Getting float array from C++ to C#

Question

I have float array inside C++ function.

C++ function

void bleplugin_GetGolfResult(float* result)
{
    float *array = new float[20];
    for(int i=0; i < 20; i++)
      array[i]= 25;
    result = array;
    //DEBUG PRINTING1
    for(int i=0; i < 20; i++)
       cout << result[i] << endl;//Here is correct
    return;
}

Inside C#

[DllImport ("__Internal")]
private static unsafe extern void bleplugin_GetGolfResult (float* result);

public static unsafe float[] result = new float[20];

public unsafe static void GetGolfREsult(){
    fixed (float* ptr_result = result) //or equivalently "... = &f2[0]" address of f2[0]
    {           
        bleplugin_GetGolfResult( ptr_result );
        //DEBUG PRINTING2
        for(int i = 0; i < 20; i++)
            Debug.Log("result data " + ptr_result[i]);
    }
    return;
}

I called GetGolfREsult() from another function to get result.

//DEBUG PRINTING1 has correct output.

But //DEBUG PRINTING2 produced 0 only.

What could be wrong?

Where are you assigning values to the array in your C# method? — FortyTwo
– FortyTwo, Commented Dec 6, 2017 at 14:39
result is passed by value. Therefore it is pointless assigning to it. Your problems begin way before there is any C# code. Your C++ function is currently of no use whatsoever. You are getting ahead of yourself. Make sure your C++ code works before you try to call it from C++. And there's no need for unsafe code here. — David Heffernan
– David Heffernan, Commented Dec 6, 2017 at 14:52
If you can add a hint about the array size (and fix your C++ code as David pointed out), you can use the solution provided in this answer. — Leonardo Herrera
– Leonardo Herrera, Commented Dec 6, 2017 at 14:52

jHN · Accepted Answer · 2017-12-06 15:01:07Z

4

As UnholySheep and nvoigt already stated,

result = array;

overrides the address of the passed pointer, making you lose the reference to the calling function.

Directly writing to your parameter should solve this.

result[i] = 25;

Further you dont actually have to use pointers in c#. You can actually do the following:

Declare your Import like this:

private static extern void bleplugin_GetGolfResult (float arr[]);

Then you can call it like this:

float arr = new float[20];
bleplugin_GetGolfResult(arr);

answered Dec 6, 2017 at 15:01

jHN

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Comments

nvoigt · Accepted Answer · 2017-12-06 14:42:10Z

3

This line in your C++ code:

float *array = new float[20];

creates a new array, which you operate on in C++. Then control returns to C#, who has it's own array and that's still unchanged. Why don't you write to the array you got?

answered Dec 6, 2017 at 14:42

nvoigt

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4 Comments

UnholySheep Over a year ago

I think the actual flaw in OP's thinking is that result = array; would assign (copy?) this array back to C#

batuman Over a year ago

Because I like to copy array data to result. What you meant to write the array i got. I don't get it.

batuman Over a year ago

Actually I used float *array as a simple example. That array is already there for some reason and I can't use parameter result to create array at C++ function.

UnholySheep Over a year ago

@batuman result = array; does not modify the passed in array (it wouldn't do that if you were working exclusively in C++ either), it just redirects the local pointer to the array. You need to write directly into the passed in array (by result[i] = 25; instead of array[i] = 25;)

Jake Freeman · Accepted Answer · 2017-12-06 14:52:48Z

0

The problem is that you use the assignment operator on the parameter result which prevents the data from being transferred to the C# array on return.

Using the following C++ example:

void z(int * x)
{
  x = new int(4);
}

int main()
{
  int * x = new int(-2);
  z(x);
  cout<<*x<<endl;
}

The output for this is -2 not 4 because you use the assignment operator on the parameter.

edited Dec 6, 2017 at 14:52

answered Dec 6, 2017 at 14:43

Jake Freeman

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2 Comments

batuman Over a year ago

What should I do for that?

Jake Freeman Over a year ago

@batuman could pass an array to the function with a length of 20 then you would just change the value of each spot in the array

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