Collection was modified; enumeration operation might not exe

Notice also that if there's a possibility that you might need to remove more than one row then you should iterate backwards through the collection using a 'for' statement.

This is because each time you delete a row it alters the indices of the rows still to be examined which all move down one. If you iterate forwards, you will therefore fail to examine the next element in the collection.

Iterating backwards is the cleanest way to avoid this problem though you can still iterate forwards if you decrement the loop control variable before the next iteration.

You cant remove a row from rows collection when you are in loop.

Try to do something like this.

Dim dt1 As New DataTable()
dt1.Columns.Add("Name")
dt1.Rows.Add("Apple")
dt1.Rows.Add("Banana")
dt1.Rows.Add("Orange")

Dim dt2 As New DataTable()
dt2.Columns.Add("Name")
dt2.Rows.Add("Apple")
dt2.Rows.Add("Banana")

Dim rows_to_remove As New List(Of DataRow)()
For Each row1 As DataRow In dt1.Rows
For Each row2 As DataRow In dt2.Rows
If row1("Name").ToString() = row2("Name").ToString() Then
rows_to_remove.Add(row1)
End If
Next
Next

For Each row As DataRow In rows_to_remove
dt1.Rows.Remove(row)
dt1.AcceptChanges()
Next