Extension Methods

How often do you want to modify existing classes to have extra methods? Quite often. Right? When you have source code of the classes under consideration at your hand then things are easy. You probably just add the extra methods to those classes. What if you don't have source code? Well. In such cases one approach is to inherit the existing classes and add extra methods to the child classes. However, this way may not be always correct and possible in terms of your application design and OO principles. Luckily, C# offers a quick way to extend your class functionality through a feature known as Extension Methods. Extension methods allow you to extend existing types without inheriting them

Will explain with example
public static class DateTimeHelper
{
public static DateTime ToIST(this DateTime dt)
{
DateTime utc = dt.ToUniversalTime();
return utc.AddMinutes(330);
}

public static bool IsDate(this string s)
{
DateTime dt;
return DateTime.TryParse(s, out dt);
}
}

there is static class named DateTimeHelper
All the extension methods must reside inside a static class
The extension methods themselves must be static

The ToIST() static method is an extension method that returns a DateTime instance. Observe how the parameter of the method is specified. An extension method can have any number of parameters but the first parameter indicates the data type to which that extension is applicable. In case of ToIST() method we specify that this method is an extension method of DateTime structure. The ToIST() method simply adjusts the date and time with required offset.

The IsDate() method is another extension method that will be applicable to string data type. Inside it checks if the supplied string is a valid date and returns a Boolean value.

Now, if you use DateTime structure then the Visual Studio will show you ToIST() method in the IntelliSense. Similarly, IsDate() method will be shown for string class.

More about extension methods
Though extension methods allow you to quickly extend an existing type this feature should not be used extensively as a replacement to other elegant approaches. Additionally, the following points about extension methods are worth noting.

Extension methods can chain themselves. For example you may write something like this:
dt.ToIST().ToDDMMYYYY()
The ToIST() and ToDDMMYYYY() are assumed to be extension methods.
Extension methods can be accessed only when its namespace is in scope
If an extension method has the same signature as any of the instance method of the class it extends then the instance methods take precedence
If two extension methods of a type have same signature them they must be called like normal static methods. For example, if you define two extension methods with same name and signature in two classes say DateTimeHelper1 and dateTimeHelper2, then they can be called like this - DateTimeHelper1.ToIST(dt)