There is already an analyzer for it: https://github.com/microsoft/RoslynClrHeapAllocationAnalyzer

That Roslyn analyzer linked by notanaverageman below is really cool, I didn't know about it. But I don't think that even that analyzer will be able to tell if you call a method which itself allocates memory.

But it's not very complicated to write a simple IL analyzer, using Mono.Cecil to go over the raw IL, instead of using Roslyn. The idea is to look for the newobj or box IL opcodes, but also to recurse to called methods (when meeting the callX opcodes), by finding using Cecil where the call leads us, and loading that method.

This could be used after compilation instead of during compilation, but it can be used without actually running the code, so it's a sort of static analysis.

I threw together a simple demo, that has some bugs but is generic enough and should work. It's not very efficient though.

See here :

https://dotnetfiddle.net/EyuYSh

(It can also be run from there since dotnetfiddle supports nugets).

And also in godbolt:

https://godbolt.org/z/P3rK1voKP

(it doesn't run there since Godbolt apparently doesn't support nugets)

It's self contained and will run as a console application by just copying and pasting the code into an IDE. You need to add Mono.Cecil as a nuget package though.