What is Func, how and when is it used

Maybe it is not too late to add some info.

Sum:

The Func is a custom delegate defined in System namespace that allows you to point to a method with the same signature (as delegates do), using 0 to 16 input parameters and that must return something.

Nomenclature & how2use:

Func<input_1, input_2, ..., input1_6, output> funcDelegate = someMethod;

Definition:

public delegate TResult Func<in T, out TResult>(T arg);

Where it is used:

It is used in lambda expressions and anonymous methods.

Both C# and Java don't have plain functions only member functions (aka methods). And the methods are not first-class citizens. First-class functions allow us to create beautiful and powerful code, as seen in F# or Clojure languages. (For instance, first-class functions can be passed as parameters and can return functions.) Java and C# ameliorate this somewhat with interfaces/delegates.

Func<int, int, int> randInt = (n1, n2) => new Random().Next(n1, n2); 

So, Func is a built-in delegate which brings some functional programming features and helps reduce code verbosity.

Aforementioned answers are great, just putting few points I see might be helpful:

• Func is built-in delegate type

• Func delegate type must return a value. Use Action delegate if no return type needed.

• Func delegate type can have zero to 16 input parameters.

• Func delegate does not allow ref and out parameters.

• Func delegate type can be used with an anonymous method or lambda expression.

  Func<int, int, int> Sum = (x, y) => x + y;