F*cked by memory corruption after assiging value to associative array

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After a while my program crashes.

I'm inspecting in the debugger that some strings are overwritten 
after a struct is assigned to an associative array.

- I have disabled the GC.
- All happens in the same thread.
- The strings belong to an object collection inside an object 
created from a d-DLL.
- The object returned by the DLL function is added to the GC with 
GC.addRoot().
- This object also lives in a static array the whole time.
- Not all objects are affected but many.
- The struct itself looks okay also the key for the associative 
array has normal form.

The data is not overwritten by another Thread (only one is 
running) but by the compiler. I'm watching it by memory location. 
It gets always visible first after that assignment. But how is 
this even possible? In theory, how could I ran into this issue?