You’ve just created a Console app in the latest Visual Studio, and wrote some C# code that allocates some non-negligible quantity of memory, say 6 GB. The machine you’re developing has a decent amount of RAM – 16GB – and it’s running 64-bit Windows 10.

You hit F5, but are struck to find the debugger breaking into your code almost immediately and showing:

What’s going on here ? You’re not running some other memory-consuming app. 6 GB surely should have been in reach for your code. The question that this post will set out to answer thus becomes simply: “Why do I get a System.OutOfMemoryException when running my recently created C# app on an idle, 64-bit Windows machine with lots of RAM ?“.

TL;DR (small scroll bar => therefore I deduct a lot of text => I’ve got no time for that, and need the answer now): The default build settings for Visual Studio limit your app’s virtual address space to 4 GB. Go into your project’s Properties, go to Build, and choose Platform target as x64. Build your solution again and you’re done.

Not so fast ! Tell me more about what goes on under the hood: Buckle up, since we’re going for a ride. First we’ll look at a simple example of code that consumes a lot of memory fast, then uncover interesting facts about our problem, hit a “Wait, what ?” moment, learn the fundamentals of virtual memory, find the root cause of our problem then finish with a series of Q&A.

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