The CodeDOM is a very handy .NET API which allows you to programatically compile code using the .NET compilers and programatically construct code without just pasting together strings. 

With the new version of the language, we've heard a numer of questions about how to use the CodeDOM with the new compiler.

There are two aspects to the inegration of C#3.0 with CodeDOM:

• Programatically compiling C# source code: This is supported in .NET3.5.  The existing CodeDOM is augmented with an overload of CSharpCodeProvider which takes a "providerOptions" argument.  This can be used to point CodeDOM at the new .NET Framework 3.5 C# compiler (which supports C#3.0), by passing a "providerOptions" dictionary containing "CompilerVersion" as "v3.5".  This can also be controlled via the .config file, which is done  for example in Orcas ASP.NET websites targeting .NET3.5. 
  
  
• Programatically constructing C#3.0 source code:  There won't be support for the this in the .NET Framework 3.5 CodeDOM that ships with Orcas.  Luckily, very few of the C# 3.0 features need CodeDOM support - since most new features are expressions, and the CodeDOM doesn't go down to the expression level. 

Example of programatically compiling C#3.0 source code:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.CSharp;
using System.CodeDom.Compiler;
class Program
{
    static void Main(string[] args)
    {
        var csc = new CSharpCodeProvider(new Dictionary<string, string>() { { "CompilerVersion", "v3.5" } });
        var parameters = new CompilerParameters(new[] { "mscorlib.dll", "System.Core.dll" }, "foo.exe", true);
        parameters.GenerateExecutable = true;
        CompilerResults results = csc.CompileAssemblyFromSource(parameters,
        @"using System.Linq;
            class Program {
              public static void Main(string[] args) {
                var q = from i in Enumerable.Rnge(1,100)
                          where i % 2 == 0
                          select i;
              }
            }");
        results.Errors.Cast<CompilerError>().ToList().ForEach(error => Console.WriteLine(error.ErrorText));
    }
}