Question

I get asked this question a lot and I thought I’d solicit some input on how to best describe the difference.

Answer 1

They are actually two very different things. "Delegate" is actually the name for a variable that holds a reference to a method or a lambda, and a lambda is a method without a permanent name.

Lambdas are very much like other methods, except for a couple subtle differences.

1. A normal method is defined in a "statement" and tied to a permanent name, whereas a lambda is defined "on the fly" in an "expression" and has no permanent name.
2. Some lambdas can be used with .NET expression trees, whereas methods cannot.

A delegate is defined like this:

delegate Int32 BinaryIntOp(Int32 x, Int32 y); 

A variable of type BinaryIntOp can have either a method or a lambda assigned to it, as long as the signature is the same: two Int32 arguments, and an Int32 return.

A lambda might be defined like this:

BinaryIntOp sumOfSquares = (a, b) => a*a + b*b; 

Another thing to note is that although the generic Func and Action types are often considered "lambda types", they are just like any other delegates. The nice thing about them is that they essentially define a name for any type of delegate you might need (up to 4 parameters, though you can certainly add more of your own). So if you are using a wide variety of delegate types, but none more than once, you can avoid cluttering your code with delegate declarations by using Func and Action.

Here is an illustration of how Func and Action are "not just for lambdas":

Int32 DiffOfSquares(Int32 x, Int32 y) {   return x*x - y*y; }  Func<Int32, Int32, Int32> funcPtr = DiffOfSquares; 

Another useful thing to know is that delegate types (not methods themselves) with the same signature but different names will not be implicitly casted to each other. This includes the Func and Action delegates. However if the signature is identical, you can explicitly cast between them.

Going the extra mile…. In C# functions are flexible, with the use of lambdas and delegates. But C# does not have "first-class functions". You can use a function’s name assigned to a delegate variable to essentially create an object representing that function. But it’s really a compiler trick. If you start a statement by writing the function name followed by a dot (i.e. try to do member access on the function itself) you’ll find there are no members there to reference. Not even the ones from Object. This prevents the programmer from doing useful (and potentially dangerous of course) things such as adding extension methods that can be called on any function. The best you can do is extend the Delegate class itself, which is surely also useful, but not quite as much.

Update: Also see Karg’s answer illustrating the difference between anonymous delegates vs. methods & lambdas.

Update 2: James Hart makes an important, though very technical, note that lambdas and delegates are not .NET entities (i.e. the CLR has no concept of a delegate or lambda), but rather they are framework and language constructs.