如何将C数组映射和编组为C＃代码

Question

I am writing C# code that call a C library and something it is not quite clear in my mind.

The C function has this signature:

double* DoSomeStuff(double* input,int numberOfElements);

I have mapped the function as:

[System.Runtime.InteropServices.DllImportAttribute("myDll.dll", EntryPoint="DoSomeStuff")]
public static extern  System.IntPtr DoSomeStuff(ref double input, int numberOfElements) ;

The input value is an array, so the C function will expect a contiguous memory layout. I am more familiar with C++ than C#. In C++ I'd used a std::vector to store the data and then I would use the data() method to get the pointer and exchange the information with C code. std::vector guarantees a contiguous layout memory.

Which data structure can I used in C#? Is there anything like std::vector in C#?

I have faced before the same problem for a string (In C++ std::string is just a std::vector with some make up). And I have solve the problem using:

System.IntPtr stringExample = Marshal.StringToHGlobalAnsi("StringExample"); 

The static function does the job for me. There is anything like this function for other types?

I have asked already too many questions, I think the most important one is: what is the best practise to solve this kind of problem?

Thanks

Answer 1

1)

Define the input as IntPtr:

[System.Runtime.InteropServices.DllImportAttribute("myDll.dll", EntryPoint="DoSomeStuff")]
public static extern  System.IntPtr DoSomeStuff(IntPtr input, int numberOfElements) ;

2)

Create an array in a fixed block, then create an IntPtr from the Pointer and then pass it to DoSomeStuff .

double[] input = new double[20];
IntPtr result = IntPtr.Zero;
fixed(double* d = &input[0])
{
    result = DoSomeStuff(new InptPtr(d), 20);
}

...

Reason for the fixed block is so that GC does not move the array while unmanaged code is populating it.

Answer 2

To make your example work you should define the siganture of the extern function as follows:

[System.Runtime.InteropServices.DllImportAttribute("myDll.dll", EntryPoint="DoSomeStuff")]
public static extern  System.IntPtr DoSomeStuff([MarshalAs(UnmanagedType.LPArray, SizeParamIndex=1)], int numberOfElements);

The second (named) parameter of MarshalAs attaribute tells to the marshaller where the size of the array stored.

Regarding the second question, C# has List<Type> class that behaves similary to std:vector<Type> . However, I don't think you can directly provide it to the marshaller. What you can do is to use ToArray() methor of the List class, to get an array.