Question

I’m exploring the idea of building a bridge between a DLL plugin for a 3rd party app and a C# app. I’m writing both the plugin DLL and the C# application. The plugin will be loaded into the 3rd party app and then I want to use call the plugin from C# to indirectly get data from the 3rd party app.

I am able to successfully call an exported function from the DLL from C#. For example:

C++ DLL:

extern "C" __declspec(dllexport) char * HelloFromDll()
{
    char *result;
    result = "Hello from my DLL";
    return result;
}

C#:

using System.Runtime.InteropServices;

[DllImport(@"MyDll.dll")]
private static extern string HelloFromDll();

I can then call this DLL function from C# and display the string in the UI. However, as soon as I create an export function that calls a function from my 3rd party app, I get an AccessViolationException. For example,

extern "C" __declspec(dllexport) char * GetData()
{
    char *result;
    result = 3rdPartyLibrary::SomeFunction();
    return result;
}

Through some testing, the error seems to occur as soon as I make a call to a 3rd party function. How can I fix this?

Answer 1

From your question it seems that this is the scenario:

ProcessA (3rd party App) –> loads X.DLL –> initializes the plugin –> does other stuff.

ProcessB (Your C# App) –> loads X.DLL –> calls GetData();

Does X.DLL loaded in ProcessA have any mechanism to talk to X.DLL loaded in ProcessB?

if not then this approach is flawed. Your code probbably crashes because “3rdPartyLibrary” class hasn’t been initialised in your C# app as it is completely different copy of the DLL.

For you to extract this data you need a query interface defined by X.DLL which can talk across processes, maybe sockets?

Then ProcessB talks to this interface and extracts the data. if using sockets, then your X.DLL would implement both server and client code, where your GetData() would use this mechanism (maybe sockets) and query the data and return it.

So : X.DLL in ProcessA should act like a server.
And: X.DLL (or write a Y.DLL) in ProcessB should act like a client and get this information from ProcessA.

btw, if the query is only needed to be done once, just hard code this is in X.DLL and dump to disk, and then explore at your convinience 🙂