C# in Depth

Delegates39without worrying about consistency, thread safety, or anyone trying to change theiractions. This is just the same with delegate instances as it is with strings, which arealso immutable. The reason for mentioning this is that Delegate.Combine is just likeString.Concat—they both combine existing instances together to form a new onewithout changing the original objects at all. In the case of delegate instances, theoriginal invocation lists are concatenated together. Note that if you ever try to combinenull with a delegate instance, the null is treated as if it were a delegateinstance with an empty invocation list.You’ll rarely see an explicit call to Delegate.x += y;x = x+y;x = Delegate.Combine(x, y);Figure 2.2 The transformation processused for the C# shorthand syntax forcombining delegate instancesCombine in C# code—usually the + and += operatorsare used. Figure 2.2 shows the translationprocess, where x and y are both variables of thesame (or compatible) delegate types. All of this isdone by the C# compiler.As you can see, it’s a straightforward transformation,but it does make the code a lot neater.Just as you can combine delegate instances,you can remove one from another with theDelegate.Remove method, and C# uses the shorthandof the - and -= operators in the obvious way. Delegate.Remove(source, value)creates a new delegate whose invocation list is the one from source, with the list fromvalue having been removed. If the result would have an empty invocation list, nullis returned.Table 2.1 shows some examples of the results of combining and removing delegateinstances. I’ve used the notation [a, b, c] to indicate a delegate with an invocationlist of actions a, b, and c (whatever they may happen to be).Table 2.1A selection of examples of combining and removing delegates, showing what the resultis and whyOperation Result Notes[a] + [b] [a, b] --[a] + null [a] null counts as an empty invocation list.null + [a] [a] --[a] + [b, c] [a, b, c] --[a, b] + [b, c] [a, b, b, c] Duplicates are allowed.[a, b, c] - [b] [a, c] The removal list doesn’t have to be at the end…[a, b, c, d, b, c] - [b, c] [a, b, c, d] … but the last occurrence of the removal list isremoved.[a] - [b] [a] No-op removal of nonexistent list.Licensed to Rhona Hadida