[liversage]

I want to point out that disposing resources (using the IDisposable) interface does not free memory (unless the memory isn't managed but then it's not related to garbage collection).

A System.Drawing.Image holds operating system resources (GDI+) and these resources are released when the Image instance is disposed. If the instance isn't explicitly disposed then the finalizer will do it but this only happens when the garbage collector collects the Image instance.

Allocating many Image instances without disposing them might exhaust the available resources (Windows bitmap handles or whatever) but the garbage collector doesn't see any memory pressure and does not perform any collection so the finalizer does not dispose Image instances that are no longer used.

The garbage collector has an API (GC.AddMemoryPressure) where an object that has unmanaged resources can signal that it's consuming additional memory to inform the garbage collector's decision of when to perform a collection.

[pjmlp]

Ideally such handles are also wrapped in SafeHandle classes.