Winform DataGridView with a pair of cascaded comboboxes

I am trying to set up a 2 comboboxes in a DataGridView, one with a State, the second holding a list of counties for that particular state.  I have bound the state combobox to an array of state names, and that works fine.  I have county column bound to a filtered collection of counties.  I update the filtered list in the 'RowEnter' and the 'CellValueChanged" event.  This seems to work fine, as I can select a state, and the county list displays the correct list of counties.  However, when I try and go to the next row(which is a newly created row), I get the following error message:
System.ArgumentException: DataGridViewComboBoxCell value is not valid.

The form load looks like this
        private void frmEditor_Load(object sender, EventArgs e)
        {
           /* omit code to fetch list of states and counties */
            colState.DataSource = oStates;
            colState.ValueMember = "StateFipsCode";
            colState.DisplayMember = "State";

             colCounty.DataSource = oFiltered;
             colCounty.DisplayMember = "County";
             colCounty.ValueMember = "CountyFipsCode";
        }

        // function called on RowEnter and CellValueChanged event (in state column)
        void UpdateFilter()
        {
            oFiltered.Clear();
            int nStateFips = SafeObjToInt(dgFarms.Rows[e.RowIndex].Cells[1].Value);
            // adding a blank county row did not help anything
            //oFiltered.Add(new FarmSimData.cCountyFips("", -1, "", -1));
            for (int i = 0; i <= oCountyByState.GetUpperBound(0); i++)
                if (nStateFips == (int)oCountyByState[i][0].StateFipsCode)
                {
                    for (int j = 0; j <= oCountyByState[i].GetUpperBound(0); j++)
                        oFiltered.Add(oCountyByState[i][j]);
                    break;
                }
        }

Thanks for any help
bnonnemannAsked:
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ArkConnect With a Mentor Commented:
This is because your filtered list applies to all rows. Try to populate countly combo directly (without datasource) in EditingControlShowing event like
cboCountly.Items.Clear();
//.............
   for (int j = 0; j <= oCountyByState[i].GetUpperBound(0); j++)
          cboCountly.Items.Add(oCountyByState[i][j]);
                        break;

BTW, unique code is a good idea to save table data and initialy populating it (you can use CellFormatting event for this)
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Kelvin McDanielSr. Developer/ConsultantCommented:
Is a default "State" selected when you enter go the newly created row?
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bnonnemannAuthor Commented:
Nothing specific was selected for the State
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Kelvin McDanielSr. Developer/ConsultantCommented:
That sounds like the problem then... I recommend that you build in a test to see if the State has a valid selected index before you try to use it. I would do it something like this...
// assumes C# 9.0 ... you'll need to adjust the syntax if you're using an earlier version.
// function called on RowEnter and CellValueChanged event (in state column) 
void UpdateFilter()
{
    if (!string.IsNullOrEmpty(dgFarms.Rows[e.RowIndex].Cells[1].Value))
    {
        var intCheck = default(int);
        int.TryParse(dgFarms.Rows[e.RowIndex].Cells[1].Value, intCheck);
 
        if (intCheck > 0)
        {
            oFiltered.Clear();
            int nStateFips = SafeObjToInt(dgFarms.Rows[e.RowIndex].Cells[1].Value);
            // adding a blank county row did not help anything
            //oFiltered.Add(new FarmSimData.cCountyFips("", -1, "", -1));
            for (int i = 0; i <= oCountyByState.GetUpperBound(0); i++)
            {
                if (nStateFips == (int)oCountyByState[i][0].StateFipsCode)
                {
                        for (int j = 0; j <= oCountyByState[i].GetUpperBound(0); j++)
                            oFiltered.Add(oCountyByState[i][j]);
                        break;
                }
            }
        }
    }
}

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Kelvin McDanielSr. Developer/ConsultantCommented:
A couple of syntax updates (my bad)...

if (!string.IsNullOrEmpty(dgFarms.Rows[e.RowIndex].Cells[1].Value))
SHOULD BE
if (!string.IsNullOrEmpty((string)dgFarms.Rows[e.RowIndex].Cells[1].Value))


int.TryParse(dgFarms.Rows[e.RowIndex].Cells[1].Value, intCheck);
SHOULD BE
int.TryParse((string)dgFarms.Rows[e.RowIndex].Cells[1].Value, intCheck);

Try those and see what you get.
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bnonnemannAuthor Commented:
It turns out that your suggestion was only part of the solution.   The problem comes up when you wwitch between rows that have different states.  For example, say in the first row you select the state 'Alabama', and then pick 'Butler' county.  You then go to the next row, and you select 'Illinois', and the 'Grundy'as the county.  So far, so good -- but when you return to the first line, the county list reverts back to those for Alabama, and the second row (containing the Illinois data) now displays 'Greene' county -- the county in Alabama that matches the FIPS code for 'Grundy' county in Illinois.  When you switch back to the row with Illinois as the state, then 'Grundy' county reappears in the county column.  Argh!   I'm going to try to see if making the county fips codes unique helps things, but I have the sinking feeling that the end result will be blank county entries when you switch states.

Thanks for you help so far!!
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bnonnemannAuthor Commented:
As I expected, changing the county fips codes to be unique (by combining state and county codes into a composite value) results in county columns going blank in rows whose state does not make the state in the current row.
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Kelvin McDanielSr. Developer/ConsultantCommented:
Sorry; was away from the Internet yesterday.
I agree with Ark's logic. That's actually the technique I would have used for my own needs.
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bnonnemannAuthor Commented:
Sorry, was away from the office for a while -- this suggestion works just fine.  Thanks!
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