We warn about this in the "Getting Started" PDF, but it comes up so often, we thought a FAQ was in order.

Do not call MarkAsDeleted within a "foreach" loop.

According to the MSDN docs, you cannot do anything inside a "foreach" that adds, removes, or alters the order of the list.

To delete all entities within a collection, use collection.MarkAllAsDeleted(). It will honor a filter, if one is applied:

EmployeesCollection collection = new EmployeesCollection();
collection.LoadAll();

collection.Filter = "LastName = 'Doe'";
collection.MarkAllAsDeleted();
collection.Save();

If you need to use more complex criteria than can be done with a filter, then you must loop back wards through the list using the indexer:

C#

EmployeesCollection collection = new EmployeesCollection();
collection.LoadAll();

for (int i = collection.Count - 1; i >= 0; i--)
{
    if (collection[ i ].LastName == "Doe")
    {
        if (collection[ i ].Age.Value <= 30)
        {
            collection[ i ].MarkAsDeleted();
        }
    }
}
collection.Save();

VB.NET

Dim collection As New EmployeesCollection() 
collection.LoadAll()

For i As Integer = collection.Count - 1 To 0 Step -1

    If collection(i).LastName = "Doe" Then
        If collection(i).Age.Value <= 30 Then
            collection(i).MarkAsDeleted()
        End If
    End If
Next

collection.Save()