C# Large objects and heap

This article has a lot of details, although you should be aware of changes coming in .NET 4.5 too.

The only types which are likely to end up on the LOH are strings and arrays - because they're the only types which can basically be given a size at execution time. I'm not sure it's even valid to create a type with so many fields that it would end up on the LOH as a single object - it may well be, but I can't imagine it happening in reality.

According to the linked article, the limit is currently 85,000 bytes. It's an implementation detail really though - you should rarely need to think about it.

The general rule is: If the size of the object is 85000 bytes or more it is considered large and will be place on the LOH.

For some reason double[] is treated differently, so any array of doubles with 1000 or more elements go on the LOH as well. I haven't seen any official documentation for this implementation detail, but it is fairly easy to verify.