Why Does My App_Code Assembly Reference Error Point to the Wrong Assembly?

Tracking Down the Missing Assembly Reference in App_Code

The enigmatic error message "CS0012: The type 'Project.Rights.OperationsProvider' is defined in an assembly that is not referenced" plagues developers using the App_Code directory. Despite attempts to resolve the issue using conventional solutions, the cause remains elusive for some.

Upon closer inspection, it becomes evident that the application is not searching for the missing type in the App_Code directory, despite deploying the relevant .cs file to that location. To understand why, let's delve into the error's underlying cause.

Understanding the Error

When encountering this error, it's crucial to recognize that it doesn't always indicate a missing reference to the stated assembly (in this case, 'Project.Rights'). Instead, the problem lies in a missing reference to a related type.

To illustrate, consider the following code:

MyObjectType a = new MyObjectType("parameter");

While "MyObjectType" may be referenced correctly, a problem arises if one of its constructor overloads accepts a type that is not referenced. For instance, if the constructor is overloaded as:

public MyObjectType(TypeFromOtherAssembly parameter) {
    // ... constructor code ...
}

In such cases, the error message will arise due to the missing reference to "TypeFromOtherAssembly."

Resolving the Issue

To resolve this issue, meticulously examine method calls and property references to identify potential missing references. By attending to these subtle oversights, you can rectify the error and restore functionality to your application.

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