Add a piece of dynamically compiled code in memory with the help of Roslyn. (After this the Greeter class will be available and can be run from example from the immediate window in visual studio).

RoslynDynamicCompilation.cs

public static void AddGreeter()
{
    AddInmemory("Greeter", @"using System;

class Greeter
{
public string Greet()
{
return ""Hello World"";
}
}");
}

public static void AddInmemory(string inMemoryAssemblyName, string code )
{
    var syntaxTree = SyntaxTree.ParseText(code);

    var refs = new[] {
        MetadataReference.CreateAssemblyReference("mscorlib"),
    };

    var compilation = Compilation.Create(inMemoryAssemblyName,
    syntaxTrees: new[] { syntaxTree },
    references: refs,
    options: new CompilationOptions(OutputKind.DynamicallyLinkedLibrary));

    using (var memoryStream = new MemoryStream())
    {
        EmitResult result = compilation.Emit(memoryStream);
        memoryStream.Flush();
        var assembly = Assembly.Load(memoryStream.GetBuffer());
    }
            
}