Question

I have a dll containing a function that returns a structure with a byte, and an array of 7 doubles:

public struct myStruct
{
    public byte v1;     
public Blob v2;
}

the dll function:

[DllImport("myDLL", EntryPoint = "?myDLLFuntion@@YA?AUmyStruct@@XZ", ExactSpelling = true)]
public static extern MyStruct myDLLFunction();

this is my blob:

[StructLayout(LayoutKind.Sequential, Size = ((sizeof(double))*7))]
public struct Blob
{
// Intentionally left empty. It's just a blob
}

I created a function to get the array:

public double[] GetArray(Blob NameBlob)
{

    IntPtr dPtr = IntPtr.Zero;  
    try
    {
        dPtr = Marshal.AllocHGlobal(((sizeof(double))*7));
        Marshal.StructureToPtr(NameBlob, dPtr, false);
        double[] r = new double[((sizeof(double))*7)];

        Marshal.Copy(dPtr, r, 0, ((sizeof(double))*7));

        return r;
    }
    finally
    {
        if (dPtr != IntPtr.Zero) 
        {
            Marshal.FreeHGlobal(dPtr);
        }
    }
}

I called it like this:

myStruct s1=myDLLFunction();

GetArray(s1.v2)[0]; // print it

I get the wrong value, please if someone can help me that would be great, I’m stating the use of Marshal… I think there is where I’m doing it wrong.

Answer 1

As Hans Passant pointed out, StructureToPtr() doesn’t do anything when the struct doesn’t have any fields. I think we was talking about this:

[StructLayout(LayoutKind.Sequential)]
public struct Blob
{
    [MarshalAs (UnmanagedType.ByValArray, SizeConst = 7)]
    double[] array;
}