帮菜手解决问题，combobox cmbXsBj 总出现问题

private void cmbXsBj_SelectedIndexChanged(object sender, EventArgs e)
{
   if (this.cmbXsBj.SelectedIndex > -1)
   {
      this.xsxxBindingSource.Filter = string.Format("[ssbj]={0}", this.cmbXsBj.SelectedValue);
   }
}
        private void updatecmbXsBj()
        {
            this.cmbXsBj.SelectedIndexChanged -= new System.EventHandler(this.cmbXsBj_SelectedIndexChanged);
            if (this.cmbXsNj.Tag != null && this.cmbXsNj.Tag.ToString() == this.cmbXsNj.SelectedValue.ToString())
            {
                return;
            }
            this.cmbXsNj.Tag = this.cmbXsNj.SelectedValue;
            pmain.updateStatus("");
            DataTable xsBj = new DataTable();
            SqlDataAdapter sda = new SqlDataAdapter();
            cmd.CommandText = string.Format("SELECT top 100 percent [ID] [bjid],[BJMC] FROM [BJVIEW] WHERE [ZYID]={0} AND [NJMC]='{1}' ORDER BY [BJMC]", this.cmbXsZy.SelectedValue, this.cmbXsNj.Text, conn);
            sda.SelectCommand = cmd;
            sda.Fill(xsBj);
            this.cmbXsBj.DataSource = xsBj;
            this.cmbXsBj.DisplayMember = "bjmc";
            this.cmbXsBj.ValueMember = "bjid";
            this.cmbXsBj.SelectedIndexChanged += new System.EventHandler(this.cmbXsBj_SelectedIndexChanged);
            this.cmbXsBj_SelectedIndexChanged(new object(), new EventArgs());
        }
我做的这个combobox，的selectindex总是从0跳到-1，经常要选多次才能成功，请高手指点。