一个构造函数的问题
class   Class1
{
[STAThread]
static   void   Main(string[]   args)
{
Rectangle   rect   =   new   Rectangle(0,   0,   20,   50);
Square   sqr   =   new   Square(0,   0,   20,   50);
rect.ShowArea();
sqr.ShowArea();
Console.WriteLine( "\n\n按回车键退出 ");
Console.ReadLine();
}
}

class   Rectangle  
{
public   int   left,   top,   width,   height;
protected   string   type;
public   Rectangle(int   aLeft,int   aTop,   int   aWidth,int   aHeight)
{
left   =   aLeft;
top   =   aTop;
width   =   aWidth;
height   =   aHeight;
type   =   "矩形 ";
}
public   void   ShowArea()
{
Console.WriteLine( "当前形状为{0},   面积是{1} ",  
type,   width*height);
}
}

class   Square   :   Rectangle
{
public   Square(int   aLeft,int   aTop,int   aWidth
):base(0,0,0,0)  
{
type   =   "正方形 ";
}
       
public   Square(int   aLeft,int   aTop,int   aWidth,int   aHeight):this()
{
left   =   aLeft;
top   =   aTop;
if   (aWidth   >   aHeight)  
{
width   =   aHeight;
height   =   aHeight;
}
else
{
width   =   aWidth;
height   =   aWidth;
}
}  
      }  

其中有一个构造函数:public   Square(int   aLeft,int   aTop,int   aWidth
):base(0,0,0,0)  
{
type   =   "正方形 ";
}
可是这样会报错,要改成这样:public   Square():base(0,0,0,0)  
{
type   =   "正方形 ";
}
程序才不会报错.请问这是为什么?为什么要这样改?

------解决方案--------------------
public Square(int aLeft,int aTop,int aWidth,int aHeight):this()

你这里的:this()调用的是哪个构造函数？！！