Creating delegates manually vs using Action/Func delegates

The advantage is clarity. By giving the type an explicit name it is more clear to the reader what it does.

It will also help you when you are writing the code. An error like this:

cannot convert from Func<string, int, double> to Func<string, int, int, double>

is less helpful than one which says:

cannot convert from CreateListAction to UpdateListAction

It also means that if you have two different delegates, both of which take the same types of parameters but conceptually do two entirely different things, the compiler can ensure that you can't accidentally use one where you meant the other.

The advent of Action and Func family of delegates has rendered custom delegates less used, but the latter still finds uses. Advantages of custom delegates include:

1. As others have pointed, conveys intent clearly unlike generic Action and Func (Patrik has a very good point about meaningful parameter names).

2. You can specify ref/out parameters unlike the other two generic delegates. For eg, you can have

   public delegate double ChangeListAction(out string p1, ref int p2);
   

   but not

   Func<out string, ref int, double> ChangeListAction;
   

3. Also, with custom delegates you need to write ChangeListAction (I mean the definition) only once somewhere in your code base, whereas if you don't define one you will have to litter everywhere Func<string, int, double> all over. Changing the signature will be a hassle in the latter case - a bad case of not being dry.

4. Can have optional parameters.

   public delegate double ChangeListAction(string p1 = "haha", int p2);
   

   but not

   Func<string, int, double> ChangeListAction = (p1 = "haha", p2) => (double)p2; 
   

5. You can have params keyword for parameters of a method, not so with Action/Func.

   public delegate double ChangeListAction(int p1, params string[] p2);
   

   but not

   Func<int, params string[], double> ChangeListAction;
   

6. Well, if you're truly out of luck and need parameters more than 16 (for the moment) :)

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As to merits of Action and Func:

1. It's quick and dirty, and I use it all over. It makes code short if the use-case is trivial (custom delegates have gone out of fashion with me).

2. More importantly, its type compatible across domains. Action and Func are framework defined, and they operates seamlessly as long as the parameter types match. You can't have ChangeSomeAction for ChangeListAction. Linq finds great use of this aspect.