Lambda expression that returns a delegate

Question

Hi I am trying to achive something like this;

Method<TObjectType>(m=>m.GetData); //m is the instance of TObjectType

if I can succeed on that, then I can visit this expression and get the GetData Method and use it for creating a dynamic sql query. I could do this by giving method name as a string however I don't want to break the strongly type world of my developer friends.

I know I have to give the exact definition of the delegate but this still doesn't helped me;

void Method<TObjectType>(Expression<Func<TObjectType, Delegate>> ex){/**/}

Do you have an idea?

Roncesvalles

Do you know anything about GetData()? Do you know that it will always have zero parameters, or something like that? – Stationer

If you do not know the signature of the GetData method you will be leaving the strongly typed world. – Pox

This might help you as well #2840938 – Manilla

@Stationer signature of the method should be generic because this could be any method. – Roncesvalles

@Erno I thought so I am afraid – Roncesvalles

Answer 1

Unfortunately, there's barely such a thing as "a delegate". For most purposes (and especially: for resolving a MethodInfo), it must be a strongly-typed delegate. This is because GetData isn't a method, it is a method group. You really need to specify the method precisely, or have a known delegate-type (which ultimately does the same thing).

You have two practical options; work to object, or add a generic. For example:

void Method<TObjectType>(Expression<Func<TObjectType,object>> ex) {...}

would work, as would:

void Method<TObjectType, TValue>(Expression<Func<TObjectType,TValue>> ex) {...}

The caller would use:

Method<Foo>(x => x.GetData());

---

If you really want to use a delegate, the type must be predictable, for example:

void Method<TObjectType>(Expression<Func<TObjectType,Func<int>>> ex)

allowing:

Method<Foo>(x => x.GetData);

Alternatively, if you know (for example) that the method is always parameterless, but you don't know the return type; maybe something like:

void Method<TObjectType, TValue>(Expression<Func<TObjectType,Func<TValue>>> ex)

which allows:

Method<Foo, int>(x => x.GetData);

Oshea

In this scenario I suppose you are assuming that the GetData method is parameterless so that you can define it using the Func<T>. Which means, I have to make lots of Method overrides, am I correct? – Roncesvalles

@lazycoder I would advise against it, because you'll end up making a method with an insane number of generic type arguments, that is rather inconvenient to call. I would actually advise the simpler object/TValue approach, with the caller specifying x.GetData(). For more complex overloads, they can always specify x.GetData(123, "abc", DateTime.Now) - the expression tree will tell you what overload they used, and (if you want them) the parameter values. – Oshea

@lazycode Worst case, you then need two overloads; one Func<TObjectType,object>, one Action<TObjectType> (for void methods) – Oshea

thanks, I suppose I have two ways; one way is to force the developer to define at least one parameterless override and second is to as Erno said "leaving the strongly typed world." :) – Roncesvalles

Answer 2

Are you looking for something like this?

object InvokeMethod(Delegate method, params object[] args){
    return method.DynamicInvoke(args);
}

int Add(int a, int b){
    return a + b;
}

void Test(){
    Console.WriteLine(InvokeMethod(new Func<int, int, int>(Add), 5, 4));
}

Manilla

thank you for the answer but I don't want to invoke this method I just want developers to define something with strongly typed – Roncesvalles