Me gustaría hacer el equivalente de:

object result = Eval("1 + 3");
string now    = Eval("System.DateTime.Now().ToString()") as string

Siguiendo a Biri senlaza, Obtuve este fragmento (modificado para eliminar el método obsoletoICodeCompiler.CreateCompiler():

private object Eval(string sExpression)
{
    CSharpCodeProvider c = new CSharpCodeProvider();
    CompilerParameters cp = new CompilerParameters();

    cp.ReferencedAssemblies.Add("system.dll");

    cp.CompilerOptions = "/t:library";
    cp.GenerateInMemory = true;

    StringBuilder sb = new StringBuilder("");
    sb.Append("using System;\n");

    sb.Append("namespace CSCodeEvaler{ \n");
    sb.Append("public class CSCodeEvaler{ \n");
    sb.Append("public object EvalCode(){\n");
    sb.Append("return " + sExpression + "; \n");
    sb.Append("} \n");
    sb.Append("} \n");
    sb.Append("}\n");

    CompilerResults cr = c.CompileAssemblyFromSource(cp, sb.ToString());
    if (cr.Errors.Count > 0)
    {
        throw new InvalidExpressionException(
            string.Format("Error ({0}) evaluating: {1}", 
            cr.Errors[0].ErrorText, sExpression));
    }

    System.Reflection.Assembly a = cr.CompiledAssembly;
    object o = a.CreateInstance("CSCodeEvaler.CSCodeEvaler");

    Type t = o.GetType();
    MethodInfo mi = t.GetMethod("EvalCode");

    object s = mi.Invoke(o, null);
    return s;

}