Passing strings from C# to C++ DLL and back -- minimal example

c#, c++, pinvoke

Solution

You cannot pass a C++ `std::string` across an interop boundary. You cannot create one of those in your C# code. So your code can never work.

You need to use interop friendly types at the interop boundary. For instance, null-terminated arrays of characters. That works well when you allocate and deallocate the memory in the same module. So, it's simple enough when passing data from C# to C++.

C++

void foo(const char *str)
{
    // do something with str
}

C#

[DllImport("...", CallingConvention = CallingConvention.Cdecl)
static extern void foo(string str);

....

foo("bar");

In the other direction you would typically expect the caller to allocate the buffer, into which the callee can write:

C++

void foo(char *str, int len)
{
    // write no more than len characters into str
}

C#

[DllImport("...", CallingConvention = CallingConvention.Cdecl)
static extern void foo(StringBuilder str, int len);

....

StringBuilder sb = new StringBuilder(10);
foo(sb, sb.Capacity);

Problem

I am trying to make the absolute simplest minimal example of how to pass strings to and from a C++ DLL in C#. My C++ looks like this: ``` using std::string; extern "C" { string concat(string a, string b){ return a + b; } } ``` With a header like ``` using std::string; extern "C" { // Returns a + b __declspec(dllexport) string concat(string a, string b); } ``` My C# is ``` [DllImport("*****.dll", CallingConvention = CallingConvention.Cdecl)] static extern string concat(string a, string b); } ``` And I am calling it with: Console.WriteLine(concat("a", "b")); But this gives a System.AccessViolationException. This seems like it out to be the most trivial thing to deal with, but I am completely stuck on it. When I tried to do a similar experiment with a function "Add" that took two doubles and returned a double I had no problems.

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