Question

This a moot question, as the amount I was allocating was wrong. It was supposed to be 400MB not 400GB… Sorry! The answers are good though.

I am trying to unit test a method of mine which throws an OutOfMemoryException if a large file is uploaded.

My problem is my unit test is also throwing this exception (for the same reasons I guess).

According to Task Manager, if I am reading the information correctly, I do have enough RAM so am not really sure why my unit test is failing in this way.

Here is the respective unit test code:

var data = new List<byte>();
for(UInt64 i = 0; i < 400ul * 1024ul * 1024ul * 1024ul; i += 65536ul)
{
    data.AddRange(new byte[65536]); // <-- throws exception here when i = 536870912
}
this.UploadedFileData = data.ToArray();

I tried the following before the above code:

var data = new byte[400ul * 1024ul * 1024ul * 1024ul];

This though, results in the error: Arithmetic operation resulted in an overflow.

Answer 1

Are you on a 32 bit machine?

The process limit is 2GB on 32-bit, unless you use the /3GB boot switch.

From C# Increase Heap Size – Is It Possible

It is possible that you need to do the below

For very large List objects, you can increase the maximum capacity to 2 billion elements on a 64-bit system by setting the enabled attribute of the gcAllowVeryLargeObjects configuration element to true in the run-time environment.

From http://msdn.microsoft.com/en-us/library/6sh2ey19.aspx

As David mentions, the code will require a lot of reallocations when it grows. To prevent performance costs and memory fragmentation, you should at the very least set Capacity to an appropriate amount, if not rewrite the code.