Question

The code I have written below will do iteration of the loop properly. It tests the correct row against the condition, adds to the “secondary” DataTable and removes from the “master” as it is supposed to. However, upon the second iteration of the loop I get the following error:

Collection was modified; enumeration operation might not execute.

Here is the code I am using

            For Each row As DataRow In tblAgencyEdInfo.Rows
                Dim rDate As DateTime = row.Item("ExpirationDate")

                If rDate < DateTime.Now Then
                    tblExpEdInfo.ImportRow(row)
                    tblAgencyEdInfo.Rows.Remove(row)
                End If
            Next row

Answer 1

You cannot modify a collection during enumeration. That means you cannot add or remove items. In this case you want to remove a DataRow from a DataTable in a For Each loop.

The solution is either to

1. use a For-Loop and iterate the items backwards, removing the rows via index or to
2. use another collection (f.e. a List(Of DataRow)) and add the matching rows you want to remove in the For Each. Afterwards you just need to iterate this list and call tblAgencyEdInfo.Rows.Remove(rowToRemove)

For example:

Dim rowsToRemove = New List(Of DataRow)
For Each row As DataRow In tblAgencyEdInfo.Rows
    Dim rDate As DateTime = row.Item("ExpirationDate")
    If rDate < DateTime.Now Then
        rowsToRemove.Add(row)
    End If
Next row

For Each rowToRemove As DataRow In rowsToRemove 
    tblExpEdInfo.ImportRow(row)
    tblAgencyEdInfo.Rows.Remove(row)
Next rowToRemove