using System;

public class Test
{
    public String HelloWorld()
    {
        return "Hello World!";
    }
}

var cp = new CompilerParameters();

cp.ReferencedAssemblies.Add("System.Web.UI.WebControls");

var cp = new CompilerParameters();
cp.ReferencedAssemblies.Add("System.Web.dll");

var assemblies = AppDomain.CurrentDomain
                            .GetAssemblies()
                            .Where(a => !a.IsDynamic)
                            .Select(a => a.Location);

  cp.ReferencedAssemblies.Add(assemblies.ToArray());

var assemblies = someType.Assembly.GetReferencedAssemblies().ToList();
   var assemblyLocations =
assemblies.Select(a =>
     Assembly.ReflectionOnlyLoad(a.FullName).Location).ToList();

assemblyLocations.Add(someType.Assembly.Location);

cp.ReferencedAssemblies.AddRange(assemblyLocations);