protected void btnFilter_Click(object sender, EventArgs e)
 {
     if (ddlPayment.SelectedItem.Text == "All")
     {
         BindGridView();
         txtEndDate.Enabled   = false;
         txtStartDate.Enabled = false;
         btnFilter.Enabled    = false;
     }
     else
     {
         _invMstr.CompanyId     = Convert.ToInt16(Session["CompanyId"]);
         _invMstr.PaymentStatus = Convert.ToInt16(ddlPayment.SelectedItem.Value);
         DateTime StartDate = Convert.ToDateTime(txtStartDate.Text);
         DateTime EndDate   = Convert.ToDateTime(txtEndDate.Text);
         _ds.Tables.Add(_sobj.SelectInvoiceDate(_invMstr, StartDate, EndDate));
         gvInvoice.DataSource = _ds;
         gvInvoice.DataBind();
         txtEndDate.Enabled   = true;
         txtStartDate.Enabled = true;
         btnFilter.Enabled    = true;
     }
 }