How Do I Fix the 'Type is Defined in an Assembly That is Not Referenced' Error in My App_Code Directory?

Resolving the Unreferenced Assembly Error in App_Code Directory

The dreaded "type is defined in an assembly that is not referenced" error can be perplexing, especially when using the App_Code directory. While there are numerous existing questions on Stack Overflow, this unique scenario often calls for a tailored approach.

Analyzing the Error Message

The error message typically indicates the missing assembly and its details. In this case, the problematic assembly is 'Project.Rights', highlighting a discrepancy between the assembly referenced and the one required by the code.

Troubleshooting Steps

Despite following established suggestions, the issue persists. Here are additional troubleshooting measures:

• Verify File Location: Ensure that the '.cs' file containing the 'Project.Rights.OperationsProvider' type is correctly deployed to the App_Code directory.
• Check Build Action: Confirm that the build action for '.cs' files is set to "Compile."
• Inspect Property/Parameter Types: Examine the code for potential constructors or method parameters that utilize types from an unreferenced assembly.
• Exclude Erroneous Assemblies: As mentioned, delete any instances of 'Project.Rights.dll' from system folders.

Root Cause: Missing Type References

The error message suggests that a referenced type is missing. This could occur when the type used is part of a method parameter or property that requires a type from a separate assembly. For example, if 'MyObjectType' has an overload that accepts a 'TypeFromOtherAssembly' parameter, and the latter assembly is not referenced, the error will be triggered.

Resolution

Therefore, thoroughly inspect your code for any types used in properties or parameter lists that may be missing references. Add the appropriate references to successfully resolve the issue and avoid this confusing error in the future.

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