Prepopulating when adding new row to DataGridView

This will do it for an unbound grid with three columns
in repsonse to a button click or some other event where
the user has not added the row directly to the grid

private void addRowButton_Click(object sender, EventArgs e)
{
    int rowIndex = dataGridView1.Rows.Add();
    DataGridViewRow row = dataGridView1.Rows[rowIndex];
    row.Cells[0].Value = "test1";
    row.Cells[1].Value = "test2";
    row.Cells[2].Value = "test3";
    dataGridView1.CurrentCell = row.Cells[0];
}

When the user does add a row to the grid by entering
data in one of the columns on the new row line the
following will work

private void dataGridView1_RowsAdded(object sender, DataGridViewRowsAddedEventArgs e)
{
    DataGridViewRow row = dataGridView1.Rows[e.RowIndex - 1];
    row.Cells[0].Value = "test1";
    row.Cells[1].Value = "test2";
    row.Cells[2].Value = "test3";
}

Note that any user input will override values set here. This can be avoided by making
columns read only where required.

I had same issue : how to have the blank row of the DataGrid to
have computed values. I posted the question, and answer here :

http://stackoverflow.com/questions/12485664/set-the-content-of-a-new-row-datagrid[^]

i copy the answer here :

When a DataTable is bound to a DataGrid, a CollectionView is created in order to see it. You can get it by using the (static/shared) CollectionView.GetDefaultView(ThePropertyThatIsBound) method.
Since it implements ICollectionChanged, you can add an event handler to the CollectionChangedEvent.

In the CollectionChanged event handler, if you have a new item (e.NewItems.Count>0) you must check it against System.Windows.Data.CollectionView.NewItemPlaceholder and if it is not a place holder, then it is a brand new item, for wich i can set all default values.

You cannot modify the cells in the row in the UserAddedRow event. Use DefaultValuesNeeded[^] instead.