Question

I am having some trouble marshaling a pointer to an array of strings. It looks harmless like this:

typedef struct
{
    char* listOfStrings[100];
} UnmanagedStruct;

This is actually embedded inside another structure like this:

typedef struct
{
    UnmanagedStruct umgdStruct;
} Outerstruct;

Unmanaged code calls back into managed code and returns Outerstruct as an IntPtr with memory allocated and values filled in.

Managed world:

[StructLayout(LayoutKind.Sequential)]
public struct UnmanagedStruct
{
    [MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.LPStr, SizeConst=100)]
    public string[] listOfStrings;
}

[StructLayout(LayoutKind.Sequential)]
public struct Outerstruct
{
    public UnmanagedStruct ums;
}

public void CallbackFromUnmanagedLayer(IntPtr outerStruct)
{
    Outerstruct os = Marshal.PtrToStructure(outerStruct, typeof(Outerstruct));
    // The above line FAILS! it throws an exception complaining it cannot marshal listOfStrings field in the inner struct and that its managed representation is incorrect!
}

If I change listOfStrings to simply be an IntPtr then Marshal.PtrToStructure works but now I am unable to rip into listOfStrings and extract the strings one by one.

Answer 1

OK.. I seem to have got it to work. It should be marshaled as IntPtr[]

This seems to work:

[StructLayout(LayoutKind.Sequential)] 
public struct UnmanagedStruct 
{ 
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=100)] 
    public IntPtr[] listOfStrings; 
}

for (int i = 0; i < 100; ++i)
{
    if (listOfstrings[i] != IntPtr.Zero)
        Console.WriteLine(Marshal.PtrToStringAnsi(listOfStrings[i]));
}