System.Runtime.InteropServices.COMException – Class not registered

The System.Runtime.InteropServices.COMException with the message “Class not registered” occurs in C# when you attempt to instantiate or use a COM (Component Object Model) object, but the required COM class is not registered on the system. This typically happens due to missing or incorrect registration of the COM component.

Common Causes and Solutions

1. COM Component Not Registered:
   If the COM component is not registered on the system, this exception will occur.

   var comObject = new MyComClass(); // COMException – Class not registered

Fix: Register the COM component using regsvr32 or equivalent.

   regsvr32 MyComLibrary.dll

2. Incorrect Bitness (32-bit vs 64-bit):
   If your application is running as a 64-bit process but the COM component is 32-bit (or vice versa), this exception can occur. Fix: Ensure the application and COM component match in terms of 32-bit or 64-bit.

   // Set the platform target in your project to match the COM component (e.g., x86 or x64)
   <PlatformTarget>x86</PlatformTarget>

3. Missing Dependencies:
   If the COM component depends on other DLLs or components that are missing, this exception can occur. Fix: Ensure all dependencies of the COM component are available and registered.

   regsvr32 DependencyLibrary.dll

4. Incorrect ProgID or CLSID:
   If the ProgID (Programmatic Identifier) or CLSID (Class Identifier) used to create the COM object is incorrect, this exception can occur.

   var comObject = Type.GetTypeFromProgID("WrongProgID"); // COMException – Class not registered

Fix: Verify the ProgID or CLSID and ensure it is correct.

   var comObject = Type.GetTypeFromProgID("CorrectProgID"); // Correct ProgID

5. User Registration vs System Registration:
   If the COM component is registered for a specific user but not system-wide, this exception can occur. Fix: Register the COM component system-wide or ensure the application runs under the correct user context.

   regsvr32 /i MyComLibrary.dll

6. Missing Type Library:
   If the type library for the COM component is missing or not registered, this exception can occur. Fix: Register the type library using regtlibv12 or equivalent.

   regtlibv12 MyComLibrary.tlb

7. Registry Corruption:
   If the registry entries for the COM component are corrupted or missing, this exception can occur. Fix: Re-register the COM component or repair the registry entries.

   regsvr32 /u MyComLibrary.dll
   regsvr32 MyComLibrary.dll

8. Using CoCreateInstance Directly:
   If you use CoCreateInstance directly and the class is not registered, this exception can occur. Fix: Ensure the class is registered or use Type.GetTypeFromProgID and Activator.CreateInstance.

   var comType = Type.GetTypeFromProgID("MyComClass");
   var comObject = Activator.CreateInstance(comType); // Works if the class is registered