C# calling Delphi DLL (that loads additional dlls) FaultExecutionEngineError

Question

I have a C# Application (VS 2017 .Net 4.6.1) that passes a string to a Delphi DLL (Delphi 10.2) which processes the inputs and returns outputs.
I am receiving a FaultExecutionEngineError "Common sources of this bug include user marshaling errors for COM-interop or PInvoke".

I have a few questions:

If My Delphi DLL is compiled with IMAGE_FILE_LARGE_ADDRESS_AWARE will my c# App also need to be compiled with editbin /largeaddressaware?

Could my Delphi DLL Loading child DLLs be causing the FaultExecutionEngineError? In my test I am not actually calling anything in the child DLLs except their initial startup.

Is there a tool to maybe debug these in Delphi?

Delphi:

procedure Go(Inputs : WideString; var Rates : WideString); stdcall;
begin
  Rates := Inputs + 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA';
end;

c#

[DllImport("DelphiDLL.dll",
            CharSet = CharSet.Unicode,
            EntryPoint = "LoadLibraries")]
public static extern void _LoadLiabraries(
    out int res);        

[DllImport("DelphiDLL.dll",
            CharSet = CharSet.Unicode,
            EntryPoint = "Go")]
public static extern void _Go(
    [MarshalAs(UnmanagedType.BStr)]
    string Inputs,
    [MarshalAs(UnmanagedType.BStr)]
    out string Outputs);        

public static string CallGo(string Inputs) {
    string outputs;
    _Go(Risk, out outputs);
    return outputs;
}

If I run this code:

{
    string inputs = "Test Input";
    int i = 0;
    do {
      string output = CallGo(inputs);
      Console.WriteLine(String.Format("{0} {1} {2}", i, inputs, output)
    } while (i++ < 10000) 
}

The I don't get the message but if I run this:

{
    int llres = 0;
    _LoadLibraries(llres);
    string inputs = "Test Input";
    int i = 0;
    do {
      string output = CallGo(inputs);
      Console.WriteLine(String.Format("{0} {1} {2}", i, inputs, output)
    } while (i++ < 10000); 
}

I get the error after N tries. The Number of tries has been consistent between tries. If I change the code the Number of successful tries has changed.

I just tried this passing and receiving ints instead of strings and still get the error, Although it is after thousands of more calls to the Delphi DLL — Darby Jacobsen
What about this question and answer: stackoverflow.com/questions/4151772/… — Graymatter
I have looked at that one and others as well. I tried a lot of different approaches I’ve found here. They all work for a number of calls then fail. The last thing I’ve tried is to just pass and receiving an integer. It failed after a number of calls to the dll. None of them were failing when I skip loading the additional dlls. One thing I didn’t mention before that might be causing a problem is the dll uses fastmm. Maybe fastmm memory management is causing the problem. — Darby Jacobsen
Large address aware is a process attribute. Pointless to mark a dll with it. Your question seems to implicate _LoadLiabraries but we can't see what it does. — David Heffernan
Thanks David that was my impression about Large Address Aware but I wanted to confirm that was the case. I'll see what I can do about putting the _LoadLibraries code up. — Darby Jacobsen

Darby Jacobsen Darby Jacobsen · Accepted Answer · 2019-03-15T17:08:51

I need to thank David for this. I went looking into the _LoadLibraries code commenting out sections to see what I could post here.
In doing so I found the problem. In my actual code _LoadLibraries passes a string (an ini file name) to the Delphi.DLL. This string was not being passed correctly. I didn't think to look at _loadLibraries because the error was thrown when calling the CallGo function.

So the answer is check every entry point to your DLL, the marshalling error may be caused by something you called earlier.

C# calling Delphi DLL (that loads additional dlls) FaultExecutionEngineError

1 Answers