Why do we need C# delegates

Question

I never seem to understand why we need delegates? I know they are immutable reference types that hold reference of a method but why can't we just call the method directly, instead of calling it via a delegate?

Thanks

Answer 1

Anything that can be done with delegates can be done without them, but delegates provide a much cleaner way of doing them. If one didn't have delegates, one would have to define an interface or abstract base class for every possible function signature containing a function Invoke(appropriate parameters), and define a class for each function which was to be callable by pseudo-delegates. That class would inherit the appropriate interface for the function's signature, would contain a reference to the class containing the function it was supposed to represent, and a method implementing Invoke(appropriate parameters) which would call the appropriate function in the class to which it holds a reference. If class Foo has two methods Foo1 and Foo2, both taking a single parameter, both of which can be called by pseudo-delegates, there would be two extra classes created, one for each method.

Without compiler support for this technique, the source code would have to be pretty heinous. If the compiler could auto-generate the proper nested classes, though, things could be pretty clean. Dispatch speed for pseudo-delegates would probably generally be slower than with conventional delegates, but if pseudo-delegates were an interface rather than an abstract base class, a class which only needs to make a pseudo-delegate for one method of a given signature could implement the appropriate pseudo-delegate interface itself; the class instance could then be passed to any code expecting a pseudo-delegate of that signature, avoiding any need to create an extra object. Further, while the number of classes one would need when using pseudo-delegates would be greater than when using "real" delegates, each pseudo-delegate would only need to hold a single object instance.

Answer 2

You asked for an example of why you would pass a function as a parameter, I have a perfect one and thought it might help you understand, it is pretty academic but shows a use. Say you have a ListResults() method and getFromCache() method. Rather then have lots of checks if the cache is null etc. you can just pass any method to getCache and then only invoke it if the cache is empty inside the getFromCache method:

_cacher.GetFromCache(delegate { return ListResults(); }, "ListResults");

public IEnumerable<T> GetFromCache(MethodForCache item, string key, int minutesToCache = 5)
    {
        var cache = _cacheProvider.GetCachedItem(key);
        //you could even have a UseCache bool here for central control
        if (cache == null)
        {
            //you could put timings, logging etc. here instead of all over your code
            cache = item.Invoke();
            _cacheProvider.AddCachedItem(cache, key, minutesToCache);
        }            

        return cache;
    }