Extension Methods in C#

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? And what if the class is sealed class?

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

Example Scenario

Let's try to undertand what extension methods are with an example.

Assume that you are developing a discussion forum application. The forum displays date and time at which posts and replies are made. The discussion will be used primarily in India and hence you wish to show these timestamps in Indian Standard Time (IST). However, your servers are hosted in some different geographical region (and hence date-time offset) and hence you must do some adjustment to display all the times in IST.

You then decide to get the PST using the following lines of code:

DateTime dt = DateTime.Now;
DateTime utc = dt.ToUniversalTime();
DateTime PST = utc.AddMinutes(300);

The above code calls ToUniversalTime() method of the DateTime structure that returns date and time in Universal Standard Time (earlier known as GMT). You then add 300 minutes (5 hrs.) to the UTC and get PST. Though the code is simple enough and your team can call it wherever they need, won't it be nice if you can have this functionality to DateTime structure itself? That way all team members will find it convenient to quickly get the PST from a specified time. This is where we will use a C# extension method to get our job done.

Creating Extension Methods

Have a look carefully at the following class that defines two extension methods:

namespace Utils
{
    public static class DateTimeHelper
    {
        public static DateTime ToPST(this DateTime dt)
        {
            DateTime utc = dt.ToUniversalTime();
            return utc.AddMinutes(300);
        }

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

The above code defines a static class named DateTimeHelper. All the extension methods must reside inside a static class. The extension methods themselves must be static (In fact that is why compiler forces you to declare the class as static).

The ToPST() 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 ToPST() method we specify that this method is an extension method of DateTime structure. The ToPST() method simply adjusts the date and time with required offset.

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.ToPST().ToDDMMYYYY()
The ToPST() 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.ToPST(dt)