.Net C++ Com

Introduction

When working on a .NET project that references a COM DLL, it’s crucial to ensure that your project maintains the correct version of the interop DLL. This is essential not only for tracking compatibility but also for ensuring that your application functions seamlessly across updates and patches. In this blog post, we will explore how to properly manage versions of your COM DLL references in a .NET environment and address common pitfalls.

The Problem at Hand

The issue arises when you use Visual Studio to add a COM reference. By default, the version assigned may not align with the version of the original COM DLL. This discrepancy can lead to difficulties, especially when patches are involved. You may find yourself asking:

  • How can I ensure the interop DLL created has the correct version?
  • Is there a way to incorporate a build number alongside major and minor versions?

Let’s dive into the solution!

Understanding COM Reference Versions

Before we tackle the solution, it’s important to understand where the versioning comes from:

  1. Type Libraries (TypeLibs): In COM, the versioning is primarily associated with the Type Library rather than the DLL itself. The GUID refers to the TypeLib, and the version numbers correspond to the VersionMajor and VersionMinor attributes of that TypeLib.

  2. Registry Storage: The version numbers are stored in the Windows registry under:

    HKEY_CLASSES_ROOT\Typelib\{typelib uuid}\Major.Minor

    Here, the major version must be something, while the minor version can be set to zero, meaning it will import the latest matching version.

Steps to Manage Your COM DLL Versioning

1. Reference the Type Library Instead of the DLL

Instead of directly referencing the COM DLL, reference the Type Library. This way, you inherit the versioning associated with it. Ensure that when using TlbImp, you include the /asmversion flag to specify the desired version.

2. Using Visual Studio to Set Version

To manually set the version while using Visual Studio, editing the .vcproj file can help. Here’s an example to guide you:

<ItemGroup>
    <COMReference Include="MYDLLLib">
        <Guid>{459F8813-D74D-DEAD-BEEF-00CAFEBABEA5}</Guid>
        <VersionMajor>1</VersionMajor> 
        <VersionMinor>0</VersionMinor> 
        <Lcid>0</Lcid>
        <WrapperTool>tlbimp</WrapperTool>
        <Isolated>False</Isolated>
    </COMReference>
</ItemGroup>

3. Verify Registry Settings

After making changes, verify that the new version numbers appear correctly in the registry. This confirmation step will provide peace of mind that your application will continue to function correctly across updates.

4. Limitations of Versioning Support

Unfortunately, the version system supports only major and minor version numbers and does not accommodate a build version. This is determined by the IDL file associated with your TypeLib. If you need more granular version information, consider following a naming convention in your project management system.

Conclusion

Ensuring that your interop DLL from a referenced COM DLL in a .NET project maintains the correct version can be complex, but by following the steps outlined above, you can achieve a more manageable approach. Remember, the key is focusing on the TypeLib version rather than the DLL version directly and checking registry settings after modifications.

By using these methods, you’ll not only avoid the frustration of mismatching versions but also streamline your updates and maintain the integrity of your application through patches.

If you have further questions or insights regarding COM DLL version management in .NET projects, feel free to share your experiences!