2
The IEnumerator<T>.Current property must have only a getter. I.e., you are not allowed to change the Current value from outside. Only MoveNext() and Reset() are allowed.
public TData Current => _current.Data;
The IEnumerator.Current property can slightly be simplified. Add the expression body to the property instead of the getter (this defines the ...
1
License counting
foreach (IGrouping<int, Installation> group in groupUsersQuery)
{
if (group.Count() == 1)
{
copiesNeeded += 1;
} else if (group.Count() == 2)
{
if (group.ContainsComputerType("LAPTOP")
&& group.ContainsComputerType("DESKTOP"))
{
...
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