How do I avoid the loader lock?

Posted by Mark0978 on Stack Overflow See other posts from Stack Overflow or by Mark0978
Published on 2010-03-19T21:10:22Z Indexed on 2010/03/19 21:21 UTC
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managed-code

We have a managed app, that uses an assembly. That assembly uses some unmanaged C++ code.

The Managed C++ code is in a dll, that depends on several other dlls. All of those Dlls are loaded by this code. (We load all the dll's that ImageCore.dll depends on first, so we can tell which ones are missing, otherwise it would just show up as ImageCore.dll failed to load, and the log file would give no clues as to why).

class Interop
{
    private const int DONT_RESOLVE_DLL_REFERENCES = 1;
    private static log4net.ILog log = log4net.LogManager.GetLogger("Imagecore.NET");

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr LoadLibraryEx(string fileName, IntPtr dummy, int flags);
    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr FreeLibrary(IntPtr hModule);

    static private String[] libs = { "log4cplus.dll", "yaz.dll", "zlib1.dll", "libxml2.dll" };

    public static void PreloadAssemblies()
    {
        for (int i=0; i < libs.Length; ++i) {
            String libname = libs[i];

            IntPtr hModule = LoadLibraryEx(libname, IntPtr.Zero, DONT_RESOLVE_DLL_REFERENCES);
            if(hModule == IntPtr.Zero) {
                log.Error("Unable to pre-load '" + libname + "'");
                throw new DllNotFoundException("Unable to pre-load '" + libname + "'");
            } else {
                FreeLibrary(hModule);
            }
        }

        IntPtr h = LoadLibraryEx("ImageCore.dll", IntPtr.Zero, 0);
        if (h == IntPtr.Zero) {
            throw new DllNotFoundException("Unable to pre-load ImageCore.dll");
        }
    }
}

And this code is called by

public class ImageDoc : IDisposable {
    static ImageDoc()
    {
        ImageHawk.ImageCore.Utility.Interop.PreloadAssemblies();
    }
    ...
}

Which is static constructor.

As near as I can understand it, as soon as we attempt to use an ImageDoc object, the dll that contains that assembly is loaded and as part of that load, the static constructor is called which in turn causes several other DLLs to be loaded as well. What I'm trying to figure out, is how do we defer loading of those DLLs so that we don't run smack dab into this loader lock that is being kicked out because of the static constructor.

I've pieced this much together by looking at:

But I just can't seem to find a way to get these external DLLs to load without it happening at the point the class is loading. I think I need to get these LoadLibrary calls out of the static constructor, but don't know how to get them called before they are needed (except for how it is done here). I would prefer to not have to put this kind of knowledge of the dlls into every app that uses this assembly. (And I'm not sure that would even fix the problem....

The strange thing is that the exception only appears to be happening while running within the debugger, not while running outside the debugger.

Developer IT