Pass a c# array to a c++ method

Question

I have a CLR class library in c++:

namespace ANN_Lib {

    public ref class ANN_FF_BP
    {
    private:
        int neurons;
        int inputs;
        int outputs;

        double **wi;
        double *wl;

    public:
        ANN_FF_BP(int neurons, int inputs, int outputs);

        void Train(double **p, double *t, int pairsCount, int epochs, double goal);
    };
}

I am using this class as a reference in a WPF project:

ANN_FF_BP neuralNetwork = new ANN_FF_BP(neurons, inputs, outputs);

Now I want to call the Train() method (in WPF) from that class and pass in some arguments. I have a problem to pass the first two parameters (pointers) to the method. My current code in C#:

ANN_FF_BP neuralNetwork = new ANN_FF_BP(neurons, inputs, outputs);
double[,] P = new double[3, 2] { { 1, 2 }, { 3, 4 }, { 5, 6 } };
double[] T = new double[] { 2, 4, 6 };
neuralNetwork.Train(P, T, 3, 20, 0.01);

I get the following error in C#:

enter image description here

Can anybody explain me how to pass an C# array to a c++ class library method?

Don't expose native interface from ref C++/CLI class. Use .NET arrays in the class interface, which map to C# double[,] and double[] types: array<double, 2> and array<double> msdn.microsoft.com/en-us/library/dtbydz1t.aspx functionx.com/cppcli/arrays/multidimension.htm Inside C++/CLI class, if you want to access native data, use pin_ptr — Alex F
– Alex F, Commented Jan 1, 2015 at 10:52

t3chb0t · Accepted Answer · 2015-01-01 11:06:41Z

2

As your ANN_FF_BP is a ref class and thus a CLR type it's better to pass .NET arrays to the C++/CLI method and do the hardwork there. This way you don't have to declare your C# code as unsafe what would be necessary here if you used pointers in C#.

This should work:

*.h:

void Train(cli::array<double, 2>^ ps, cli::array<double>^ ts, int pairsCount, int epochs, double goal);

*.cpp:

void ClrClass::Train(cli::array<double, 2>^ ps, cli::array<double>^ ts, int pairsCount, int epochs, double goal) {

    // pin pointer to first element in ps
    pin_ptr<double> _ps = &ps[0,0];   
    double* p = _ps;

    // pin pointer to first element in ts
    pin_ptr<double> _ts = &ts[0];
    double* t = _ts;

    // Now with the two arrays being pinned you can call you native method.
    NativeClass::Train((double**)p, t, pairsCount, epochs, goal);
}

To protect the arrays from being moved around in memory by the garbage-collector you need to use the pin_ptr:

A pinning pointer is an interior pointer that prevents the object pointed to from moving on the garbage-collected heap. That is, the value of a pinning pointer is not changed by the common language runtime. This is required when you pass the address of a managed class to an unmanaged function so that the address will not change unexpectedly during resolution of the unmanaged function call.

edited Jan 1, 2015 at 11:06

answered Jan 1, 2015 at 10:54

t3chb0t

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1 Comment

t3chb0t Over a year ago

@EmirAhmetović you've also deserved a thank you because it is a really high quality question that contains all relevant information so it wasn't difficult I know it's relative ;-) to answer it. Keep it that way ;-]

IS4 · Accepted Answer · 2015-01-01 12:29:02Z

1

If you wonder how the solution in unsafe C# looks:

unsafe{
    fixed(double* ptr1 = &P[0,0])
    fixed(double* ptr2 = &T[0])
    {
        neuralNetwork.Train(ptr1, ptr2, 3, 20, 0.01);
    }
}

edited Jan 1, 2015 at 12:29

answered Jan 1, 2015 at 12:05

IS4

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1 Comment

t3chb0t Over a year ago

I think it'd be a good idea if you could extend the example by wrapping it in a method with the unsafe keyword so that it's clear that it's necessary.

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