if (DataGridView.Rows[0].Cells["Rizwan"].Value == textBox1.text)
{
}
and if you uesd textBox wiyh in gridView then first find it and compare
value than anater textbox.
TextBox NameTextBox = (TextBox)fvPerson.FindControl("NameTextBox");
if(NameTextBox .text==rizText.text)
{
}