this base

public class BASE
{
    public BASE() 
    {
        Console.WriteLine(1);
    }
    public BASE(string str)
    {
        Console.WriteLine(2);
    }
    public BASE(string str, string str1): this(str)
    {
        Console.WriteLine(3);
    }
}
public class Class1:BASE
{
    public Class1() : base()
    {
        Console.WriteLine(4);
    }
    public Class1(string str) : this()
    {
        Console.WriteLine(5);
    }
}

 Class1 class1 = new Class1();//打印  1  4
 Class1 class2 = new Class1("");//打印  1  4  5