Framework Design Guidelines: Avoiding custom delegates

Continuing in our weekly blog post series that highlights a few of the new additions to the Framework Design Guidelines 2nd edition.. This content is found in the Events and Callbacks section of Chapter 6: Designing for Extensibility. I am impressed by how simple new additions to the BCL can have such a large (and positive) effect on framework design. As these new advances come out, learn them and use them!

DO use the new Func<…>, Action<…>, or Expression<…> instead of custom delegates, when defining APIs with callbacks.

Func<…> and Action<…> represent generic delegates. The following is how .NET Framework defines them:

public delegate void Action()
public delegate void Action<T1, T2>(T1 arg1, T2 arg2)
public delegate void Action<T1, T2, T3>(T1 arg1, T2 arg2, T3 arg3)
public delegate void Action<T1, T2, T3, T4>(T1 arg1, T2 arg2, T3 arg3, T4 arg4)
public delegate TResult Func<TResult>()
public delegate TResult Func<T, TResult>(T arg)
public delegate TResult Func<T1, T2, TResult>(T1 arg1, T2 arg2)
public delegate TResult Func<T1, T2, T3, TResult>(T1 arg1, T2 arg2, T3 arg3)
public delegate TResult Func<T1, T2, T3, T4, TResult>(T1 arg1, T2 arg2, T3 arg3, T4 arg4)

They can be used as follows:

Func<int,int,double> divide = (x,y)=>(double)x/(double)y;
Action<double> write = (d)=>Console.WriteLine(d);
write(divide(2,3));

Expression<…> represents function definitions that can be compiled and subsequently invoked at runtime but can also be serialized and passed to remote processes. Continuing with our example:

Expression<Func<int,int,double>> expression =(x,y)=>(double)x/(double)y;
Func<int,int,double> divide = expression.Compile();
write(divide2(2,3));

Notice how the syntax for constructing an Expression<> object is very similar to the one used to construct a Func<> object; in fact, the only difference is the static type declaration of the variable (Expression<> instead of Func<>).

RICO MARIANI

Most times you’re going to want Func or Action if all that needs to happen is to run some code. You need Expression when the code needs to be analyzed, serialized, or optimized before it is run. Expression is for thinking about code, Func/Action is for running it.