C＃代表具有不同可选参数的方法

Question

I know that I can declare delegates that have default constant parameters such as:

delegate void MyDelegate(int x, int y = 5);

and then call it anywhere with any method that matches the signature.

Ok, I've got LOTS of methods declared like this way:

public Something FirstMethod(float val = 10, int skip = 0){ ... return sth; }
public Something SecondMethod(float val = 20, int skip = 0){ ... return sth; }
public Something ThirdMethod(float val = 5, int skip = 0){ ... return sth; }

... this list goes up all the way down, anyway, they all have that signature structure. The point here is that, they all have floating point argument, that defaults to something different.

Then, I want to create a delegate that will point to one of these methods:

delegate Something ProblematicDelegateType(<<WHAT WILL GO HERE>>);
ProblematicDelegateType myFunc;
if(someValue == someParameter){
    myFunc = FirstMethod;
}else if(...){
    myFunc = SecondMethod;
}else...
...
}

myFunc();
myFunc(skip:100);

I want to be able to call myFunc both parameterless or with the skip parameter. In this part of code, the first parameter, val , is NOT used. (they are used elsewhere).

What will go to the delegate's argument list? I want to preserve that method's default val argument, whatever it is.

Answer 1

If I understand you correctly, your myFunc will leave out the val parameter entirely, always using the default for whichever method you're ultimately calling, correct?

If so, this should accomplish what you're looking for:

delegate Something ProblematicDelegateType(int skip = 0);
ProblematicDelegateType myFunc;
if (someValue == someParameter) {
    myFunc = skip => FirstMethod(skip: skip);
} else if (...) {
    myFunc = skip => SecondMethod(skip: skip);
} else ...
    ...
}

Answer 2

Use a lambda to bind the value that you want. Default values are effectively overloads that have bound the value you desire to the function parameter. Since you have the various if's separating out the functions, just pass around a delegate to a function that only takes a integer.

In other words,

delegate Something ProblematicDelegateType(int skip = 0)

//...

ProblematicDelegateType myDelegate;
if (someCondition)
  myDelegate = (_1) => FirstMethod(10, _1);
else if (someOtherCondition)
  myDelegate = (_1) => SecondMethod(20, _1);

and so on..

Answer 3

Your requirement that the delegate have an optional parameter that varies depending on the method it invokes can't be done. Optional parameters are implemented by rewriting the call site of the method when it is compiled. If the method the delegate points to is determined at run-time what would it write at compile-time? Eric Lippert's series on optional arguments is very enlightening.

However, you could also look into using reflection to get the default value. Use MethodInfo.GetParameters() to get the parameter info and ParameterInfo.DefaultValue to get the default value.