Anonymous methods/delegates and lambda expressions

Question

With the advent of new features like lambda expressions (inline code), does it mean we dont have to use delegates or anonymous methods anymore? In almost all the samples I have seen, it is for rewriting using the new syntax.

Any place where we still have to use delegates and lambda expressions won't work?

Answer 1

lambda is shortcut for anonymous delegate, but you will always be using delegates. the delegate specifies the methods signature. you can just do this:

 delegate(int i) { Console.WriteLine(i.ToString()) }

can be replaced with

f => Console.WriteLine(f.ToString())

Answer 2

Lambda expressions are just "syntactic sugar", the compiler will generate appropriate delegates for you. You can investigate this by using Lutz Roeder's Reflector.

Answer 3

Lamda's are just syntactic sugar for delegates, they are not just inline, you can do the following:

s.Find(a =>
{
    if (a.StartsWith("H"))
     return a.Equals("HI");
    else
     return !a.Equals("FOO");
});

And delegates are still used when defining events, or when you have lots of arguments and want to actually strongly type the method being called.

Answer 4

Yes there are places where directly using anonymous delegates and lambda expressions won't work.

If a method takes an untyped Delegate then the compiler doesn't know what to resolve the anonymous delegate/lambda expression to and you will get a compiler error.

public static void Invoke(Delegate d)
{
  d.DynamicInvoke();
}

static void Main(string[] args)
{
  // fails
  Invoke(() => Console.WriteLine("Test"));

  // works
  Invoke(new Action(() => Console.WriteLine("Test")));

  Console.ReadKey();
}

The failing line of code will get the compiler error "Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type".

Answer 5

Delegate have two meanings in C#.

The keyword delegate can be used to define a function signature type. This is usually used when defininge the signature of higher-order functions, i.e. functions that take other functions as arguments. This use of delegate is still relevant.

The delegate keyword can also be used to define an inline anonymous function. In the case where the function is just a single expression, the lambda syntax is a simpler alternative.

Answer 6

Lambda expression is not (and was not meant to be) a silver bullet that would replace (hide) delegates. It is great with small local things like:

List<string> names = GetNames();
names.ForEach(Console.WriteLine);

1. it makes code more readable thus simple to understand.
2. It makes code shorter thus less work for us ;)

On the other hand it is very simple to misuse them. Long or/and complex lambda expressions are tending to be:

1. Hard to understand for new developers
2. Less object oriented
3. Much harder to read

So “does it mean we don’t have to use delegates or anonymous methods anymore?” No – use Lambda expression where you win time/readability otherwise consider using delegates.