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c#cpointerspinvokemarshalling
PaeneInsula
PaeneInsula

Reputation: 2100

How to marshal a double pointer?

I am trying to use pinvoke to marshal a C structure to C#. While I am able to marshal an intptr I cannot find the syntax to marshal a double pointer. Both the int pointer and double pointer are used on the C side to alloc an array of ints or doubles.

Here is the C struct:

struct xyz
{
      int *np;  // an int pointer works fine
      double *foo;
};

And here is the c# class:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public class xyz
{
    Intptr np;  // works fine
            // double *foo   ?? 
   }

I am unable to find any instructions on how to marc

Upvotes: 2

Views: 7241

Answers (2)

Dutts
Dutts

Reputation: 6191

Check out this description for what an IntPtr is. Have you tried using:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public class xyz
{
    IntPtr np;
    IntPtr foo;
}

Upvotes: 1

David Heffernan
David Heffernan

Reputation: 612993

You seem to think that IntPtr is a pointer to an int. That is not the case. An IntPtr is an integer that is the same width as a pointer. So IntPtr is 32 bits wide on x86, and 64 bits wide on x64. The documentation makes all this clear.

The closest equivalent native type to IntPtr is void*, an untyped pointer.

So your class in C# should be:

[StructLayout(LayoutKind.Sequential)]
public class xyz
{
    IntPtr np;
    IntPtr foo;
}

To read the scalar value that np refers to call Marshal.ReadInt32. And to write it call Marshal.WriteInt32. But more likely, since this is a pointer, the pointer refers to an array. In which case you use the appropriate Marshal.Copy overload to read and write.

For the pointer to double, if the value is a scalar there is no method in Marshal to read or write the value. But again, it is surely an array in which case use Marshal.Copy to access the contents.

Upvotes: 0

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