how to generate auto number in textbox every time after user click on insert after

You can problably use any one of the two approaches.

1. Create a static class like below

public static class SequentialNumber
{
       private static int _currentNumber=0;
       public static string GetNextNumber()
       {
            _currentNumber++;
            return "C"+_currentNumber.ToString();
       }
}

and then in insert buttons click, write a code like below to get the sequential number in the text box

string ContractNo=SequentialNumber.GetNextNumber();

2 approach.

Declare a variable _currentnumber in the code behind page
in the click event of the insert button write code as below.

if(string.IsNullOrEmpty(ViewStatte["Num"]))
   _currentNumber=0;
else
{
   _currentNumber=(int)ViewState["Num"];
}
_currentNumber++;
ViewState["Num"]=_currentNumber;
string ContractNo=_currentNumber;

Approach 1 can be used in all types of project where as approach 2 can be used in asp.net only.

Hope this helps.

--sunil

Hi hope this will help You

public void Method()
         {
             Connection();
             int Num1 = 0;
             cmd = new SqlCommand("Select MAX(Column) from TableName", cs);
             dr = cmd.ExecuteReader();
             while (dr.Read())
             {
                 Num1 = int.Parse(dr[0].ToString());
             }
             dr.Close();
             TextBox.Text ="C" + (Num1 + 1).ToString();
             cs.Close();


         }

SqlDataAdapter sda = new SqlDataAdapter("Select * from Newentry2", con);
DataTable dt = new DataTable();
sda.Fill(dt);
int i = dt.Rows.Count;
string cnt = Convert.ToString(i + 1);
textBox3.Text = "NIRU-" + cnt;

add this code on your Insert button...

private void btnInsert_Click(System.Object sender, System.EventArgs e)
{
	string txt = TextBox1.Text;
	TextBox1.Text = "C" + Convert.ToInt32(txt.Substring(1, txt.Length - 1)) + 1;
}