Auto populating textbox with incremented column value

Hi Star I made all the things for you check and place comment if necessary.And Adapt to your solution little bit customiztion may required or no customiztion needed.

In Aspx

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="AddNewStudentWebForm.aspx.cs" Inherits="WebApplicationUpdatePanel.AddNewStudentWebForm" %>
<asp:content id="Content1" contentplaceholderid="HeadContent" runat="server" >
</asp:content>
<asp:content id="Content2" contentplaceholderid="MainContent" runat="server" >
    <asp:scriptmanager id="ScriptManager1" runat="server">
    </asp:scriptmanager>
    <asp:updatepanel id="UpdatePanel1" runat="server">
    <contenttemplate>
        <asp:gridview id="GridView1" runat="server">
        </asp:gridview>
    <hr />
    Select a Department:
<asp:dropdownlist runat="server" id="ddlDept" autopostback="true" onselectedindexchanged="ddlDept_SelectedIndexChanged"></asp:dropdownlist>

<h3>Insert Panel</h3>
Selected Dept ID:  <asp:label id="lblDeptID" font-bold="true" runat="server"></asp:label><br />
Auto Incremented StudentID:
<asp:textbox id="txtAutoId" readonly="true" runat="server"></asp:textbox><br />
Enter Name:
<asp:textbox id="txtName" runat="server"></asp:textbox><br />
    </contenttemplate>
    <triggers>
    <asp:asyncpostbacktrigger controlid="btnAdd" eventname="Click" />
    <asp:asyncpostbacktrigger controlid="ddlDept" eventname="SelectedIndexChanged" />
    </triggers>
    </asp:updatepanel>

<asp:button id="btnAdd" text="Register" onclick="btnAdd_Click" runat="server" />

</asp:content>

In Code Behind .cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
namespace WebApplicationUpdatePanel
{
    public partial class AddNewStudentWebForm : System.Web.UI.Page
    {
        SqlConnection conn = new SqlConnection(@"Data Source=ADDIE-ANIS\ANISSQLSERVER12;Initial Catalog=BooksDB;Persist Security Info=True;User ID=sa;Password=Sa123");
        protected void Page_Load(object sender, EventArgs e)
        {
            
            if (!IsPostBack)
            {
                
                bindgrid();
               bindDropDown();
            }

        }
        void bindgrid()
        {
           
          
                string sqlSelectCmd = "SELECT  [StudentID],Registration.[Name],Registration.[DId],Department.[Name]FROM Registration inner join Department on Registration.[DId]=Department.[DId] order by Registration.[DId]";
                SqlCommand cmd = new SqlCommand(sqlSelectCmd, conn);

                SqlDataAdapter dap = new SqlDataAdapter(cmd);
                DataSet ds = new DataSet();
                dap.Fill(ds);

                GridView1.DataSource = ds;
                GridView1.DataBind();

            
            conn.Close();
        }
        void bindDropDown()
        {
            conn.Open();
           
             string sqlSelectCmd="SELECT  [DId],[Name] FROM [BooksDB].[dbo].[Department]";
             SqlCommand cmd = new SqlCommand(sqlSelectCmd, conn);

             SqlDataAdapter dap = new SqlDataAdapter(cmd);
             DataSet ds = new DataSet();
             dap.Fill(ds);
            
             ddlDept.DataSource = ds;
             ddlDept.DataTextField = "Name";
             ddlDept.DataValueField = "DId";
             ddlDept.DataBind();

             
            conn.Close();
        }

        protected void ddlDept_SelectedIndexChanged(object sender, EventArgs e)
        {
            lblDeptID.Text = ddlDept.SelectedItem.Value;
            if (lblDeptID.Text != "")
            {
              txtAutoId.Text= NextStudentID(Convert.ToInt32(lblDeptID.Text));
            }
        }

        protected void btnAdd_Click(object sender, EventArgs e)
        { 
        
        }

        string NextStudentID(int DeptID)
        {
            string sqlCmd = "SELECT  MAX(cast([StudentID] as int)) from Registration where [DId]=" + DeptID;
            conn.Open();
            SqlCommand cmd = new SqlCommand(sqlCmd, conn);
            object nextrollObj = cmd.ExecuteScalar();
            int Incremented = 0;
            Incremented = Convert.ToInt32(nextrollObj.ToString()) + 1; ;
            string Nextroll = "0" + Incremented.ToString();

            return Nextroll;
        }
    }
}

Full Solution is <a href="https://drive.google.com/file/d/0B2Hxs8nJ709CSkJhR2Nud3lPTjA/view?usp=sharing">Here</a> You Can download

Database Schema and sample data

--Department Table
CREATE TABLE [dbo].[Department](
	[DId] [int] IDENTITY(1,1) NOT NULL,
	[Name] [nchar](10) NULL,
 CONSTRAINT [PK_Department] PRIMARY KEY CLUSTERED 
(
	[DId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

--Registration Table
CREATE TABLE [dbo].[Registration](
	[StudentID] [nvarchar](10) NOT NULL,
	[Name] [nvarchar](50) NULL,
	[DId] [int] NOT NULL,
 CONSTRAINT [PK_Registration] PRIMARY KEY CLUSTERED 
(
	[StudentID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[Registration]  WITH CHECK ADD  CONSTRAINT [FK_Registration_Registration] FOREIGN KEY([StudentID])
REFERENCES [dbo].[Registration] ([StudentID])
GO

ALTER TABLE [dbo].[Registration] CHECK CONSTRAINT [FK_Registration_Registration]
GO

-- Data insert Before Testing

SET IDENTITY_INSERT [dbo].[Department] ON 

INSERT [dbo].[Department] ([DId], [Name]) VALUES (1, N'ICE       ')
INSERT [dbo].[Department] ([DId], [Name]) VALUES (2, N'CSE       ')
INSERT [dbo].[Department] ([DId], [Name]) VALUES (3, N'AECE      ')
SET IDENTITY_INSERT [dbo].[Department] OFF
INSERT [dbo].[Registration] ([StudentID], [Name], [DId]) VALUES (N'0405088', N'Esha', 3)
INSERT [dbo].[Registration] ([StudentID], [Name], [DId]) VALUES (N'0705043', N'Neha', 2)
INSERT [dbo].[Registration] ([StudentID], [Name], [DId]) VALUES (N'0705044', N'Tamim', 2)
INSERT [dbo].[Registration] ([StudentID], [Name], [DId]) VALUES (N'0718004', N'Anis', 1)
INSERT [dbo].[Registration] ([StudentID], [Name], [DId]) VALUES (N'0718005', N'Toma', 1)

You can download full soluion here

Then before test using .sql file create tables and insert some sample data.Change Connection string.happy coding