I've just spent the best part of a week trying to track down why my bridge from C# to C++ was working on Windows XP but crashing sporadically on Windows 7, and the answer is that .NET marshalling is trickier than you think for strings.
Essentially, I had this:
private static extern String LookupCorrespondingString(Int32 key);
and in the DLL, I had
__declspec(dllexport) const char *LookupCorrespondingString(int key);
Whenever I called this, it would get all the way into my DLL, I could tell it was going to return a value, but during the return operation, it would crash. When I ran it in the debugger, I got output messages about how memory was being free'd into the wrong heap.
Eventually I found this article:
which contained the useful quote (emphasis mine):
When a string buffer allocated by native code is marshaled to managed code, CLR Interop marshaller will allocate a managed string object and copy the contents of native buffer to the managed string object. Now in order to prevent a potential memory leak, CLR Interop Marshaller will try to free allocated native memory. It does so by calling CoTaskMemFree. The decision to call CoTaskMemFree is by-design. This can at times lead to crash, if memory was allocated by the called-native function using any API other than CoTaskMemAlloc family of API’s as custom allocators may allocate on different heaps.And there was the answer. .NET was freeing the block of memory I had passed to it to help me "prevent a potential leak". The problem being the lack of ability to communicate the 'const-ness' of the underlying DLL entry point's return value in the MarshalAs() attribute.
The solution was to declare the entry point differently:
private static extern IntPtr LookupCorrespondingString(Int32 key);
and then when I call it, do the marshalling explicitly.
String s = Marshal.PtrToStringAnsi( LookupCorrespondingString(k) );
After I bitched and moaned about how this stuff isn't documented anywhere, Manish Jawa kindly pointed out that it is, in fact, documented in the very first sentence on this page: http://msdn.microsoft.com/en-us/library/f1cf4kkz.aspx - you can't ask for more than that.
Actually, you can and they give it to you here: http://msdn.microsoft.com/en-us/library/x3txb6xc.aspx - the problem I was experiencing and its solution spelled out.
So, the moral of the story is: if you pass native strings to .NET via the marshalling interface, make sure you use IntPtr and PtrToStringAnsi() unless you want them to be free'd for you.