Sadly it was not just the GC, there were plenty of problems in Mono itself that adversely affected GC performance. A significant one was a problem in the way t

Sadly it was not just the GC, there were plenty of problems in Mono itself that adversely affected GC performance. A significant one was a problem in the way that the Mono C# compiler generated foreach loops, which caused it to allocate an object for the enumerator that the regular .NET compiler was able to avoid. This caused foreach() to produce unnecessary garbage and resulted in a lot of Unity developers writing manual for() indexing loops instead of a more straightforward foreach() loop. Another problem was a bug in the Mono standard library where one of the array containers -- I forget whether it was ArrayList or List<T> -- would fail to clear elements at the end on removals. This was innocuous for arithmetic types, but for reference types it would result in hidden entries in unused capacity holding onto stale references. In some cases this caused rather large trees of objects to persist unnecessarily, pinned invisibly by containers that had already been deleted from or were even empty.