Windows Forms

ComboBox hookedCombo = null; // this is a class level field
private void OnEditControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
    if (dataGrid.CurrentCell.ColumnIndex = 0) // use the index for your comboboxcolumn
    {
        hookedcombo = (ComboBox)e.Control;
        if (hookedCombo == null)
        {
            return;
        }

        hookedCombo.SelectedIndexChanged += this.OnComboIndexChanged // use any of the events for the combobox
    }
}

Dim HookedCombo as ComboBox
Private Sub OnEditControlShowing(ByVal sender As Object, ByVal e As DataGridViewEditingControlShowingEventArgs) Handles m_Grid.EditingControlShowing
    If m_Grid.CurrentCell.ColumnIndex = 0 Then 'use the index for your comboboxcolumn
        HookedCombo = TryCast(e.Control, ComboBox)
        If HookedCombo Is Nothing Then
            Return
        End If
        AddHandler HookedCombo.SelectedIndexChanged, AddressOf OnComboIndexChanged
    End If
End Sub

<br />
   Private Sub DataGridView1_EditingControlShowing(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewEditingControlShowingEventArgs) Handles DataGridView1.EditingControlShowing<br />
<br />
        If DataGridView1.CurrentCell.ColumnIndex = 0 Then<br />
<br />
<br />
<br />
<br />
        End If<br />
    End Sub<br />

Public Class Form1

    Private Sub DataGridView1_RowValidating(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellCancelEventArgs) Handles DataGridView1.RowValidating

        Dim row As DataGridViewRow = DataGridView1.Rows(e.RowIndex)

        If Not row.Cells("Column1").Value Is Nothing Then

            If row.Cells("Column1").Value.ToString = "Prod 01" Then

                Dim coid As String = "Product Description 01"
                row.Cells("Column2").Value = coid
                row.Cells("Column3").Value = "1.00"
                row.Cells("Column4").Value = "10.00"
                e.Cancel = False
            End If

            If row.Cells("Column1").Value.ToString = "Prod 02" Then

                Dim coid As String = "Product Description 02"
                row.Cells("Column2").Value = coid
                row.Cells("Column3").Value = "1.00"
                row.Cells("Column4").Value = "11.00"
                e.Cancel = False
            End If

        End If
    End Sub

End Class

Dim HookedCombo As ComboBox

Public Sub OnComboIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
    Dim strItemCode As String = Trim(HookedCombo.Text) 'Value Of ComboBox

    '#NOTES#-----------------------------------------------------------------------
    'I have populated the combobox with some dummy string values
    'Item 01, Item 02, Item 03 etc....
    'In my final atempt I will just read this off a database but I first
    'need to understand how to get this to work without complicating it with
    'a database.
    '
    'The next line does not work I keep geting the error;
    '
    'Index was out of range. Must be non-negative and less than the size of the collection.
    'Parameter name: index
    '
    'I have noted that when the DataGridView loads (blank) the row count is 1
    'using that I tried to deduct the row count (i) by 1 (i-1) however i still get the same
    'problem.


    If strItemCode = "Item 01" Then
        Dim i As Integer = DataGridView1.SelectedRows(0).Index
        DataGridView1.Rows(i).Cells(1).Value = "Item Description 01"
    End If
End Sub


Private Sub DataGridView1_EditingControlShowing(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewEditingControlShowingEventArgs) Handles DataGridView1.EditingControlShowing

    If DataGridView1.CurrentCell.ColumnIndex = 0 Then
        HookedCombo = TryCast(e.Control, ComboBox)
        If HookedCombo Is Nothing Then
            Return
        End If

        AddHandler HookedCombo.SelectedIndexChanged, AddressOf OnComboIndexChanged

    End If
End Sub

Public Sub OnComboIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
    Dim TheCombo As ComboBox = CType(sender, ComboBox)
    Dim strItemCode As String = Trim(TheCombo.Text) 'Value Of ComboBox
    ' This means that you always get the "Text" of the ComboBox that caused this event
    ' As I said above, in this particular case that is the same thing as HookedCombo, but that won't always be true

Public Sub OnComboIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
    Dim strItemCode As String = Trim(HookedCombo.Text) 'Value Of ComboBox

If strItemCode = "Item 01" Then
      Dim i As Integer = DataGridView1.SelectedRows(0).Index
      DataGridView1.Rows(i).Cells(1).Value = "Item Description 01"
  End If

      If strItemCode = "Item 01" Then    
            DataGridView1.CurrentRow.Cells(1).Value = "Item Description 01"
'The point is, and probably the reason you are getting the error, although you are 'using' a particular row
' it may not be selected and therefore won't appear in the SelectedRows collection. However CurrentRow
' always points at the row with the cursor in.
      End If

Public Class DGVComboIndexChangedForm
	Private productListMember As List(Of String) = New List(Of String)
	Dim HookedCombo As ComboBox

	Private Sub DGVComboIndexChangedForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
		productListMember.Add("Product 1")
		productListMember.Add("Product 2")
		productListMember.Add("Product 3")

		CType(Me.dgvItems.Columns("ItemID"), DataGridViewComboBoxColumn).DataSource = Me.ProductList()
	End Sub

	Private Sub OnEditControlShowing(ByVal sender As Object, ByVal e As DataGridViewEditingControlShowingEventArgs) Handles dgvItems.EditingControlShowing
		If dgvItems.CurrentCell.ColumnIndex = 0 Then 'use the index for your comboboxcolumn
			HookedCombo = TryCast(e.Control, ComboBox)
			If HookedCombo Is Nothing Then
				Return
			End If
			AddHandler HookedCombo.SelectedIndexChanged, AddressOf OnComboIndexChanged
		End If
	End Sub

	Public Property ProductList()
		Get
			Return Me.productListMember
		End Get
		Set(ByVal value)
			Me.productListMember = value
		End Set
	End Property

	Private Sub OnComboIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)
		Dim TheCombo As ComboBox = CType(sender, ComboBox)
		Dim strItemCode As String = Trim(TheCombo.Text)	'Value Of ComboBox

		Select Case (strItemCode)
			Case "Product 1"
				dgvItems.CurrentRow.Cells("ItemStock").Value = "1"
				dgvItems.CurrentRow.Cells("ItemPrice").Value = 1.1
			Case "Product 2"
				dgvItems.CurrentRow.Cells("ItemStock").Value = "2"
				dgvItems.CurrentRow.Cells("ItemPrice").Value = 2.2
			Case "Product 3"
				dgvItems.CurrentRow.Cells("ItemStock").Value = "3"
				dgvItems.CurrentRow.Cells("ItemPrice").Value = 3.3
		End Select

	End Sub
End Class