How to get checkbox data after form submit using c#

Try this also.

protected void btnProcess_Click(object sender, EventArgs e)
{
StringBuilder sbCheck_val = new StringBuilder();
foreach (GridViewRow row in grid.Rows)
{
CheckBox chk = (CheckBox)row.FindControl("chk");
if (chk.Checked)
{
Label lblId = (Label)row.FindControl("lblId");
sbCheck_val.Append(lblId.Text + ",");
}
}

lblmsg.Text = "Selected Roll Numbers are : " + sbCheck_val.ToString().TrimEnd(',');
}