multicast delegate apart, using Delegate.GetInvocationList():

foreach (Func<string, string> func in funcSum.GetInvocationList())
{
    Console.WriteLine(func("!"));
}

Answer 2

The multicast delegate will always return the result of the last function. Because there is no predefined way to combine or chain the T results.

If you want to obtain all the results along the chain, try this :

    var result = "!";
    foreach (Func<string, string> func in funcSum.GetInvocationList())
    {
        result = func(result);
    }