Why Does My C# Unit Test Fail with an 'Assembly Manifest Mismatch' Error?

Decoding the C# "Assembly Manifest Mismatch" Error in Unit Tests

Encountering the error "The located assembly's manifest definition does not match the assembly reference" while testing your C# Windows Forms application? This guide unravels the mystery behind this common .NET assembly loading issue.

The core problem lies within the .NET Assembly Loader, responsible for locating and loading external assemblies. This loader meticulously checks the assembly's version, culture, and public key token against the information in your project's references. A mismatch, as indicated by the error, leads to the incorrect assembly being loaded.

The error message often highlights the conflict: for instance, a reference to version 1.2.0.203 of a "Utility" assembly, but the loader finds version 1.2.0.200.

The solution involves ensuring the correct version (1.2.0.203 in this example) is accessible to your application. There are two primary approaches:

1. Global Assembly Cache (GAC): Install the correct assembly into the GAC using the gacutil command:

   gacutil /i "path/to/my.dll"

2. Application's Path: Alternatively, place the correct my.dll file directly in your application's execution path.

Troubleshooting with AssemblySniffer:

If you're unsure of the assembly's location, tools like AssemblySniffer can help. This utility searches your file system for assemblies matching specific criteria (version, public key token, etc.), pinpointing the conflicting version.

By resolving the assembly version discrepancy, the .NET Assembly Loader will correctly load the intended assembly, resolving the "Assembly Manifest Mismatch" error and allowing your unit tests to execute without issue.

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