vb.net change value of a cell in a databound datagridview - vb.net

Question

I have a datagridview that has been populated from a datatable using the datasource property. I need to change the value of a cell without affecting the database. So far I could only find that I have to first change the databound item but no information about how to do that. Besides, I don't want to change the databound item.
The reason for wanting to do this is that I have a list of records with a start and end time and there is a computed field that gives the difference in seconds. I need to change the value of the computed field and change the height of the row accordingly, that is, the height of each row is proportional to the duration it represents. Changes are done from a text box which increments quickly if the "increase" button is kept down and therefore I cannot change the database with each increment. Rather I would change the value in the table frequently and update the database once the increments stops.
I have tried updating the datagridview directly using this code:
dgvTasks.Rows(SelectedRowIndex).Cells(5).Value = DateAdd(DateInterval.Minute, DateDiff(DateInterval.Minute, CDate("00:00:00"), CDate(txtDuration.Text & ":00")), dgvTasks.Rows(SelectedRowIndex).Cells(4).Value)
But receive the error:
System.Data.ReadOnlyException: Column 'Column1' is read only.
I also tried updating the datatable, and allowing the calculated column to update:
dtblTasks.Rows(SelectedRowIndex)(5) = DateAdd(DateInterval.Minute, DateDiff(DateInterval.Minute, CDate("00:00:00"), CDate(txtDuration.Text & ":00")), dtblTasks.Rows(SelectedRowIndex)(4))
dgvTasks.DataSource = dtblTasks
But the calculated value doesn't change.
I am also providing an additional calculated value (in the SQL) where I find the difference in minutes and the append a string literal to that:
( CAST ( DATEDIFF(MINUTE, Tasks.TaskStart, Tasks.TaskEnd) AS NVARCHAR(10) ) + ' mins') AS TaskDurationString
Is there a way to detach the grid from the datasource but still keep the data visible in the datagrid and manipulate it (without affecting the databound object)? Or perhaps some other approach to this problem?

Answer 1

Updating the calculated column in the DataTable should work - I've just tried it, and both when I edit one of the values that are part of the calculated value, and when I change the value in code, that column is recalculated and the grid UI updates.
One thing to check is that you are calculating the value correctly. You should be using the DataTable~ column [Expression` property]1. Something like this:
dt.Columns["FullName"].Expression = "FirstName + LastName";
Since it appears that you don't actually need to bind any of these values to the DataTable or persist them to the database, one possible solution is to do all of this in an unbound DataGridView calculation column.
To do that add a column using the designer, then in the cell validating event have something like this:
void dataGridView1_CellValidated(object sender, DataGridViewCellEventArgs e)
{
dataGridView1.Rows[e.RowIndex].Cells["TargetColumn"].Value = dataGridView1.Rows[e.RowIndex].Cells["FirstName"].Value.ToString() + dataGridView1.Rows[e.RowIndex].Cells["LastName"].Value.ToString();
dataGridView1.Rows[e.RowIndex].Height = dataGridView1.Rows[e.RowIndex].Cells["LastName"].Value.ToString().Length + dataGridView1.Rows[e.RowIndex].Cells["FullName"].Value.ToString().Length;
}