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COM Interop in c# 4.0

// Code simplified for this example
using Microsoft.Office.Interop;
using Microsoft.Office.Interop.Word;

object foo = "MyFile.txt";
object bar = Missing.Value;
object optional = Missing.Value;

Document doc = (Document)Application.GetDocument(ref foo, ref bar, ref optional);
doc.CheckSpelling(ref optional, ref optional, ref optional, ref optional);

There are (at least) three problems with the code above. First, you have to declare all your variables as objects and pass them with the ref keyword. Second, you can't omit parameters and must also pass the Missing.Value even if you are not using the parameter. And third, behind the scenes, you are using huge (in file size) interop assemblies just to make one method call.

C# 4.0 will allow you to write the code above in a much simpler form that ends up looking almost exactly like 'normal' C# code. This is accomplished by using some of the features already discussed; namely dynamic support and optional parameters.

// Again, simplified for example.
using Microsoft.Office.Interop.Word;

var doc = Application.GetDocument("MyFile.txt");
doc.CheckSpelling();