Convert from double array to pointer

Question

I have such class

public unsafe class EigenSolver
{
   public double* aPtr
   {get; private set;}
   public EigenSolver(double* ap)
   {
      aPtr = ap;
   }
   public EigenSolver(double[] aa)
   {
     // how to convert from aa double array to pointer?
   }

   public void Solve()
   {
     Interop.CallFortranCode(aPtr);
   }
}

As you can guess, I need to convert from double array to pointer. How to do it?

Note: the interop function Interop.CallFortranCode(double* dPtr) is something I can't change.

Note 2: Both the constructors are needed because some of my API users want to pass in pointers, and some would like to pass in array. I can't force them to choose.

Answer 1

Use the fixed statement:

fixed (double* aaPtr = aa) {
   // You can use the pointer in here.
}

While in context of fixed, the memory for your variable is pinned so the garbage collector will not try to move it around.

I would take this approach instead:

public class EigenSolver
{
   public double[] _aa;
   /*
   There really is no reason to allow callers to pass a pointer here, 
   just make them pass the array.
   public EigenSolver(double* ap)
   {
      aPtr = ap;
   }
   */
   public EigenSolver(double[] aa)
   {
     _aa = aa;
   }

   public void Solve()
   {
     unsafe {
        fixed (double* ptr = _aa) {
           Interop.CallFortranCode(ptr);
        }
     }
   }
}

This assumes of course CallFortranCode does not try to use the pointer outside of the call. Once the fixed statement goes out of scope the pointer is no longer valid...

UPDATE:

There is no way you can take the address of your argument double[] aa and store it in your instance field. Even if the compiler would let you, the GC is bound to move that memory around leaving your pointer useless.

You could probably do this: Use Marshal.AllocHGlobal to allocate enough memory to store all the elements of the array (aa.Length * sizeof(double))). Then, use Marshal.Copy to copy the contents of the array to your newly allocated memory:

bool _ownsPointer; 
public EigenSolver(double[] aa) {
   IntPtr arrayStore = (double*)Marshal.AllocHGlobal(aa.Length * sizeof(double));
   Marshal.Copy(aa, 0, arrayStore, aa.Length);
   this.aPtr = (double*)arrayStore.ToPointer();
   _ownsPointer = true;
}

~EigenSolver {
   if (_ownsPointer) {
      Marshal.FreeHGlobal(new IntPtr(this.aPtr));
   }
}

Hopefully this works...

Andrew

Answer 2

While fixed is indeed the answer you're looking for, you can't use it in the way you're trying to. Once the fixed block ends (at the closing brace) the pointer is invalidated. So you can't use it in a constructor to store the pointer. Nor should you want to, because pinning a managed object in memory for extended periods of time leads to drastic performance degradation.

What I'd do instead is pass the array (as a managed object) to your Solve function. This way you can pin it down just to make your fortran interop call then let the gc take care of it.