Converting from int[] to int*

Question

I have a C++ .DLL and C# application. In DLL I have a function like:

namespace Sample { public ref class SampleClass { public: int f(int arr[], int length); }; }

How can I call it from C# application? Problem is that in C++ I can use only int* argument and cannot declare int[], and only int[] variable in c#.

I have tried to declare int*[] type in C# but it is impossible due to

Cannot take address of a managed type

error. I have no idea how can I put an array into this function.

Upd: trying to dllimport like

[DllImport("samplelib.fll", EntryPoint = "Sample.SampleClass.f")]

but that seems to be wrong.

Why would you even have that signature in C++? How do you know how many elements are in `arr`? — crashmstr, Aug 27 '14 at 12:14
DllImport to...import that and signature simply int[]. See examples on MSDN. — Adriano Repetti, Aug 27 '14 at 12:14
@crashmstr `int f(int arr[])` is the same as `int f(int arr*)` in C++. The [] syntax is just there to confuse. — juanchopanza, Aug 27 '14 at 12:16
@juanchopanza but then the OP's argument about only being able to use `int*` (and **not** `int[]`) in C++ doesn't make sense.. He should be able to use both — default, Aug 27 '14 at 12:20
@Default No, you can't declare an `int[]` (without a brace-encluses initializer list), but you can declare an `int*`. The two are the same in function parameter lists only. — juanchopanza, Aug 27 '14 at 12:23
@juanchopanza but in C# he wants to pass an integer array (i.e. `int[]`). I think the user is confusing C++ arrays and C# arrays a bit. — crashmstr, Aug 27 '14 at 12:27
@crashmstr Right. And my first comment should be a hint to OP that there is no length information being passed with that signature in C++. — juanchopanza, Aug 27 '14 at 12:43

score 0 · Answer 1 · edited Aug 27 '14 at 12:53

0

I'm not sure, if this is what you want. Your example seems at bit incomplete ...

Let's say you have something like this in C/C++:

void TakesAnArray(int size, int array[]) {
   printf_s("[unmanaged]\n");
   for (int i=0; i<size; i++)
      printf("%d = %d\n", i, array[i]);
}

Then you want to call ist from C#, so you create a wrapper class.

internal static class NativeMethods
{
   [DllImport("SomeLib.dll" )]
   public static extern void TakesAnArray(int size, [In, Out] int[] array);
}

edited Aug 27 '14 at 12:53

Sam Harwell

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answered Aug 27 '14 at 12:24

Mithrandir

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Thank you, that works. But i can't find the proper syntax for entry point. I`m trying Sample.SampleClass.f, but that doesn`t work. – AndrewR Aug 27 '14 at 13:33
@maniac98066 how to find entry point for C++ .DLL and use it: http://stackoverflow.com/a/12885084/3908097 – Rimas Aug 27 '14 at 14:51

Sam · Answer 2 · 2014-08-27T13:05:51.927

0

int[] arr = new int[10];
int ptrsize = arr.Length * Marshal.SizeOf(typeof(int));
IntPtr arrptr = Marshal.AllocHGlobal(ptrsize);
Marshal.Copy(arr, 0, arrptr, ptrsize);
NativeFunction(arr.Length, arrptr);
//after
Marshal.FreeHGlobal(arrptr);

Use the array as a pointer

void NativeFunction(int length, int* array) { /*...*/ }

Import(.\\ is executable path)

[DllImport(".\\MyLib.dll")]
static extern void NativeFunction(int length, IntPtr array);

edited Aug 27 '14 at 13:05

answered Aug 27 '14 at 12:37

Sam

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Are you sure you want the memory size of the array instead of the number of elements being passes into the C++ function? – crashmstr Aug 27 '14 at 12:40
@crashmstr yeah you are right fixed with length – Sam Aug 27 '14 at 12:42
-1: This code is already more complicated than using a proper P/Invoke signature, and doesn't handle releasing memory in all the exception cases where a proper signature would handle it for you. – Sam Harwell Aug 27 '14 at 12:54

Converting from int[] to int*

2 Answers2