C# Marshalling unsigned char* array from C++ DLL (in and out)

Question

I am having trouble marshalling data between a C# application and an existing C++ DLL. The caveats that are making this difficult are that it is an unsigned char pointer to an array, and I need access to the data (from all fields) after the call in C#. I would like to avoid using unsafe code, if at all possible.

Here is the C++ signature:

BYTE GetData(unsigned char *Data_Type, unsigned char *Data_Content, unsigned int *Data_Length);

I've tried a bunch of things in C#, but here's what I have now:

[DllImport("somecpp.dll")]
public static extern byte GetData([In][Out] ref byte Data_Type, [In][Out] ref byte[] Data_Content, [In][Out] ref int Data_Length);

Then calling it, I am attempting this:

byte retrievedData = GetData(ref data_type, ref data_content, ref data_length);

This is definitely not working, and I'm not sure what to try next. Any ideas? Thanks!

uint would be the correct type for Data_Length, but I don't know why that would cause this problem. As always when talking about things not working, please be specific, are you receiving an error message if so what is the message. — Guvante
– Guvante, Commented Jan 22, 2015 at 19:58

David Heffernan · Accepted Answer · 2015-01-22 20:01:23Z

2

Your ref byte[] parameter matches unsigned char**. That's one level of indirection too many.

The p/invoke should be

[DllImport("somecpp.dll")]
public static extern byte GetData(
    ref byte Data_Type,
    [In,Out] byte[] Data_Content,
    ref uint Data_Length
);

It is plausible that the function uses cdecl. We can't tell that from here.

I also suspect that the Data_Type argument should be out rather than ref.

answered Jan 22, 2015 at 20:01

David Heffernan

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