Maximum size of .NET arrays

Question

Not that I would ever need to do this, but I want to understand how it works/doesn't work. I googled quite a bit for the maximum length of an array and can't really find anything.

long[] hugeArray = new long[long.MaxValue];

//No exceptions
Console.WriteLine("Init");

//Overflow exception
Console.WriteLine(hugeArray.LongLength.ToString());

hugeArray = new long[int.MaxValue];

//OutOfMemoryException
Console.WriteLine( hugeArray.Length.ToString());

And I guess a follow up question would be, if there is a limit and I am initializing outside that limit, why no exception when creating only when using? And is this something the compiler should catch?

Answer 1

According to SpankyJ, .NET 2.0 at least had a limit of 2 GB per array. 8 bytes (sizeof long) * ~2^31 = 16 gigabytes (not to mention the actual memory required). As for the Overflow, I agree with marcc that is probably because arrays are expected to be int-indexed (see e.g. this method Though I'm a bit uncertain regarding this, as this overload takes an array of Int64 lengths. This may be an extension only available in later versions.

Overall, though, this is mostly a theoretical issue. Anyone actually relying on this is likely doing something wrong.

Answer 2

The underlying IL opcodes for arrays deal with IntPtr (known as native int in the IL documentation):

• newarr creates an array, and accepts an IntPtr
• ldelem and stelem read and write elements, and they use an IntPtr index
• ldlen returns the length of an array, and it returns an unsigned IntPtr

I'd expect a theoretical limit on the number of elements of 2^31-1 on 32-bit platforms, and 2^63-1 on 64-bit platforms.

However, I just tried this code on my 64-bit PC, and I got an OutOfMemoryException on the first allocation, so clearly the practical limits are smaller.

Answer 3

The index of an array is an int, not long. So, I guess technically new long[int.MaxValue] would be the maximum. But you could still get the OutOfMemoryException you are seeing if you really do run out of memory. Are you running this on a 32-bit OS? There isn't a set maximum length, other than the index value is an int, not a long.