winform textBox光标进入时自动选中全部内容

如果只是在textBox的enter事件中用 txt.SelectAll()

   或     txt.SelectionStart = 0;  //设置起始位置 

            txt.SelectionLength = txt.TextLength;  //设置长度

则不管用。

step1: 它的enter事件中   
                          private void txtStart_Enter(object sender, EventArgs e)
                          {                            TextBox txt = sender as TextBox;
                             txt.SelectAll();                          }
step2: 它的mouseClick事件中
                         private void txtStart_MouseClick(object sender, MouseEventArgs e)
                          {                     TextBox txt = sender as TextBox;
                              txt.Tag = 1;
                              txt.SelectAll();                          }

step 3: 它的leave事件中
                       private void txtEnd_Leave(object sender, EventArgs e)
                         {                           TextBox txt = sender as TextBox;
                            txt.Tag = 0;                        }