经过网上查找,终于找到了选中文本本框的内容方法

textBox2.Focus(); 

textBox2.SelectionStart = 0;  

 textBox2.SelectionLength = textBox2.Text.Length;

 这是找到的代码, 自己要用做函数,就写了一下

private void button2_Click(object sender, EventArgs e)       

 {            

                   SelectAll(textBox2);     

   }       

 public static TextBox SelectAll(TextBox TB)      

  {           

                 TB.Focus();      

                TB.SelectionStart = 0;  

                TB.SelectionLength = TB.Text.Length;       

                return TB;     

   }