how to increment textboxvalue

First get the textbox value in integer variable and than add 1 to it..

int txtboxText = Convert.ToInt32(txt1.Text);
int result = txtboxText + 1;

OR U can do this in a single line.

int result = Convert.ToInt32(txtbox.text) + 1;

and to restrict user to enter only numeric values to textbox You can use Filtered textbox extender.

You should first try to convert the TextBox.Text property from its own string type to int type:

int gpno;
if (int32.TryParse(txtbox.Text, out gpno) == false)
{
  // handle error: Text content does NOT represent a valid int
}
else
{ // succesful parse, here you may increment the value.
  gpno++;
}

Convert the text to a numerical value, and than add one
http://msdn.microsoft.com/en-us/library/f02979c7.aspx[^]

Would look something like:

Int32 number = new Int32();
if Int32.TryParse(txtbox.text,number) then
{
//it is a number
gpon = number + 1;
}
else
{
//The value is not a number
}

If the textbox can contain only numerical values :

int value = int.Parse(textBox1.Text) + 1;

else use :

if (int.TryParse(textBox1.Text, out value))
            {
                value += 1;
            }

yes you can

gpno = Convert.Toint32(txtbox1.text)+1;