Removing Items in a List While Iterating Through It with For Each Loop

Question

I have a list named NeededList I need to check each item in this list to see if it exists in my database. If it does exist in the database I need to remove it

Answer 1

How about this (no iteration needed):

NeededList = (NeededList.Where(Function(Needed) IsNeeded(Needed)).ToList

Function IsNeeded(Needed As ...) As Boolean
    ...
    Return Not dr.HasRows
End Function

Answer 2

You can also invert the order of the list's elements and still use For Each using the IEnumerable Cast and Reverse extensions.

Simple example using a List(Of String):

For Each Needed In NeededList.Cast(Of List(Of String)).Reverse()
    If dr.HasRows Then
        NeededList.Remove(Needed)
    End If
Next

Answer 3

You can use a For loop iterating through every index with Step -1.

For i as Integer = NeededList.Count - 1 to 0 Step -1

    Dim Needed = NeededList(i)

    'this is a copy of your code
    Dim Ticker = Needed.Split("-")(0).Trim()
    Dim Year = Needed.Split("-")(1).Trim()
    Dim Period = Needed.Split("-")(2).Trim()
    Dim Table = Needed.Split("-")(3).Trim()

    Dim dr As OleDbDataReader
    Dim cmd2 As New OleDb.OleDbCommand("SELECT * FROM " & Table & " WHERE Ticker = ? AND [Year] = ? AND Period = ?", con)
    cmd2.Parameters.AddWithValue("?", Ticker)
    cmd2.Parameters.AddWithValue("?", Year)
    cmd2.Parameters.AddWithValue("?", Period)
    dr = cmd2.ExecuteReader

    'MODIFIED CODE
    If dr.HasRows Then NeededList.RemoveAt(i)

Next i

Answer 4

Go for safe and make a copy with ToList():

For Each Needed In NeededList.ToList()
    Dim Ticker = Needed.Split("-")(0).Trim()
    ...
    If dr.HasRows Then
        NeededList.Remove(Needed)
    End If
Next

Answer 5

The contents of an array (or anything else you can fast enumerate with For Each can not be modified with a For Each loop. You need to use a simple For loop and iterate through every index.

Hint: Because you'll be deleting indexes, I suggest starting at the last index and work your way toward the first index so you don't skip over one every time you delete one.

Answer 6

No you can not remove from a List that you are working on e.g. For Each Str As String In listOfStrings If Str.Equals("Pat") Then Dim index = listOfStrings.IndexOf(Str) listOfStrings .RemoveAt(index) End If Next

But this way will work make a copy of your list and delete from it e.g. For Each Str As String In listOfStrings If Str.Equals("Pat") Then Dim index = listOfStringsCopy.IndexOf(Str) listOfStringsCopy.RemoveAt(index) End If Next