Datagridview Combobox

Hi Mehmet try this one 

private void dataGridView1_CellValidating(object sender, DataGridViewCellValidatingEventArgs e)
{
    DataGridViewComboBoxColumn comboBoxColumn = dataGridView1.Columns["kbaskani"] as DataGridViewComboBoxColumn;

    if (comboBoxColumn != null && e.ColumnIndex == comboBoxColumn.Index)
    {
        string newValue = e.FormattedValue.ToString();
        string oldValue = dataGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Value.ToString();

        // Validate the selected value
        if (newValue == "Yes")
        {
            // Check if another "Yes" option already exists in the column
            int yesCount = 0;
            foreach (DataGridViewRow row in dataGridView1.Rows)
            {
                if (row.Index != e.RowIndex && row.Cells[e.ColumnIndex].Value.ToString() == "Yes")
                {
                    yesCount++;
                    break;
                }
            }

            if (yesCount > 0)
            {
                MessageBox.Show("You have chosen more than one Yes option. You can choose only one Yes option.", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                e.Cancel = true; // Cancel cell validation
                return;
            }
        }
        else if (newValue == "No")
        {
            // Check if no "Yes" option is chosen
            int yesCount = 0;
            foreach (DataGridViewRow row in dataGridView1.Rows)
            {
                if (row.Index != e.RowIndex && row.Cells[e.ColumnIndex].Value.ToString() == "Yes")
                {
                    yesCount++;
                    break;
                }
            }

            if (yesCount == 0)
            {
                MessageBox.Show("You haven’t chosen any Yes option. You have to choose one of the Yes options.", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                e.Cancel = true; // Cancel cell validation
                return;
            }
        }
    }
}