After I wrote Get row data from WinForm DataGridView (https://chanmingman.wordpress.com/2014/01/01/get-row-data-from-winform-datagridview/) I encountered this question in Visual C# forum “c#.net to assign a value to the cell in datagrid pro grammatically when clicking radio button” (http://social.msdn.microsoft.com/Forums/en-US/447264b9-920c-4dd5-875e-2ff9e4c1117d/cnet-to-assign-a-value-to-the-cell-in-datagrid-pro-grammatically-when-clicking-radio-button?forum=csharpgeneral)
So I added the Button to the form and the following code.
privatevoid button1_Click(object sender, EventArgs e)
dataGridView1.Rows[intIndex].Cells.Value = textBox1.Text;
dataGridView1.Rows[intIndex].Cells.Value = textBox2.Text;