What is the difference between lambdas and delegates in the .NET Framework?

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

A delegate is a reference to a method with a particular parameter list and return type. It may or may not include an object.

A lambda-expression is a form of anonymous function.

Answer 2

lambdas are simply syntactic sugar on a delegate. The compiler ends up converting lambdas into delegates.

These are the same, I believe:

Delegate delegate = x => "hi!";
Delegate delegate = delegate(object x) { return "hi";};

Answer 3

Lambdas are simplified versions of delegates. They have some of the the properties of a closure like anonymous delegates, but also allow you to use implied typing. A lambda like this:

something.Sort((x, y) => return x.CompareTo(y));

is a lot more concise than what you can do with a delegate:

something.Sort(sortMethod);
...

private int sortMethod(SomeType one, SomeType two)
{
    one.CompareTo(two)
}

Answer 4

Heres an example I put up awhile on my lame blog. Say you wanted to update a label from a worker thread. I've got 4 examples of how to update that label from 1 to 50 using delegates, anon delegates and 2 types of lambdas.

 private void button2_Click(object sender, EventArgs e) 
     { 
         BackgroundWorker worker = new BackgroundWorker(); 
         worker.DoWork += new DoWorkEventHandler(worker_DoWork); 
         worker.RunWorkerAsync(); 
     } 

     private delegate void UpdateProgDelegate(int count); 
     private void UpdateText(int count) 
     { 
         if (this.lblTest.InvokeRequired) 
         { 
             UpdateProgDelegate updateCallBack = new UpdateProgDelegate(UpdateText); 
             this.Invoke(updateCallBack, new object[] { count }); 
         } 
         else 
         { 
             lblTest.Text = count.ToString(); 
         } 
     } 

     void worker_DoWork(object sender, DoWorkEventArgs e) 
     {   
         /* Old Skool delegate usage.  See above for delegate and method definitions */ 
         for (int i = 0; i < 50; i++) 
         { 
             UpdateText(i); 
             Thread.Sleep(50); 
         } 

         // Anonymous Method 
         for (int i = 0; i < 50; i++) 
         { 
             lblTest.Invoke((MethodInvoker)(delegate() 
             { 
                 lblTest.Text = i.ToString(); 
             })); 
             Thread.Sleep(50); 
         } 

         /* Lambda using the new Func delegate. This lets us take in an int and 
          * return a string.  The last parameter is the return type. so 
          * So Func<int, string, double> would take in an int and a string 
          * and return a double.  count is our int parameter.*/ 
         Func<int, string> UpdateProgress = (count) => lblTest.Text = count.ToString(); 
         for (int i = 0; i < 50; i++) 
         { 
             lblTest.Invoke(UpdateProgress, i); 
             Thread.Sleep(50); 
         } 

         /* Finally we have a totally inline Lambda using the Action delegate 
          * Action is more or less the same as Func but it returns void. We could 
          * use it with parameters if we wanted to like this: 
          * Action<string> UpdateProgress = (count) => lblT…*/ 
         for (int i = 0; i < 50; i++) 
         { 
             lblTest.Invoke((Action)(() => lblTest.Text = i.ToString())); 
             Thread.Sleep(50); 
         } 
     }

Answer 5

I assume that your question concerns c# and not .NET, because of the ambiguity of your question, as .NET does not get alone - that is, without c# - comprehension of delegates and lambda expressions.

A (normal, in opposition to so called generic delegates, cf later) delegate should be seen as a kind of c++ typedef of a function pointer type, for instance in c++ :

R (*thefunctionpointer) ( T ) ;

typedef's the type thefunctionpointer which is the type of pointers to a function taking an object of type T and returning an object of type R. You would use it like this :

thefunctionpointer = &thefunction ;
R r = (*thefunctionpointer) ( t ) ; // where t is of type T

where thefunction would be a function taking a T and returning an R.

In c# you would go for

delegate R thedelegate( T t ) ; // and yes, here the identifier t is needed

and you would use it like this :

thedelegate thedel = thefunction ;
R r = thedel ( t ) ; // where t is of type T

where thefunction would be a function taking a T and returning an R. This is for delegates, so called normal delegates.

Now, you also have generic delegates in c#, which are delegates that are generic, i.e. that are "templated" so to speak, using thereby a c++ expression. They are defined like this :

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

And you can used them like this :

Func<double, double> thefunctor = thefunction2; // call it a functor because it is
                                                // really as a functor that you should
                                                // "see" it
double y = thefunctor(2.0);

where thefunction2 is a function taking as argument and returning a double.

Now imagine that instead of thefunction2 I would like to use a "function" that is nowhere defined for now, by a statement, and that I will never use later. Then c# allows us to use the expression of this function. By expression I mean the "mathematical" (or functional, to stick to programs) expression of it, for instance : to a double x I will associate the double x*x. In maths you write this using the "\mapsto" latex symbol. In c# the functional notation has been borrowed : =>. For instance :

Func<double, double> thefunctor = ( (double x) => x * x ); // outer brackets are not
                                                           // mandatory

(double x) => x * x is an expression. It is not a type, whereas delegates (generic or not) are.

Morality ? At end, what is a delegate (resp. generic delegate), if not a function pointer type (resp. wrapped+smart+generic function pointer type), huh ? Something else ! See this and that.