Why Does My App Show 'The type {Type Name} is defined in an assembly that is not referenced' When Using App_Code, Despite Apparent Referencing?

Identifying the Cause Behind "Assembly Not Referenced" Error

The infamous "The type {Type Name} is defined in an assembly that is not referenced" error has perplexed many developers, particularly those utilizing App_Code directories. While solutions for similar errors abound, the following steps aim to uncover the root cause specifically within this context.

The Missing Reference Conundrum

Despite attempts to remove lingering remnants of previous referenced assemblies (Project.Rights.dll) and ensuring "Compile" build actions, the application stubbornly refuses to recognize the type in question (Project.Rights.OperationsProvider) within the App_Code directory. The error message itself provides a valuable clue: an assembly is missing.

Beyond the Obvious: Examining Nested Dependencies

Inspecting the error message closely reveals a potential oversight. While the primary type (MyObjectType) may have been referenced properly, this only accounts for direct dependencies. The constructor overloading scenario exposes a crucial blindspot: missing references to types used within method parameters or properties.

Tracing the Breadcrumbs

To resolve the issue, developers must meticulously scrutinize the type's constructor overloads. One may find that an overload exists which requires a type not currently referenced. By pinpointing this missing dependency, the solution becomes clear: the missing assembly must be referenced to resolve the error and allow the application to locate the desired type within the App_Code directory.

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