C# calling C++ DLL returns incorrect value

Question

I have no further ideas how to debug this bug, so asking for advice.

Essentially, I have a C++ DLL, which has a boolean function

class Point
{
   Public: 
   double x;
   double y;

   // lots of operators
}

typedef struct SomeStructParent
{
    bool isValid;

    int64_t StartTime;
    int64_t EndTime;
} SomeStructParent;

typedef struct SomeStruct : SomeStructParent
{
    Point Center;
} SomeStruct;

bool func(SomeStruct & arg)
   { return some_var; }

some_var is initiated to false. The only place in the code where it is set to true has a logger printing. The return value of func is also printed to logger regularly.

C# imports the function like so:

public struct EtPoint
    {
        public double x;
        public double y;
    }

[StructLayout(LayoutKind.Sequential)]
public struct SomeStruct 
    {
        bool isValid;

        // in milliseconds using tracker epoch
        public long StartTime;
        long EndTime;

        public EtPoint Center;       
    }

[DllImport("ETHAL.dll", EntryPoint = "func", CallingConvention = CallingConvention.Cdecl)]
public static extern bool func(ref SomeStruct data);

Under some conditions, calling func from C# returns true, even though the logger shows it as false - the regular prints are false and the line that shows it was set does not occur. With a modified function bool funcArg(SomeStruct & arg, bool & ret) { ret = some_var; }, ret is reliably false under the same conditions.

Perhaps the most annoying/perplexing part: when adding a debug print inside func, it returns the correct value. The same happens when I add Sleep(500). This makes me suspect some kind of multithreading shenanigans, but as I said - only one place in the code where the some_var is assigned true, and it isn't reached in this run.

Is there something with the interaction of C# and C++ that could account for this?

Incorrect function signature causing UB? Multithreading? Some other part of the program writing in memory it shopuldn't write? Hard to tell without more info.. — stijn
– stijn, Commented Sep 1, 2016 at 7:29
I've added the C# calling. The rest of it... well, I don't expect SO to find multithreading bugs in my code, I was specifically asking about C# and C++ interactions to see if there's some common pitfall there that I'm not aware of. — dkb
– dkb, Commented Sep 1, 2016 at 7:36
What is SomeStruct, does it have the exact same layout in C# and C++? — stijn
– stijn, Commented Sep 1, 2016 at 7:49

Community · Accepted Answer · 2017-05-23 12:01:03Z

2

A solution to your problem can be to precede your declaration withe the following marshaling.

[return:MarshalAs(UnmanagedType.I1)]

This may happen because C defines bool as 4 bytes int and C++ defines it as 1 byte. C# team decided to use 4 byte bool as default during PInvoke because most of the system API function use 4 bytes values as bool. If you want to change this behavior you have to do it with marshaling specifying that you want to use 1 byte value. Also another solution can be to try changing the return type from bool to int, that should also fix the issue.

For more information, have a look at the answer mentioned here: C# DllImport with C++ boolean function not returning correctly

edited May 23, 2017 at 12:01

CommunityBot

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answered Sep 1, 2016 at 10:27

Omer Kahoot

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