gridview prerendra even in asp.net c#

change textbox size on edit mode of bound field

protected void GridView1_PreRender(object sender, EventArgs e)
    {
        if (GridView1.EditIndex > -1)
        {
            TextBox tb1;
            tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[1].Controls[0];
            tb1.Width = Unit.Parse("1.5cm");
            tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[2].Controls[0];
            tb1.Width = Unit.Parse("1.5cm");
            tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[3].Controls[0];
            tb1.Width = Unit.Parse("1.5cm");
            tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[4].Controls[0];
            tb1.Width = Unit.Parse("2cm");
            tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[0].Controls[0];
            tb1.Width = Unit.Parse("2cm");
            tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[7].Controls[0];
            tb1.Width = Unit.Parse("1.5cm");
            tb1.MaxLength = 3;
            tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[8].Controls[0];
            tb1.Width = Unit.Parse("0.5cm");
            tb1.MaxLength = 1;
            //tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[10].Controls[0];
            //tb1.Width = Unit.Parse("1.5cm");
            tb1 = (TextBox)GridView1.Rows[GridView1.EditIndex].Cells[11].Controls[0];
            tb1.Width = Unit.Parse("1.5cm");

        }