using System;
public partial class MyClass
{
    public void MyMethod1()
    {
        Console.WriteLine("MyMethod1 人工编写的代码");
    }
}

using System;
public partial class MyClass
{
    public void MyMethod2()
    {
        Console.WriteLine("MyMethod2 通常是自动生成的代码，如webService 之类");
    }
}

csc /out:lib.dll  /t:library file1.cs file2.cs
csc /out:App.exe /r:lib.dll app.cs

using System;
public static class UtilityClass
{
    public static void Foo()
    {
        Console.WriteLine("Hello");
    }            
}

using System;
class App
{
    static void Main()
    {
        UtilityClass.Foo();
    }
}

object nullObj = null; 
object obj = new Object(); 
return nullObj ?? obj; // returns obj