Sequential number based on DropDownList using C# in ASP.Net

Hi Priya8,

I have created sample, refer the below code.

HTML

<div>
    <asp:DropDownList ID="DropDownList2" runat="server" OnSelectedIndexChanged="GenerateCode"
        AutoPostBack="true">
        <asp:ListItem Text="Select" Value="0" />
        <asp:ListItem Text="HAG01" Value="HAG01" />
        <asp:ListItem Text="HEM08" Value="HEM08" />
    </asp:DropDownList>
    <br />
    <asp:TextBox ID="TextBox4" runat="server">
    </asp:TextBox>
</div>

Code

private static string hasValue = string.Empty;
private static string hemValue = string.Empty;

protected void GenerateCode(object sender, EventArgs e)
{
    string contractNo;
    string selected = DropDownList2.SelectedItem.Value;
    string code = string.Empty;
    if (selected == "HAG01")
    {
        if (hasValue == string.Empty)
        {
            contractNo = "1";
            code = string.Format("ABC/{0}/{1}/" + contractNo + "/2016-17", selected.Substring(0, 1).Equals("H") ? "HQ" : "RD", selected.Substring(1, 2));
            hasValue = code;
        }
        else
        {
            contractNo = (Convert.ToInt32(hasValue.Split('/')[3]) + 1).ToString();
            code = string.Format("ABC/{0}/{1}/" + contractNo + "/2016-17", selected.Substring(0, 1).Equals("H") ? "HQ" : "RD", selected.Substring(1, 2));
            hasValue = code;
        }
    }
    else if (selected == "HEM08")
    {
        if (hemValue == string.Empty)
        {
            contractNo = "1";
            code = string.Format("ABC/{0}/{1}/" + contractNo + "/2016-17", selected.Substring(0, 1).Equals("H") ? "HQ" : "RD", selected.Substring(1, 2));
            hemValue = code;
        }
        else
        {
            contractNo = (Convert.ToInt32(hemValue.Split('/')[3]) + 1).ToString();
            code = string.Format("ABC/{0}/{1}/" + contractNo + "/2016-17", selected.Substring(0, 1).Equals("H") ? "HQ" : "RD", selected.Substring(1, 2));
            hemValue = code;
        }
    }
    TextBox4.Text = code;
}

Screenshot