Microsoft Roslyn Example

using Roslyn.Compilers;
using Roslyn.Compilers.CSharp;

[...]

var text = @"class Calc { public static object Eval() { return 42; } }";

var tree = SyntaxTree.ParseCompilationUnit(text);
var compilation = Compilation.Create(
    "calc.dll",
    options: new CompilationOptions(assemblyKind: AssemblyKind.DynamicallyLinkedLibrary),
    syntaxTrees: new[] { tree },
    references: new[] { new AssemblyFileReference(typeof(object).Assembly.Location) });

Assembly compiledAssembly;
using (var stream = new MemoryStream())
{
    EmitResult compileResult = compilation.Emit(stream);
    compiledAssembly = Assembly.Load(stream.GetBuffer());
}

Type calc = compiledAssembly.GetType("Calc");
MethodInfo eval = calc.GetMethod("Eval");
string answer = eval.Invoke(null, null).ToString();

Assert.AreEqual("42", answer);