Using 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. This article is going to throw some light on this handy feature.

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 IST using the following lines of code:

DateTime dt=DateTime.Now;
DateTime utc = dt.ToUniversalTime();
DateTime ist = utc.AddMinutes(330);

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 330 minutes (5 hrs. 30 min.) to the UTC and get IST. 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 IST 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 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);

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 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.

Note that the extension methods will be shown for the underlying type only if the class defining the extension methods is available to your application. That means you must refer the appropriate assembly for DateTimeHelper class and import its namespace before using the extension methods.

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:
    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)

That's it! Will be back with something interesting. Till then keep coding!


Bipin Joshi is an independent software consultant, trainer, author, and meditation teacher. He has been programming, meditating, and teaching for 25+ years. He conducts instructor-led online training courses in ASP.NET family of technologies for individuals and small groups. He is a published author and has authored or co-authored books for Apress and Wrox press. Having embraced the Yoga way of life he also teaches Ajapa Yoga to interested individuals. To know more about him click here.

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Posted On : 04 August 2008

Tags : .NET Framework C#