LINQ Sum OverflowException?

Question

I've implemented a custom IEqualityComparer for EventLogEntry.

public class EventLogEntryListComparison :
    IEqualityComparer<List<EventLogEntry>>,
    IEqualityComparer<EventLogEntry>

For IEqualityComparer<List<EventLogEntry>>, the GetHashCode function is very simple.

public int GetHashCode(List<EventLogEntry> obj)
{
    return obj.Sum(entry => 23 * GetHashCode(entry));
}

However, this throws an OverflowException for certain entries.

"Arithmetic operation resulted in an overflow."
   at System.Linq.Enumerable.Sum(IEnumerable`1 source)
   at System.Linq.Enumerable.Sum[TSource](IEnumerable`1 source, Func`2 selector)
   at <snip>.Diagnostics.EventLogAnalysis.EventLogEntryListComparison.GetHashCode(List`1 obj) in C:\dev\<snip>Diagnostics.EventLogAnalysis\EventLogEntryListComparison.cs:line 112
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.set_Item(TKey key, TValue value)
   at <snip>.Diagnostics.EventLogAnalysis.Program.AnalyseMachine(String validMachineName) in C:\dev\<snip>.Diagnostics.EventLogAnalysis\Program.cs:line 104
   at System.Threading.Tasks.Parallel.<>c__DisplayClass2d`2.<ForEachWorker>b__23(Int32 i)
   at System.Threading.Tasks.Parallel.<>c__DisplayClassf`1.<ForWorker>b__c()

After trying to get the same error whilst debugging and couldn't in the immediate window, I changed the code to this and bye bye OverflowException?

int total = 0;
foreach (var eventLogEntry in obj)
{
    total += GetHashCode(eventLogEntry);
}

return total;

How is it that LINQ's Sum function behaving differently?

Edit 2

Thanks to a few comments, the corrected and intended GetHashCode function is now as follows,

public int GetHashCode(List<EventLogEntry> obj)
{
    return unchecked(obj.Aggregate(17,
        (accumulate, entry) =>
        accumulate * 23 + GetHashCode(entry)));
}

Answer 1

LINQ's Enumerable.Sum(...) methods perform the additions inside a checked block. This means that they deliberately throw an exception if the sum overflows.

Your sum is not inside a checked block, so whether or not it throws an exception depends on... whether it is called from inside a checked block, or a property on the assembly I believe.