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

Question

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.

David Heffernan · Accepted Answer · 2013-12-23 22:25:00Z

97

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);

edited Dec 23, 2013 at 22:25

answered Dec 23, 2013 at 22:19

David Heffernan

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19 Comments

asutherland Over a year ago

nm you editted in an answer to my question. Trying it now, thanks!

David Heffernan Over a year ago

@orion Thanks. Normally you'd do it with one function. The size param would be ref int. Pass null to the SB to indicate you want to know length. Then call a second time.

AleX_ Over a year ago

@DavidHeffernan Thanks, Is there a reason you used stringBuilder instead of String Except for performance concerns?

fnc12 Over a year ago

How to return string as a return value not out parameter?

David Heffernan Over a year ago

@fnc12 Make sure you don't leak memory

|

Jonny · Accepted Answer · 2020-07-15 16:37:49Z

17

This is the simplest way I like - pass a string in, and use a lambda to get the response

C#

 public delegate void ResponseDelegate(string s);

 [DllImport(@"MyDLL.dll", EntryPoint ="Foo", CallingConvention = CallingConvention.StdCall)]
 public static extern void Foo(string str, ResponseDelegate response);
 ...
 
 Foo("Input", s =>
 {
    // response is returned in s - do what you want with it
 });

C++

 typedef void(_stdcall *LPEXTFUNCRESPOND) (LPCSTR s);

 extern "C"
 {
     __declspec(dllexport) void __stdcall Foo(const char *str, LPEXTFUNCRESPOND respond) 
     {
         // Input is in str
         // Put your response in respond()
         respond("HELLO");
     }
 }

edited Jul 15, 2020 at 16:37

answered Feb 6, 2019 at 21:58

Jonny

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3 Comments

David Heffernan Over a year ago

That's really unconventional, but also extremely cute!

anti Over a year ago

Can you please explain what the ResponseDelegate is in this example?

Jonny Over a year ago

@anti - apologies - added to the answer

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