Why Does My .NET App Show 'The Type is Defined in an Assembly that is Not Referenced' When Using App_Code?

Resolving "The Type is Defined in an Assembly that is Not Referenced" with App_Code Directory

In .NET development, encountering the error message "The type 'TypeName' is defined in an assembly that is not referenced" can be perplexing, especially when using the App_Code directory. Despite following recommended solutions, the error may persist.

The error message signifies a missing reference to an assembly. Consider the following code snippet:

MyObjectType a = new MyObjectType("parameter");

While the "MyObjectType" type may have been referenced correctly, another type involved in the constructor call, such as "TypeFromOtherAssembly," could be missing. This can lead to the "not referenced" error.

To resolve this issue:

1. Examine Constructor Parameters and Properties: Check if any overloaded constructors or method parameters require specific types that are not referenced yet.
2. Ensure Assembly Deployment: Verify that the .cs file containing the missing type is deployed into the App_Code directory and has its build action set to "Compile."
3. Consider Transparent Assembly: In some cases, using transparent assemblies can solve the reference issue. Transparent assemblies are different than strong-named assemblies and don't require explicit referencing in some scenarios.

By following these steps, you should be able to identify the missing reference and add it accordingly, resolving the error and allowing your code to execute smoothly.

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