private void rdPersonalCRM_CheckedChanged(object sender, EventArgs e)
        {
            if (rdPersonalCRM.Checked)
            {
                txtConnStr.Text = @"Server=.\SQLEXPRESS;Initial Catalog=Dw_Personal;Trusted_Connection=true;MultipleActiveResultSets=true;";
            }
        }

private void txtSID_MouseMove(object sender,EventArgs e)
        {
            if (!txtSID.Text.Equals(""))
            {
                txtSourceText.Text = txtConnStr.Text+"&"+txtSID.Text.Trim();
            }
            
        }

注意要在_Load里添加对应的委托事件：

txtSID.MouseMove += new MouseEventHandler(txtSID_MouseMove);

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