C# : Out of Memory exception

3 years old topic, but I found another working solution. If you're sure you have enough free memory, running 64 bit OS and still getting exceptions, go to Project properties -> Build tab and be sure to set x64 as a Platform target.

Two points:

1. If you are running a 32 bit Windows, you won't have all the 4GB accessible, only 2GB.
2. Don't forget that the underlying implementation of List is an array. If your memory is heavily fragmented, there may not be enough contiguous space to allocate your List, even though in total you have plenty of free memory.

.Net4.5 does not have a 2GB limitation for objects any more. Add this lines to App.config

<runtime>
    <gcAllowVeryLargeObjects enabled="true" />    
</runtime>

and it will be possible to create very large objects without getting OutOfMemoryException

Please note it will work only on x64 OS's!