When do you use extension methods, ext. methods vs. inheritance?

Times to use extension methods:

• when you don't control the types being extended
• where you don't want to force the implementor to provide code that can be done using the existing methods

For an example of the second point; you might have an extension method on IList<T> (for example, Sort) that can be written entirely using the existing IList<T> members... so why force anybody else to write anything? This is the foundation block of LINQ, and allowed Microsoft to provide much more functionality without breaking anything.

Times to not use extension methods:

• when polymorphism is critical; you cannot guarantee that your code will be the version that gets executed with an extension method, as methods directly on the type take precedence
• when you need access to private/protected members

Extension methods allow existing classes to be extended without relying on inheritance or having to change the class's source code. This means that if you want to add some methods into the existing String class you can do it quite easily. Here's a couple of rules to consider when deciding on whether or not to use extension methods:

• Extension methods cannot be used to override existing methods

• An extension method with the same name and signature as an instance method will not be called

• The concept of extension methods cannot be applied to fields, properties or events

• Use extension methods sparingly....overuse can be a bad thing!

I use extension methods when it makes sense. If you control a class and its code, you usually don't need extension methods.

If you don't, an extension method might be useful.

One place I frequently use extension methods is for [Flags] enumerations. When you have a flag-based enumeration, there's a rather large expression that's necessary to determine whether or not an enumeration value has a particular flag set. And so I build the following extension method whenever I build a [Flags] enumeration:

[Flags]
public enum MyEnum
{
    FlagA,
    FlagB,
    // etc.
}

public static class MyEnumExt
{
    public static bool HasFlags(this MyEnum item, MyEnum query)
    {
        return ((item & query) == query);
    }
}

That way my code looks like:

MyEnum flags = MyEnum.FlagA;
if(flags.HasFlags(MyEnum.FlagA))
{
  // handle FlagA
}

Rather than:

MyEnum flags = MyEnum.FlagA;
if((flags & MyEnum.FlagA) == MyEnum.FlagA)
{
  // handle FlagA
}

What are extension methods?

Extension methods enable you to add methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.

An extension method is a special kind of static method, but they are called as if they were instance methods on the extended type.

How to use extension methods?

An extension method is a static method of a static class, where the "this" modifier is applied to the first parameter. The type of the first parameter will be the type that is extended.

Extension methods are only in scope when you explicitly import the namespace into your source code with a using directive.

Important points for the use of extension methods:

1.An extension method must be defined in a top-level static class.

2.An extension method with the same name and signature as an instance method will not be called.

3.Extension methods cannot be used to override existing methods.

4.The concept of extension methods cannot be applied to fields, properties or events.

5.Overuse of extension methods is not a good style of programming.

Use inheritance when "Is-A" relationship makes sense. Inheritance create unwanted dependencies between the classes.

I use extension method when I feel "what good it would have been if XYZ class had ABC method".

I remember the best exammple when I used extension method when I needed to break collection into chunks. Here is the details.

https://www.codingcrest.com/extension-method-in-c/