System.AccessViolationException when calling from C# to unsafe C++

Question

I'm programming C# code that imports a C++ function similar to this:

//C++ code
extern "C" _declspec(dllexport) int foo(double *var1, double *var2){
    double dubs[10];
    RandomFunction(dubs);
    *var1 = dubs[5];
    *var2 = dubs[7];
    return 0;
}

To import this function to C# I've tried 2 methods and both have resulted in a System.AccessViolationException. That means I'm trying to access protected memory...

//C# import code
//method 1
[DllImport(example.dll,CallingConvention = CallingConvention.Cdecl)]
public static unsafe extern int foo(ref double var1, ref double var2);
//method 2
[DllImport(example.dll,CallingConvention = CallingConvention.Cdecl)]
public static unsafe extern int foo(double *var1, double *var2);

//C# calling code
//method 1
public unsafe int call_foo(){
    ....Stuff
    double var1 = 0;
    double var2 = 0;
    foo(ref var1, ref var2);
    ....More Stuff
}
//method 2
public unsafe int call_foo(){
    ....Stuff
    double var1 = 0;
    double var2 = 0;
    double *v1 = &var1;
    double *v2 = &var2;
    foo(v1, v2);
    ....More Stuff
}

Up until now, I've thought it was a problem with my C# code, but would the C++ code's use of an array cause a problem? Am I missing something really obvious here?

You did not declare the function correctly. You must use CallingConvention.Cdecl in the [DllImport] attribute since you didn't use __stdcall in the C declaration. The C++ code cannot compile and the "structs and arrays" you mention in your question title are quite invisible btw. You make it very difficult to help you. — Hans Passant
– Hans Passant, Commented Jul 7, 2014 at 17:42
I think when you are exporting functions from your C/C++ library, you should use IntPtr in .Net in order to have access to them. Whether they are safe/unsafe it's a different question. It all depends how well you write your code. — tsragravorogh
– tsragravorogh, Commented Jul 7, 2014 at 17:43
@armanali. How would an intPtr help? Do you see a reason to use it other than 'because it is there?" (PS: An intPtr lets C# do pointer arithmetic. I don't see any pointer arithmetic.) — Dale Wilson
– Dale Wilson, Commented Jul 7, 2014 at 17:50
Oops my bad; it still isn't working. Switched Calling convention to Cdecl. Still getting error. I forgot to call the function in my test script. — generic
– generic, Commented Jul 7, 2014 at 18:41
@generic this is the first time I've seen someone use both c and c++ tags correctly! — Austin Mullins
– Austin Mullins, Commented Jul 7, 2014 at 22:33

antiduh · Accepted Answer · 2014-07-09 15:48:31Z

This isn't a problem with your C# p/invokes or the signature of the native function foo. Aside from your C++ array syntax and what exactly RandomFunction does, everything works fine. I've just tested with my own little demo:

C++:

void RandomFunction( double dubs[] );

extern "C" __declspec(dllexport) int foo ( double* var1, double* var2 ) 
{
    double dubs[10];

    RandomFunction( dubs );

    *var1 = dubs[5];
    *var2 = dubs[7];

    return 0;
}

void RandomFunction( double dubs[] ) {
    for ( int i = 0; i < 10; i++ ) {
        dubs[i] = i;
    }
}

C#:

[DllImport( "Native.dll", CallingConvention = CallingConvention.Cdecl )]
private static extern int foo( ref double var1, ref double var2 );

public static void Main( string[] args )
{
    FooTest();
}

private static void FooTest()
{
    double var1 = 0;
    double var2 = 0;

    foo( ref var1, ref var2 );

    Console.Out.WriteLine( "Var1: {0:0.0}; Var2: {1:0.0}", var1, var2 );
    // Prints "Var1: 5.0; Var2: 7.0"
}

I only have a few differences from what you have:

My C++ arrays are declared using the standard syntax double dubs [10].
My compiler requires two underscores in the __declspec syntax
I'm compiling everything using Visual Studio 2012, in 64-bit, on Windows 7 SP 1.

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