C# Delegate - A Silent Hero Behind Modern Programming

Delegate plays an important role in today's programming model where it is extensively used in Parallel Programming, Asynchronous Programming, PLINQ, Events & Callback methods, Func, Action, Predicates and so on.

Introduction

Modern development tools & languages are full with easy use of tedious functionalities like threading, asynchronous programming, and so on. A developer just need to use ready-made mechanism (APIs/Functions) and rest is done by language or .net system library.

Background

Its looks like programming is now so simplified that anyone can write higher level code like multi-threading & asynchronous. It sound cool, but to write an efficient & optimized code, a programmer must have a good understanding on fundamental items like-

• In terms of Hardware - CPU architecture, memory model, IO operations and OS management on resources
• In terms of Programming – Types, Delegate, LINQ and so on.
  

.NET Types (in C# context)

Let’s understand the “type” in the following simple points-

• Almost everything in .NET is object (except interface type, pointer type, open parameter type, type & args)
• Most of types are derived from System.Object
• There are some types which are not derived from anything.

Delegate - a true legend

Let me define to delegate the following points-

• An object (variable), used to hold the address of function.
  
  
• Same type of delegate objects can be composed & discomposed using the "+" and "-" operator respectively. This is called Multi-casting.

If we further drill down on above definition, we can conclude that,

• An Object: It means delegate is of reference type (derived from System.Delegate class).
  
  
• hold the address: (a) It is like pointers to function. (b) we can also conclude here, that this address can be changed to any function (which is of same signature).

Usage of Delegate

1. Basic Usage: Callback methods & events implementations
2. Advance Usage: Func, Action, Predicates and so on.

1. Basic Usage

Suppose, we want to decide which method to call on the basis of some logic which is depend upon run-time scenario or user input. Here, at compile time we are not sure about which method to call but that particular method will decide later (at run-time).

 

Like other objects, a delegate object also follow same using approach i.e. Declare, Define and Use. Its declaration is more about the signature of function. Once it is declared, it is assigned/initialized/defined. To reach the stored address of function (it will execute/fire that function) a handy approach, Invoke() can be used if we decide to not let implicit call.

Syntax

delegate <return type of targeting function> DelegateName ([parameters of targeting function]);

Example

//a delegate to hold the address of any function which takes only one parameter as string and returns int type
delegate int MyDelegateName (string name);

We may also apply Access-modifiers on delegate, example-

public delegate void MyDelegateName (string name);

2. Advance Usage

The following are the most famous flavor of delegate- 

2.1 Func : a delegate which returns a value

2.2 Action<T> : a delegate which don't returns any value

2.3 Predicates :  a delegate which return a value of type Boolean

Summary

In modern development practices, programmers need to aware about the core components like delegate. Delegate has wider scope of usage with various flavors like multi-casting, Func, Action, Predicate and so on. Most of efficient APIs like TPL, PLINQ etc are written using the true power of delegate.