Did P/Invoke environment change in .NET 4.0?

I've started upgrading a .NET 2.0 WinForms application to .NET 4.0. Well, OK, the upgrade process was just a matter of switching platform target, but making it actually work. I assumed that's all there would be to it.

But it seems that something drastically changed in .NET 4.0 regarding interop. Using DllImport(), the application embeds a couple Delphi dlls. When the application targets .NET 2.0, everything works normally. But when I changed it to target .NET 4.0, stuff starts going haywire, like something is corrupting memory.

For example, it replaces single digits with "0" in strange places. Data passed in an IStream gets 8 characters replaced with (Hex) 00 00 00 00 00 00 00 80, but only about 70% of the time. Two consecutive calls to retrieve the same value return different results (retrieving a value from a cache in memory, succeeds the first time, fails the second time). Strings being sent to a log are showing up truncated.

I've tried lots of stuff trying to make calling conventions more explicit, none of it has any effect. All strings are handled as [MarshalAs(UnmanagedType.LPWStr)] on the .NET side and PWChar on the Delphi side.

What changed in .NET 4.0 that would break P/Invoke like this?


Here's the simplest example. It generates a PDF which sometimes works correctly, but more frequently ends up corrupt (and works correctly in .NET 2.0):

public static extern void SetDBParameters(
    [MarshalAs(UnmanagedType.LPWStr)] string Server,
    [MarshalAs(UnmanagedType.LPWStr)] string Database,
    [MarshalAs(UnmanagedType.LPWStr)] string User,
    [MarshalAs(UnmanagedType.LPWStr)] string Password,
    short IntegratedSecurity);

procedure SetDBParameters(Server, Database, User, Password: PWChar;
    IntegratedSecurity: WordBool); stdcall;

public static extern short GeneratePDF(
    [MarshalAs(UnmanagedType.LPWStr)] string Param1,
    [MarshalAs(UnmanagedType.LPWStr)] string Param2,
    [MarshalAs(UnmanagedType.LPWStr)] string Param3,
    [MarshalAs(UnmanagedType.LPWStr)] string Param4,
    out IStream PDFData);

function GeneratePDF(Param1, Param2, Param3, Param4: PWChar;
    out PDFData: IStream): WordBool; stdcall;

private byte[] ReadIStream(IStream Stream)
    if (Stream == null)
        return null;
    System.Runtime.InteropServices.ComTypes.STATSTG streamstats;
    Stream.Stat(out streamstats, 0);
    Stream.Seek(0, 0, IntPtr.Zero);
    if (streamstats.cbSize <= 0)
        return null;
    byte[] result = new byte[streamstats.cbSize];
    Stream.Read(result, (int)streamstats.cbSize, IntPtr.Zero);
    return result;

WordBool and short were originally boolean (Delphi) and bool (C#), I changed them to be more explicit, just in case.


The stuff I wrote earlier about WinForms appears to have turned out to be not completely relevant, I've recreated one of the issues without any UI. The following program generates 0,1,2,3,4,5,6,7,8,9 under 2.0/3.5, but 0,-1,-1,-1,-1,-1,-1,-1,-1 under 4.0.

using System;
using System.Runtime.InteropServices;

namespace TestNet4interop
    static class Program
        [DllImport("TestSimpleLibrary.dll", PreserveSig=true, CallingConvention = CallingConvention.StdCall)]
        public static extern void AddToList(long value);

        [DllImport("TestSimpleLibrary.dll", PreserveSig=true, CallingConvention = CallingConvention.StdCall)]
        public static extern int GetFromList(long value);

        static void Main()
            for (long i = 0; i < 10; i++)

And the Delphi side (compiled with Delphi 2007):

library TestSimpleLibrary;


{$R *.res}

   List: TStringList;

procedure AddToList(value: int64); stdcall;

function GetFromList(value: int64): integer; stdcall;
   result := List.IndexOf(IntToStr(value));


   List := TStringList.Create;


It appears to be a bug in the Visual Studio 2010 debugger. It seems to be clobbering memory that doesn't belong to it. All of the problems I've observed (all of which can be reproduced reliably) disappear completely if I run the application directly, instead of through Visual Studio 2010.

The bug is actually in the Managed Debug Assistant. If you turn it off completely (set HKLM\Software\Microsoft.NETFramework\MDA = "0"), the problem goes away. But of course you lose some debugging capability by doing so.

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