returning c string to a C# program

Question

I have a C++ dll that has a function that returns a c string and I have a C# program that calls this function and returns the data to a C# string. Here's what I mean

__declspec(dllexport) const char* function (const char* str) {
        std::string stdString( str );
        std::cout << stdString.c_str() << std::endl; // this prints fine, no data loss
        return stdString.c_str();
}

And here's the C# code

  [DllImport("MyDLL.dll")]
  public static extern string function(string data);

  string blah = function("blah");
  Console.WriteLine(blah); // doesn't print anything...

When I look into the locals it says variable 'blah' is equal to "".

What happened to the data?

It got destroyed together with stdString.

lapk
– lapk

2011-12-07 02:02:01 +00:00
Commented Dec 7, 2011 at 2:02 — lapk
– lapk, Commented Dec 7, 2011 at 2:02
C#'s string is not equivalent to const char*.

Seth Carnegie
– Seth Carnegie

2011-12-07 02:02:22 +00:00
Commented Dec 7, 2011 at 2:02 — Seth Carnegie
– Seth Carnegie, Commented Dec 7, 2011 at 2:02
What would I use to return to a C# string?

tr0yspradling
– tr0yspradling

2011-12-07 02:02:59 +00:00
Commented Dec 7, 2011 at 2:02 — tr0yspradling
– tr0yspradling, Commented Dec 7, 2011 at 2:02

Hans Passant · Accepted Answer · 2011-12-07 02:03:54Z

4

Your C++ code is broken. You are returning a pointer to a local variable. It no longer exists after the function returns. This tends to work by accident in a C++ program but is strong Undefined Behavior. It cannot possibly work in an interop scenario, the pinvoke marshaler's use of the stack will overwrite the string.

A declaration that could work:

 void function (const char* str, char* output, size_t outputLength)

Use a StringBuilder in the [DllImport] declaration for the output argument and pass an initialized one with sufficient Capacity.

answered Dec 7, 2011 at 2:03

Hans Passant

946k151 gold badges1.8k silver badges2.6k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

tr0yspradling Over a year ago

How do I convert my std::string values to char* values?

Hans Passant Over a year ago

You already did in your original snippet. But you have to copy the string to output. Use strcpy_s().

Collectives™ on Stack Overflow

returning c string to a C# program

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related