Get the cell value of a GridView row

Try changing your code to

// Get the currently selected row using the SelectedRow property.
GridViewRow row = dgCustomer.SelectedRow;

// And you respective cell's value
TextBox1.Text = row.Cells[1].Text

UPDATE: (based on my comment) If all what you are trying to get is the primary key value for the selected row then an alternate approach is to set

datakeynames="yourprimarykey"

for the gridview definition which can be accessed from the code behind like below.

TextBox1.Text = CustomersGridView.SelectedValue.ToString();

Windows Form Iteration Technique

foreach (DataGridViewRow row in dataGridView1.Rows)
{
    if (row.Selected)
    {
        foreach (DataGridViewCell cell in row.Cells)
        {
            int index = cell.ColumnIndex;
            if (index == 0)
            {
                value = cell.Value.ToString();
                //do what you want with the value
            }
        }
    }
}