How to Generate Auto-Increment number in C# ?

It's better to leave 'st' letters for your Student ID ; there is no point to using a string key rather than an integer auto-incremented one.

So you just have to make your stuID column an integer one, with the IDENTITY property. This way your database system will take care of the creation and uniqueness of your primary keys.

just have some counter and increment with any loop and concatenate it with st, below is the sample in which I am displaying the string into one label.

private void button1_Click(object sender, EventArgs e)
        {
            string Row = "";
            for (int i = 1; i <= 5; i++)
            {
                Row += "st" + i.ToString("000") + System.Environment.NewLine;
            }
            label1.Text = Row;
        }

so result would look like as below and you can using this code to generate ids and use as you like

st001
st002
st003
st004
st005