13. Unmanaged Assemblies
Isn't this a misnomer? We have learned all the way through this workshop that managed code is distributed in assemblies, so this means that assemblies must be managed, right? Fusion was created to solve the problem of DLL Hell, but much of the operating system is still unmanaged, so there must be some solution for DLL Hell for unmanaged libraries. This is the reason for unmanaged assemblies. They are not .NET assemblies, but they have similarities: they can be made up of more than one file, they are deployed as a single unit, and they have information about the files and resources in the assemblies. Unmanaged assemblies can be used to allow you to share multiple versions of DLLs between applications and to do this these 'assemblies' reside in a folder that is the native code equivalent of the .NET GAC. Finally, this cache can also contain information about version redirects, so the request for a DLL in one assembly can be directed to a different version of the DLL in another unmanaged assembly. Since this means that more than one version of an assembly can co-exist, they are also known as side-by-side assemblies.
Unmanaged side-by-side assemblies are important in this workshop because version 14 of the managed C++ compiler (supplied with Visual Studio 2005) will create .NET assemblies that use native libraries and by default, the Microsoft unmanaged libraries will be loaded from the side-by-side cache. On this page I will first describe unmanaged side-by-side assemblies, and then I will explain how this affects managed assemblies created by the C++ compiler.
13.1 Manifests and Side-by-Side Assemblies
First we need to go through a bit of terminology. The first term is an
isolated application. This is a process that uses libraries and the
description isolated means that the application is unaffected by the
installation or uninstallation of other assembles. To do this the isolated
application must use shared assemblies and private assemblies. In
most cases an isolated application is just a normal process, the only difference is that it
has a file called an application manifest that contains the dependency
information about the assemblies it uses. A shared side-by-side assembly is
one or more DLLs that can be used by any application on the machine. These are
stored under the
WinSxS folder in separate folders so that multiple
versions of an assembly can co-exist, hence the term side-by-side. An administrator or a
publisher can redirect the request for an assembly from an application to
another version of the same assembly by policy files. A private assembly is a collection of DLLs that
is deployed with an application. Such an assembly resides in the application
folder and so is not available to any other application, and they will not be
replaced when another application installs newer versions of the libraries.
Side-by-side assemblies and private assemblies are described by a manifest. There is one manifest for each assembly and it is made up of XML data that contains information about the DLLs, COM objects and other resources that are used by the assembly. (Note that when you use a manifest to specify COM objects and interfaces it means that you will not need to register the COM server with the system. Indeed, it is recommended that such COM servers do not have self register code.) The manifest for a shared assembly must be in an XML file; the manifest for an application, can be in an XML file or as a bound unmanaged resource. The manifest for a private assembly must be bound as an unmanaged resource to a DLL in the assembly that has the same name as the assembly.
|The documentation says that the manifest for a private assembly can be in an XML manifest file, however, this is not correct, a manifest for a private assembly must be bound as an unmanaged resource.|
Private assemblies must be installed in the application's folder, however, similar to .NET, they can be in a subfolder that has the name of the assembly or the locale of the assembly. A shared assembly can only be installed in the side-by-side cache and it must be installed by Windows Installer. Furthermore, so that there is no possibility of a name collision with another assembly, a shared assembly must have a publickeytoken and it must be signed so that the DLL loader can verify that it has not been altered since installation.
The manifest contains the versioning information of the resources it uses, and this versioning can be redirected using a publisher configuration file or an application configuration file.
When an application loads a library the request goes to the side-by-side
manager. This will always search the side-by-side cache first for the shared assembly.
If an appropriate shared assembly cannot be found, the side-by-side manager
searches for a private assembly in the application's folder. To do this, the
manager must get information about the assembly through the manifest, and
contrary to the documentation, it will only search for manifests bound to
libraries. Specifically, it will search for a DLL with the same name as the
assembly and it will look for a
RT_MANIFEST resource with an ID of
The search mechanism works like this: first the manager searches for a
subfolder with culture name if the assembly DLL does not exist there, the
manager will search in a subfolder of the culture folder which has the name of
the assembly. If there is no locale subfolder, the manager will
search the application's folder for the assembly DLL and if this cannot be found
it will search for a subfolder of the application folder which has the assembly
name. The side-by-side manager will use fallback when it searches for localized assemblies.
That is, the search will first be for a culture folder
en-GB) and if that does not exist, it will search for a language folder (eg
finally it will perform the search for a language neutral assembly. You will
recognise these steps as precisely the way that Fusion looks for assemblies.
Windows describes the concept of application context. This contains
information about the versioning of the resources in an assembly and the context
API allows you to redirect binding. When a process is started with
CreateProcess Windows looks for an application manifest and uses this to
initialize the application context. Any objects used by the application are
mapped to versioned objects as specified by this manifest.
As mentioned before, there are various configuration files used in isolated
applications. The default configuration is held in the application's manifest
(an XML file with the application's full name and the extension
app.exe.manifest) and in the manifest's of the
assemblies the application uses (bound as a resource to a DLL with the assembly
configuration can be overridden by publisher policy files, and by an application
configuration file. The publisher policy file is for a shared assembly and
it redirects versioning of an assembly to another version. The name of a policy
file is similar to the .NET policy files, that is
<minor> refers to the version being redirected, for example
policy.1.0.lib. However, unlike .NET, such versioning redirects can only
be for small versioning changes, you cannot redirect to a version that differs
by the major or minor value, you can only change the build or revision version.
You can override the versioning redirects given by a publisher policy for a specific
application using an application configuration file. This file has the name of
the application with the extension
.config (for example
app.exe.config) and is installed in the application's folder.
13.2 Unmanaged Assembly Versioning
To see how this works, here is an unmanaged DLL:
extern "C" __declspec(dllexport) void MyFunction()
Compile this to use the DLL version of the C runtime library:
When you use a system shared assembly, like the C runtime library, the
unmanaged C++ compiler will automatically create a manifest for the DLL. List
the contents of the folder and here you'll see a file called
<assembly xmlns='urn:schemas-microsoft-com:asm.v1' manifestVersion='1.0'>
<assemblyIdentity type='win32' name='Microsoft.VC80.CRT' version='8.0.50608.0'
processorArchitecture='x86' publicKeyToken='1fc8b3b9a1e18e3b' />
In fact, this is created by the linker which is invoked by the compiler. You can
suppress the manifest generation using the
switch and you can specify the name of the manifest file with the
switch. The linker finds information about system side-by-side assemblies from
information in the static import libraries for the files in the assembly. This
means that a
.lib file is fixed to a specific version of a
DLL, which is a good thing. If you use a side-by-side assembly other than the
system assemblies you will have to write the entries for those assemblies.
You can tell the linker to add an
/manifestdependency linker switch. However, as you'll
find out later, it might be a good idea to eschew the automatic generation of a
manifest file altogether and write your own.
The schema of the manifest file is similar to, but not the same as, the configuration file schema used in .NET. The
difference is the top two elements. The root element is called
and is similar to the .NET
<assemblyBinding> element, however, it has
manifestVersion attribute that is mandatory and must be set to
1.0 (this is the version of the schema). Under that element is another
<dependency> which is a collection of one or more
elements. This is similar to .NET configuration files in that it describes an
<dependentAssembly> must contain a
<assemblyIdentity> element. This element has various attributes, which are
similar to their equivalent in the .NET configuration file schema. The
type attribute is mandatory and must be
A private library's manifest must be bound it to the DLL as an unmanaged resource. Since the
manifest file is created by the linker, it means that you have to embed the
manifest as a post link step. To do this you use the
mt.exe tool to
insert the manifest as an
RT_MANIFEST resource with an ID of
2. Note that it must be a value of
2 (but, note
that an application resource must have an ID of
mt.exe tool is poorly documented, and the MSDN entry does not
even list the command line switches to perform this action (although
will list them). To embed a resource you need to pass the name of
the manifest file to
mt using the
/manifest switch and
/outputresource switch with the name of the PE file
and the resource ID in the form:
<pe file name>;#<resource id>.
This will alter the library, and so you should bear this in mind when we talk
about managed C++ later on this page.
For example, embed the manifest using the following:
Be careful when you type this because there is no colon between
and its parameter, but there has to be a colon between
and its parameter. This looks like this tool has been developed rather sloppily
in a bit by bit fashion by different teams. While I can understand this happens,
I cannot understand how it gets past code review.
Now you can load the library in VS.NET 2005, Use File, Open, File
lib.dll, this will automatically open the file in
resource view. Open the RT_MANIFEST node and double click on the single
item, 2. You will see the contents of the manifest.
There is another way to perform the same thing. Delete the library (
lib.dll) and the manifest file (
del lib.dll.manifest) so
that you have a clean slate. Now
rebuild the library (
cl /LD /MD lib.cpp), which will create the
manifest file. Now create a resource
script that uses the library manifest:
2 RT_MANIFEST lib.dll.manifest
Compile this file to create the resource:
Now link the library again, this time using the resource. The
file should still be in the build folder.
/manifest:no switch tells the linker not to generate
the manifest file. It seems a bit odd to link the file twice but in the
absence of a tool that will generate manifest files this is your only option.
Just to show how messed up the whole process is, you can use the
switch on the linker command line to tell the linker to add the supplied
parameter to the
<dependency> of the manifest file it will create.
This switch, of course, has no effect whatsoever on the output target of the
linker, which is, of course, the reason why you invoked linker. Crazy.
The whole process of manifests and embedding them is topsey-turvey and is a
complete mess. This is yet another example of Microsoft losing the plot. For a
start, MSDN library does not even document the switches to use, you have to use
|Here's a simple user of the library:
#pragma comment(lib, "lib.lib")
extern "C" void MyFunction();
Here, I have used a
pragma to indicate the import library
for the DLL that the process will use, this means that I don't have to call
the linker explicitly. You can compile this code with
(Note this will statically link the C runtime library). Run this process and
confirm that it works. The reason why it works is because the code indicates
that it uses
lib.dll and the DLL loader will load this library from the
application folder. Then the DLL loader will see that the DLL uses
msvcr80.dll. The DLL loader will try to load this DLL, from the side-by-side cache and use the assembly
manifest information to pick up the right version. Let's prove that this is the
Rename the library so that we can use it later (
rename lib.dll lib.withmanifest).
Now compile the
library without an embedded manifest:
Now run the application again.
This indicates that the DLL loader has not found the DLL and so has given up.
Now, you know that you have installed Visual C++ correctly on your machine, so
msvcr80.dll must be on your machine. This seems to indicate
that you application has broken Windows.
Even though the documentation says that the DLL loader will look for a manifest file if it cannot find an embedded resource, this example has shown that it does not do this. You must embed the manifest in the DLL.
I think it is appalling that a simple code like this will not work straight from
the compiler. At least the compiler/linker could warn you. For example, it would
be helpful if the linker issued a warning if there isn't an |
Now return to the situation of the library with the embedded manifest and to do this rename the
library without a manifest (
rename lib.dll lib.withoutmanifest) and
rename the library you made before (
rename lib.withmanifest lib.dll). Now compile the
application to use the DLL version of the CRT library:
You will find that this time the linker will create a manifest file for the
application with the same contents as the file generated for the library. Run
the application. You will find that it will work. The reason is that the DLL
loader will use the application manifest file to get information about the
version of the CRT assembly, it does not require that you
embed this manifest as an unmanaged resource, as is the case with DLLs. If you do
want to embed the manifest file you can use the same procedures that I specified
above, but the resource ID should be
Finally, go back to the library without a manifest (
rename lib.withoutmanifest lib.dll). So now you will have a
process and a library that both use the DLL version of the CRT, but neither of these
will have an embedded manifest. However, there is an application manifest file. Run
the application. This time you will find that the application will run, which
means that when
the DLL loader loads the library it uses information provided by the
application's manifest file.
13.3 The Side-By-Side Cache
So where is the side-by-side library located? Move to the windows folder (
%windir%) and list the contents. Here you'll find a folder called
WinSxS. This is the side-by-side cache. List the contents of this folder.
You'll find that there are lots of folders with long names and just three that
names. These latter three are:
Policies. The contents of these folders are altered by Windows
Installer, but you are able to have a poke around.
just a temporary folder used by the installer.
Manifests is far
more interesting. Move to this folder and list its contents. This contains
binary files (called verification catalogues) which have the extension
.cat and text files with the
.manifest. Searching through these you'll find a file
that seems to correspond to the CRT library, so print it to the console:
The contents of this file is similar to the manifest file you created for the library and the application. However, this is a manifest file for a shared assembly. Here is an edited version:
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<assemblyIdentity type="win32" name="Microsoft.VC80.CRT" version="8.0.50727.42"
This manifest file is not valid, I have edited it to make it more readable.
The important point is that there is no
<dependency> element, instead
<assemblyIdentity> element is directly under the root. This
is used to give the identity of the assembly. Note that it gives the version of
the assembly to be
8.0.50727.42. The assembly also has a
which is used to prevent name collisions. The
<file> elements are
the DLLs that make up the assembly, so if unmanaged code requests any of these
three DLLs the DLL loader will use this assembly manifest. The data I have
deleted is information about code signing, which is performed so that a manifest can be validated
before it is used. The assembly and manifest are signed, by hashing the files and then creating a
digital signature by encrypting the hash with the private key from the
publisher's certificate. The public key from the certificate is also used to create
publickeytoken in the assembly name. The operating system will have access to the public key in the certificate
and the digital signature in the security catalog (the
.cat file) so it can decrypt the digital signature and generate the hash over the file.
The operating system can then compare the two hashes, to validate the assembly and the manifest file.
Notice that the version of the CRT assembly is
however, the manifest for the library and process we built specified that they
8.0.50608.0. There is a version mismatch!
I think this is pretty shoddy. I have Visual Studio 2005 that has just been
released. The machine has never had a beta of Visual Studio 2005 on it, so there
is no possibility of an earlier build of the CRT version |
So why did the application work?
Move up a directory and move to the
Policies folder (
..\Policies). Here you will find a folder which has the name and
publickeytoken for the C runtime library, but not the complete version:
Move to this folder and list the contents. Within here is a binary
verification catalog (
8.0.50727.42.cat) and a policy file (
with the version of the assembly that is installed on the machine. Type this file
to the console:
<!-- Copyright ┬® 1981-2001 Microsoft Corporation -->
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<assemblyIdentity type="win32-policy" name="policy.8.0.Microsoft.VC80.CRT"
<assemblyIdentity type="win32" name="Microsoft.VC80.CRT"
<bindingRedirect oldVersion="8.0.41204.256-8.0.50608.0" newVersion="8.0.50727.42"/>
This gives complete information identifying the assembly installed on this
8.0.50727.42) and then, confusingly, it identifies
'dependent assemblies', with rebinding information to direct requests for all
versions of the CRT library between
8.0.50608.0 so that the DLL loader uses version
instead. This file is for a specific version of an assembly which says that any
requests for any other version should be directed to this version. This is
subtly different (and in my opinion, confusingly so) to rebinding in .NET where
the configuration file gives information for any version of an assembly
and indicates that requests for one version should be redirected to another
This is why the application worked. The application requests the CRT library,
the manifest indicates that it wants version
8.0.50608.0 and the
policy file redirects this request to
Note that version |
13.4 Side-by-Side Execution and Managed C++ Assemblies
OK, now create a managed assembly; this code is trivial, and contrived:
public ref class LibraryCode
This uses the new C++ syntax, where the
^ symbol indicates a
handle to an object on the managed heap. Compiling this code is simple:
Now list the contents of the build folder where you'll see that a manifest
was created for the assembly. This is exactly the same as the manifest created
for the unmanaged library assembly. You can get a list of the DLLs that this
library depends upon using the new
/dependents switch on
dumpbin (they are not dependents, because our library depends on them).
I get the following:
This indicates that the unmanaged and managed versions of the C runtime
library are used. Visual Studio 2005 contains a managed version of various C++
standard library classes and C runtime functions in
dumpbin /imports on the
lib assembly shows that the library
uses managed methods in
msvcm80.dll to initialize the library. The library also uses
msvcr80.dll which is the unmanaged C runtime library.
shows that our library calls functions to initialize and clean up the library
and to handle exceptions. Such initialization code is required to 'prepare' the
library to use the C runtime library, and to provide code to handle the
initialization of global, unmanaged C++ objects. This initialization code has been added to the assembly even though
we do not have code that will throw an unmanaged C++ exception, nor uses
any unmanaged code. We will return to this issue later, but for the time being,
it serves our purpose.
Create a process that calls this library:
using namespace System;
LibraryCode^ lib = gcnew LibraryCode;
Again, notice the new syntax. The object is accessed using a handle, and it
is created using the
gcnew operator. Compile this code with:
Finally, run the process and confirm that it works. You know the reason why.
The process you created also uses the managed and unmanaged C runtime library
and so the linker has created an application manifest,
The operating system uses this file to identify the side-by-side assembly that
contains the two CRT DLLs that the process uses. In addition, the values in this
file are used when the operating system loads the library assembly,
View this assembly with
ILDASM and briefly scroll through all of
the types that have been declared, then close it down before it gets too frightening
(did you know that that much initialization had to be provided so that you can
use the CRT and the C++ standard library?).
The compiler has added many types to give you support for the CRT and in this
example you do
not use them! Delete the manifest file and run the application again, you'll see the
error dialog that was shown earlier.
Now remove the dependence of the process assembly on the CRT. To do this is
to compile with the
This indicates that the compiler should create verifiable code, and hence it
will not use the CRT. The compiler has another new switch
which will create an assembly that only contains IL (native code using IJW is
not supported) but it can use platform invoke. A pure assembly can use the C
runtime library function as long as it is a function exported from the managed
version of the CRT DLL, so such an assembly will have a dependence upon
msvcm80.dll. We want no dependence on any form of the CRT, managed or
Open the library with
ILDASM and confirm
that the assembly now only contains the entry point,
ILDASM and list the contents of the build folder. You will find
that a manifest has not been created for the application because the process
assembly you just built uses no unmanaged file.
Run the application. You will find that a
is thrown. To work out why, run
fuslogvw. The default binding
errors give little information, so take a look at the list of native image
binding errors. Here, there appears to be some promising entries. The entry for
the attempt to load the
indicates that Fusion attempted to load the native image for the assembly and
could not find it.
This must be a bug. The reason for this error is that the system cannot find the
CRT libraries that are required by the library. It has nothing to do with the
native image of |
Now add the manifest of the library to the library:
Start the application, this time the application will run correctly. The
FileNotFoundException was thrown before because the DLL loader could not find
the CRT libraries, embedding the manifest gave the loader the information that it
needed to be able to find the CRT assembly.
Of course, the library does not use the CRT, nor any unmanaged types, so there is no reason for there to be support for the CRT. Compile this as verifiable code:
Now when you run the application no assembly manifest, nor application manifest is needed because neither the application, nor the library uses side-by-side assemblies.
As a final point, you should remember that if you want to create a strong
named library that uses an unmanaged assembly then you should take steps to make
sure that the library will not fail strong name validation. If you decide to use
mt.exe then you should delay sign the managed library when you
build it, then apply
mt to insert the manifest resource, and
finally re-sign the assembly. It actually makes more sense to chose the second
option identified earlier, and create the manifest file yourself and add it to
the assembly through a resource script. This way you would not have the extra
step of re-signing the assembly.
|I hope that you enjoy this tutorial and value the knowledge that you will gain from it. I am always pleased to hear from people who use this tutorial (contact me). If you find this tutorial useful then please also email your comments to email@example.com.|
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