try this..
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
TextBox1.Text = "001";
}
}
protected void btnAdd_Click(object sender, EventArgs e)
{
string print = "";
string str1 = TextBox1.Text;
Int32 minLen = 3;
Int32 intVal = Convert.ToInt32(str1);
Int32 valLen = intVal.ToString().Length;
int i = (intVal + 1).ToString().Length;
while(i < minLen)
{
print = print + "0";
i++;
}
print = print + (intVal + 1).ToString();
TextBox1.Text = print;
}
Updated..
string str1 = TextBox1.Text;
Int32 intVal = Convert.ToInt32(str1);
TextBox1.Text = (intVal + 1).ToString("00#");