Question: Is *CompileAssemblyFromDom* faster than *CompileAssemblyFromSource*?

It *should* be as it presumably bypasses the compiler front-end.

Answer: 
CompileAssemblyFromDom compiles to a .cs file which is then run through the normal C# compiler.

Example:

```
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.CSharp;
using System.CodeDom;
using System.IO;
using System.CodeDom.Compiler;
using System.Reflection;

namespace CodeDomQuestion
{
    class Program
    {

        private static void Main(string[] args)
        {
            Program p = new Program();
            p.dotest("C:\\fs.exe");
        }

        public void dotest(string outputname)
        {
            CSharpCodeProvider cscProvider = new CSharpCodeProvider();
            CompilerParameters cp = new CompilerParameters();
            cp.MainClass = null;
            cp.GenerateExecutable = true;
            cp.OutputAssembly = outputname;

            CodeNamespace ns = new CodeNamespace("StackOverflowd");

            CodeTypeDeclaration type = new CodeTypeDeclaration();
            type.IsClass = true;
            type.Name = "MainClass";
            type.TypeAttributes = TypeAttributes.Public;

            ns.Types.Add(type);

            CodeMemberMethod cmm = new CodeMemberMethod();
            cmm.Attributes = MemberAttributes.Static;
            cmm.Name = "Main";
            cmm.Statements.Add(new CodeSnippetExpression("System.Console.WriteLine('f'zxcvv)"));
            type.Members.Add(cmm);

            CodeCompileUnit ccu = new CodeCompileUnit();
            ccu.Namespaces.Add(ns);

            CompilerResults results = cscProvider.CompileAssemblyFromDom(cp, ccu);

            foreach (CompilerError err in results.Errors)
                Console.WriteLine(err.ErrorText + " - " + err.FileName + ":" + err.Line);

            Console.WriteLine();
        }
    }
}

```

which shows errors in a (now nonexistent) temp file:

> 
> ) expected - c:\Documents and Settings\jacob\Local Settings\Temp\x59n9yb-.0.cs:17
> 
> 
> ; expected - c:\Documents and Settings\jacob\Local Settings\Temp\x59n9yb-.0.cs:17
> 
> 
> Invalid expression term ')' - c:\Documents and Settings\jacob\Local Settings\Tem p\x59n9yb-.0.cs:17
> 
> 
> 

So I guess the answer is "no"
I've tried finding the ultimate compiler call earlier and I gave up. There's quite a number of layers of interfaces and virtual classes for my patience.

I don't think the source reader part of the compiler ends up with a DOM tree, but intuitively I would agree with you. The work necessary to transform the DOM to IL should be much less than reading C# source code.