AccessViolationException when calling an extern method directly

Question

I have a nuget formed from a C++ project - inside that nuget I have an extern method GetBytes inside of a static class FileAccess.cs that I call in another project's (C#) REST service call to retrieve some data. Whenever I call this GetBytes extern method from my C# REST service I get an AccessViolationException for some reason. The method takes in a two strings and several int values and returns an IntPtr.

I'm confused because I have a similar call that does not result in a violation. The key difference is that this version does not call the extern method directly. Instead, it calls a C# DataGroup class (which is part of the nuget) which contains a function that calls the extern method. The DataGroup class method returns a byte[,], but only after it calls the extern method and marshals the data.

// Version resulting in AccessViolation:

1. REST call to extern method as IntPtr (IntPtr ptr = FileAccess.getBytes(...);
2. extern method returns an IntPtr
3. AccessViolationException occurs

// Working version with extra class:

1. REST call to data group method that returns a byte[,]. (e.g. dataGroupInstance.getDataGroupBytes();
2. extern method is called inside method getDataGroupBytes
3. extern method returns an IntPtr
4. method getDataGroupBytes marshals the data from IntPtr and returns a byte[,]
5. no error

Basically, why is calling the extern method directly resulting in an AccessViolationException?

Answer 1

Figured it out. The problem was a result of my parameters - my C++ functions used std::string when I should have been using const char*. I replaced all instances of std::string (where they were used as parameters) with const char* and I no longer received the exception.