Windows Forms

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