Why Does My Project Throw a 'Could Not Load File or Assembly...Incorrect Format' Error?

Troubleshooting "Could not load file or assembly... incorrect format" Errors in .NET Projects

This article addresses the common .NET error "Could not load file or assembly... An attempt was made to load a program with an incorrect format." This often occurs when a project (e.g., ProjectB) depends on another (ProjectA), and the projects have mismatched architectures.

The Root Cause: Architectural Mismatch (32-bit vs. 64-bit)

The error typically stems from an incompatibility between the target platforms of dependent projects. For example, ProjectB might be configured for 64-bit execution, while ProjectA targets 32-bit. This architecture mismatch prevents the assembly from loading correctly.

Solution: Architectural Consistency

The solution is to ensure both projects use the same architecture. In Visual Studio:

1. Access Project Properties: Open the properties for both ProjectA and ProjectB.
2. Target Platform: Under the "Build" tab, locate the "Target platform" setting.
3. Unify Architectures: Set both projects to either "Any CPU" or the same specific platform (x86 or x64). Consistency is key.

Additional Steps for Resolution:

1. Rebuild and Restart: After changing the target platform, restart Visual Studio and rebuild the solution. This clears any cached build artifacts.
2. IIS Configuration (Web Apps): For web applications, if the main application is 64-bit and the dependent DLL is 32-bit, you'll need to enable 32-bit applications in IIS. In IIS Manager, navigate to the application pool's advanced settings and set "Enable 32-bit Applications" to "True."
3. Verify References: In ProjectB's References, check the settings for ProjectA. Ensure "Copy Local" is set to "True" to copy ProjectA.dll to the output directory.

By following these steps, you should resolve the "Could not load file or assembly" error and successfully link ProjectA and ProjectB.

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