Extension Methods in .NET

To put it in a simple manner, Extension Methods have been introduced in .NET 3.5 framework to add methods to a class without altering the code of that class. Most of the times you don't have access to create your own methods in the third party dlls and you want to write your own method in that class, then the first question comes in your mind, how will I handle it?

The answer is simple: "Extension Methods".

Why Extension Method: Another question is, why do we need to use the extension methods, we can create our own methods in client application or in other class library and we can call those methods from the client application but the important point is, these methods would not be the part of that class (class of the third party dll). So adding the extension methods in the class, you are indirectly including another behavior in that class. I am talking about the class which is the part of the third party dll (Most Important).

So when you create your own extension methods for specific types those methods would be accessible for that type.

This can be easily understood by the following diagram:



(I am object of the class)

After some time, client application wants to add another behavior in that class; let's say 'talk' then extension methods can be used to add this extra behavior in that class without altering any code.



Implementation: The following is the structure of the third party class, which is not accessible to the developer. This class contains a method 'Walk'.public class Class1{
    public String Walk()
    {        return "I can Walk";    }}

Now it's time to create the Extension Method. Just use the following steps:

Step 1: Create the Static Class

Step 2: Create the Static Method and give any logical name to it. 

Step 3: In the Extension Method Parameter, just pass this along with the object type and 
the object name as given below:



The client application can access both the method "Walk" and the extension method "TalkExtMethod":Class1 obj = new Class1 ();

Obj.TalkExtMethod ()  
obj.Walk ()

Benefits: There may be various benefits of using the Extension Methods in your code. Let's say you are using the method1 () of the class in the third party dll and the object instantiation has failed due to another reason; then you will get the object reference error while calling the method1 () if you did not handle the null reference check.Class1 obj = GetClass1Object ();//If the obj is null, you will get the object reference errorobj.Method1();

Although the following code is safer than previous one but the problem is you will have to include this check every time when you call the Method1 and if the developer forgets to include this check in the code then the program throws an object reference error.Class1 obj = GetClass1Object ();if (obj!= null)
{
    obj.Method1 ();
}
If we use the extension method to solve this problem we get outstanding results. Instead of checking the null reference every time in the code we handle it in the extension methods which is called by the client application.public static string Method1ExtMethod(this Class1 objClass)
{    //If the obj is not null, then call the Method1()

    if (objClass! = null)
    {
        return objClass.Method1 ();
    }
    return string.Empty;    
}

Class1 obj = GetClass1Object ();//Don't include any check whether the object is null or notobj.Method1ExtMethod()