Check DropDownList select value exist in database or not using Linq in ASP.Net

Hi bakhtawar,

I have created sample. Please refer the below code.

HTML

<div align="center">
    <table>
        <tr>
            <td>
                <asp:DropDownList runat="server" ID="DropDownList1" AutoPostBack="true" OnSelectedIndexChanged="DropdownChanged"
                    AppendDataBoundItems="true">
                    <asp:ListItem Text="-Select-" Value="-Select-" />
                </asp:DropDownList>
            </td>
            <td>
                <asp:GridView runat="server" ID="gvDetails" />
            </td>
        </tr>
    </table>
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[2] { new DataColumn("No"), new DataColumn("Name") });
        dt.Rows.Add("rs23", "John Hammond");
        dt.Rows.Add("rs24", "Mudassar Khan");
        dt.Rows.Add("rs25", "Suzanne Mathews");
        dt.Rows.Add("rs26", "Robert Schidner");
        DropDownList1.DataSource = dt;
        DropDownList1.DataTextField = "No";
        DropDownList1.DataValueField = "No";
        DropDownList1.DataBind();
    }
}

protected void DropdownChanged(object sender, EventArgs e)
{
    string selectedValue = DropDownList1.SelectedValue;
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[2] { new DataColumn("No"), new DataColumn("Name") });
    if (Session["Data"] != null)
    {
        dt = Session["Data"] as DataTable;
        if (selectedValue != "-Select-")
        {
            DataRow[] existRow = dt.Select("No='" + selectedValue + "'");
            if (existRow.Length > 0)
            {
                if (existRow[0].ItemArray[0].ToString().ToLower() != selectedValue.ToLower())
                {
                    dt.Rows.Add(selectedValue, selectedValue);
                    Session["Data"] = dt;
                }
                else
                {
                    ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", "alert('Value " + selectedValue + " already exist.')", true);
                }
            }
            else
            {
                dt.Rows.Add(selectedValue, selectedValue);
                Session["Data"] = dt;
            }
        }
    }
    else
    {
        dt.Rows.Add(selectedValue, selectedValue);
        Session["Data"] = dt;
    }
    gvDetails.DataSource = dt;
    gvDetails.DataBind();
}

Screenshot