Native Assembly Binding fails for ASP.NET solution

NOTE: This main purpose of the question to UNDERSTAND/EXPLAIN the assembly binding behavior of the CLR. The solution should be evident once the reason is pinned down. Please, know that I understand my setup is not optimal (nativedll not signed and versioned etc) but again I just want to investigate the binding behavior of the CLR.

I am attempting to use a native (non-COM, C++) dll into my asp.net solution. I am going to refrain from naming the dll because I think it diverts the topic in the wrong direction.

The native dll is from a blog site and does not have much information with it. I do not have any information about the native dll like architecture it was compiled on, what culture and version it is etc. Using the provided wrapper, I can use the dll in a console application properly. It works, even though we have to use partial assembly name for binding. The wrapper for the native assembly has DLL imports like [DllImport("nativedll.dll")] public static extern void someMethod([Out] BE_VERSION pbeVersion);

I have read through loads and loads of information about CLR assembly binding including:

How the Runtime Locates Assemblies
Best Practices for Assembly Loading

Per my understanding, we should be able to get the assembly loaded, but no. I get assembly binding errors.

SETUP

My helper library (compiled as MSIL (Any CPU)) that call the native dll is strong named and is in the GAC.. I am running my asp.net app on a Win 7, x64 bit machine on IIS, not the visual studio development server. The nativedll is NOT signed and NOT GACed.

THE ISSUE
The summary is, when I place the native dll in the windows directory, (c:\Windows) the solution works fine. In any other case, I get assembly binding errors.

QUESTIONS
1. I would like to know what why is the assembly getting bound when dropped in the windows directory? 2. What does it mean by error: ERR: Error extracting manifest import from file (hr = 0x80131018)? (Look at Scenario 1, Scenario 6)
3. Why does the call to Assembly.Load*() methods fail? (Scenario 3, 4, 5)

RESULTS OF MY TESTING

Scenario 1:

**Setup**
Calling Assembly: GACed.
Native Assembly: Included in project; Build Action: None; Copy To Output Directory: Copy Always.

**Description:** 
Logged into the site, home page open. Did not navigate to the page with native call.

**Result:** 
    Binding Error
    *** Assembly Binder Log Entry  (12/13/2012 @ 11:28:23 AM) ***

        The operation failed.
        Bind result: hr = 0x80131018. No description available.

        Assembly manager loaded from:  C:\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll
        Running under executable  C:\Windows\SysWOW64\inetsrv\w3wp.exe
        --- A detailed error log follows. 

        === Pre-bind state information ===
        LOG: User = NT AUTHORITY\SYSTEM
        LOG: DisplayName = nativedll
         (Partial)
        WRN: Partial binding information was supplied for an assembly:
        WRN: Assembly Name: nativedll | Domain ID: 2
        WRN: A partial bind occurs when only part of the assembly display name is provided.
        WRN: This might result in the binder loading an incorrect assembly.
        WRN: It is recommended to provide a fully specified textual identity for the assembly,
        WRN: that consists of the simple name, version, culture, and public key token.
        WRN: See whitepaper http://go.microsoft.com/fwlink/?LinkId=109270 for more information and common solutions to this issue.
        LOG: Appbase = file:///C:/WebUI/
        LOG: Initial PrivatePath = C:\WebUI\bin
        LOG: Dynamic Base = C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\webui\69d9ded4
        LOG: Cache Base = C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\webui\69d9ded4
        LOG: AppName = ca4be085
        Calling assembly : (Unknown).
        ===
        LOG: This bind starts in default load context.
        LOG: Using application configuration file: C:\WebUI\web.config
        LOG: Using host configuration file: C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet.config
        LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework\v4.0.30319\config\machine.config.
        LOG: Policy not being applied to reference at this time (private, custom, partial, or location-based assembly bind).
        LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/webui/69d9ded4/ca4be085/nativedll.DLL.
        LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/webui/69d9ded4/ca4be085/nativedll/nativedll.DLL.
        LOG: Attempting download of new URL file:///C:/WebUI/bin/nativedll.DLL.
        LOG: Assembly download was successful. Attempting setup of file: C:\WebUI\bin\nativedll.dll
        LOG: Entering download cache setup phase.
        ERR: Error extracting manifest import from file (hr = 0x80131018).
        ERR: Setup failed with hr = 0x80131018.
        ERR: Failed to complete setup of assembly (hr = 0x80131018). Probing terminated.    

---

Scenario 2:

**Setup**
Scenario 1 + Just copy the nativedll to C:\Windows folder. No reset of anytype. 

**Description**
Navigate to the page calling native.

Result: 
Success. Native Functionality is invoked. No binding failure.

---

Scenario 3:

**Setup**
Calling Assembly: GACed.
Native Assembly: Build Action: None; Copy To Output Directory: Copy Always.

Description: 
    Breakpoint at the point of native function call, use immediate window to execute the following code:

        var pat = pat = Path.Combine(AppDomain.CurrentDomain.RelativeSearchPath, "nativedll.dll");
        var a = Assembly.LoadFrom(pat);

    **Result**
    Exception raised:

    System.BadImageFormatException was unhandled
      HResult=-2146234344
      Message=Could not load file or assembly 'file:///C:\WebUI\bin\nativedll.dll' or one of its dependencies. The module was expected to contain an assembly manifest.
      Source=mscorlib
      FileName=file:///C:\WebUI\bin\nativedll.dll
      FusionLog=Assembly manager loaded from:  C:\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll
        Running under executable  C:\Windows\SysWOW64\inetsrv\w3wp.exe
        --- A detailed error log follows. 

        === Pre-bind state information ===
        LOG: User = NT AUTHORITY\SYSTEM
        LOG: Where-ref bind. Location = C:\WebUI\bin\nativedll.dll
        LOG: Appbase = file:///C:/WebUI/
        LOG: Initial PrivatePath = C:\WebUI\bin
        Calling assembly : (Unknown).
        ===
        LOG: This bind starts in LoadFrom load context.
        WRN: Native image will not be probed in LoadFrom context. Native image will only be probed in default load context, like with Assembly.Load().
        LOG: Using application configuration file: C:\WebUI\web.config
        LOG: Using host configuration file: C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet.config
        LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework\v4.0.30319\config\machine.config.
        LOG: Attempting download of new URL file:///C:/WebUI/bin/nativedll.dll.
        ERR: Failed to complete setup of assembly (hr = 0x80131018). Probing terminated.

          StackTrace:
               at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
               at System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
               at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
               at System.Reflection.RuntimeAssembly.InternalLoadFrom(String assemblyFile, Evidence securityEvidence, Byte[] hashValue, AssemblyHashAlgorithm hashAlgorithm, Boolean forIntrospection, Boolean suppressSecurityChecks, StackCrawlMark& stackMark)
               at System.Reflection.Assembly.LoadFrom(String assemblyFile)
          InnerException: 

---

Scenario 4:

**Setup**
    Calling Assembly: GACed.
    Native Assembly: Build Action: None; Copy To Output Directory: Copy Always.

**Description** 
    Breakpoint at the point of native function call, use immediate window to execute the following code:

    var pat = pat = Path.Combine(AppDomain.CurrentDomain.RelativeSearchPath, "nativedll.dll");
    var a = Assembly.Load(pat);

**Result**
    Exception raised:

        System.IO.FileLoadException was unhandled
          HResult=-2146234297
          Message=Could not load file or assembly 'C:\\WebUI\\bin\\nativedll.dll' or one of its dependencies. The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)
          Source=mscorlib
          FileName=C:\\WebUI\\bin\\nativedll.dll
          FusionLog=""
          StackTrace:
               at System.Reflection.AssemblyName.nInit(RuntimeAssembly& assembly, Boolean forIntrospection, Boolean raiseResolveEvent)
               at System.Reflection.RuntimeAssembly.CreateAssemblyName(String assemblyString, Boolean forIntrospection, RuntimeAssembly& assemblyFromResolveEvent)
               at System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean forIntrospection)
               at System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection)
               at System.Reflection.Assembly.Load(String assemblyString)
          InnerException: 

---

Scenario 5:

**Setup**
Calling Assembly: GACed.
Native Assembly: Build Action: None; Copy To Output Directory: Copy Always.

**Description: **
    Breakpoint at the point of native function call, use immediate window to execute the following code:

    var pat = pat = Path.Combine(AppDomain.CurrentDomain.RelativeSearchPath, "nativedll.dll");
    var a = Assembly.LoadFile(pat);

    **Result**
    Exception raised:

        System.BadImageFormatException was unhandled
          HResult=-2146234344
          Message=The module was expected to contain an assembly manifest. (Exception from HRESULT: 0x80131018)
          Source=mscorlib
          StackTrace:
               at System.Reflection.RuntimeAssembly.nLoadFile(String path, Evidence evidence)
               at System.Reflection.Assembly.LoadFile(String path)
          InnerException: 

---

Scenario 6:

**Setup**
Calling Assembly: GACed.
Native Assembly: Excluded from project. Manually copied into the bin folder prior to execution.

**Description: **
    Logged into the site, home page open. Did not navigate to the page with native call.

    **Result**
    Binding Error: (Output: Assembly Binding Log Viewer)

        Assembly Binder Log Entry  (12/13/2012 @ 2:19:13 PM)

        The operation failed.
        Bind result: hr = 0x80131018. No description available.

        Assembly manager loaded from:  C:\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll
        Running under executable  C:\Windows\SysWOW64\inetsrv\w3wp.exe
        --- A detailed error log follows. 

        === Pre-bind state information ===
        LOG: User = NT AUTHORITY\SYSTEM
        LOG: DisplayName = nativedll
         (Partial)
        WRN: Partial binding information was supplied for an assembly:
        WRN: Assembly Name: nativedll | Domain ID: 2
        WRN: A partial bind occurs when only part of the assembly display name is provided.
        WRN: This might result in the binder loading an incorrect assembly.
        WRN: It is recommended to provide a fully specified textual identity for the assembly,
        WRN: that consists of the simple name, version, culture, and public key token.
        WRN: See whitepaper http://go.microsoft.com/fwlink/?LinkId=109270 for more information and common solutions to this issue.
        LOG: Appbase = file:///C:/Web/WebUI/
        LOG: Initial PrivatePath = C:\WebUI\bin
        LOG: Dynamic Base = C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\webui\69d9ded4
        LOG: Cache Base = C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\webui\69d9ded4
        LOG: AppName = ca4be085
        Calling assembly : (Unknown).
        ===
        LOG: This bind starts in default load context.
        LOG: Using application configuration file: C:\WebUI\web.config
        LOG: Using host configuration file: C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet.config
        LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework\v4.0.30319\config\machine.config.
        LOG: Policy not being applied to reference at this time (private, custom, partial, or location-based assembly bind).
        LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/webui/69d9ded4/ca4be085/nativedll.DLL.
        LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/webui/69d9ded4/ca4be085/nativedll/nativedll.DLL.
        LOG: Attempting download of new URL file:///C:/WebUI/bin/nativedll.DLL.
        LOG: Assembly download was successful. Attempting setup of file: C:\\WebUI\bin\nativedll.dll
        LOG: Entering download cache setup phase.
        ERR: Error extracting manifest import from file (hr = 0x80131018).
        ERR: Setup failed with hr = 0x80131018.
        ERR: Failed to complete setup of assembly (hr = 0x80131018). Probing terminated.

---

Scenario 7:

**Setup**
Scenario 6 + Just copy the nativedll to C:\Windows folder. No reset of anytype. 

**Description**
Navigate to the page calling native.

Result: 
Success. Native Functionality is invoked. No binding failure.

As far as I can work out and with a fair amount of supposition...

The .NET assemblies in the /bin folder are automatically loaded by ASP.NET when the website starts, but any native win32 DLLs aren't loaded.

The DLLImport is failing because the DllImport is done relative to the w3wp.exe process NOT relative to the website project. An AppPool can be shared between many websites so your website's /bin folder can't be the CurrentDirectory for the w3wp.exe process even supposing there aren't security issues with doing that.

So to find the DLL, IIS is looking first in the folder where 'w3wp.exe' is located, and then checking the Windows System folders.

In theory, you should be able to use LoadLibrary() and GetProcAddress() to load the DLLs from a specific folder.

Incidentally, If you're using DllImport() in a class defined within the website project you may need to change the default compilation mode for ASP.NET to 'safe'. To allow unsafe code to be used you can update your web.config file:

<system.codedom>
    <compilers>
       <compiler ... compilerOptions="/unsafe" >
    </compilers>
</system.codedom>