Get Ready For C# 4.0!

Visual Studio 2010 is here! And of course this means that C# 4.0 is also here. Let’s do a quick review of the new language features added in this release.


The dynamic keyword is a key feature of this release. It closes the gap between dynamic and statically-typed languages. Now you can create dynamic objects and let their types be determined at run time. With the addition of the System.Dynamic namespace, you can create expandable objects and advanced class wrappers, and you can provide interoperability between different languages, including dynamic ones. Here is one quick example:

dynamic contact = new ExpandoObject();
contact.Name = “Patrick Hines”;
contact.Phone = “206-555-0144”;

I have discussed the pros and cons of this feature on this blog. If you want to know more, read here.

Covariance and Contravariance

Variance on generic type parameters in interfaces and delegates is another important feature of this release. It doesn’t add much new functionality, but rather makes things work as you expected them to in the first place. The major advantage is hidden in this simple line, which didn’t compile until C# 4.0:


Optional (or Default) Parameters

Looking at the archives of this blog, I see that people have been asking for this feature since C# 1.0. Three versions later, it’s finally here.

Now you can assign a default value to a parameter right within the method declaration. The user of the method can either pass a value or simply skip the argument. In the latter case, the default value is passed to the method.

Method declaration:

public static void SomeMethod(int optional = 0) { }

Method calls:

SomeMethod(); // 0 is used in the method.


Named Arguments

The order of parameters in a method declaration and the order of arguments you pass when calling the method don’t need to match anymore. You can provide arguments in any order you like by specifying parameter names in a method call. This might also improve the readability of your code.

var sample = new List();
sample.InsertRange(collection: new List(), index: 0);
sample.InsertRange(index: 0, collection: new List());

Read more about optional parameters and named arguments on MSDN.

Improved COM Interop

The introduction of the dynamic keyword, optional parameters and named arguments enables improvement of COM interop. So, no more ugly code like this:

var excelApp = new Excel.Application();
// . . .
excelApp.get_Range(“A1”, “B4”).AutoFormat(
Type.Missing, Type.Missing, Type.Missing,
Type.Missing, Type.Missing, Type.Missing);

You can now simply write the following:

excelApp.Range[“A1”, “B3”].AutoFormat(

By the way, this code uses one more new feature: indexed properties (take a closer look at those square brackets after Range.) But this feature is available only for COM interop; you cannot create your own indexed properties in C# 4.0.

For more information about new COM interop features, once again, refer to MSDN.

What Else?

Of course, C# benefits not only from new language features, but also from improvements to its integrated development environment (IDE) and to the .NET Framework.

Here are some links for further reading:

  • Find all calls to and from your methods with the new Call Hierarchy window.
  • Even if you have never heard of test-driven development (TDD), the Generate From Usage feature may help you to quickly prototype your application by generating method and class stubs, allowing you to concentrate on your current task rather than worrying about declarations. For TDD adepts, this feature might be a real performance booster. Check out this walkthrough for more details.
  • Don’t forget to take a look at What’s New in the .NET Framework 4 and also at What’s New in Visual C# 2010 (just to make sure I didn’t miss something).
  • And just in case, check out breaking changes for C# and .NET Framework.
  • Last, but not least, don’t forget to report any problems at Microsoft Connect.
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Vidyasagar MSC

Polyglot & Pragmatic Programmer