2.3 Extern Alias

Let’s cover extern alias in C# — a niche but important feature when dealing with multiple assemblies that have conflicting namespaces or types.

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📌 1. What Is extern alias?

• extern alias allows you to differentiate between two assemblies that contain types with the same fully qualified names.

• It essentially gives an assembly a nickname, so you can specify exactly which type you mean.

• Usually needed in legacy projects or when referencing multiple versions of the same library.

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📌 2. Scenario Where You Need It

Imagine you reference two assemblies:

1. LibraryA.dll contains MyCompany.Logging.Logger

2. LibraryB.dll also contains MyCompany.Logging.Logger

• Without extern alias, the compiler cannot distinguish which Logger you mean.

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📌 3. Setting Up extern alias

Step 1: Add alias in project references

• In Visual Studio, for one of the conflicting assemblies:

  • Right-click Reference → Properties → Aliases

  • Set it to something like LibA (default is global)

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Step 2: Use extern alias in code

extern alias LibA; // Use the alias defined in project references
extern alias LibB;
 
class Program
{
    static void Main()
    {
        LibA::MyCompany.Logging.Logger loggerA = new LibA::MyCompany.Logging.Logger();
        LibB::MyCompany.Logging.Logger loggerB = new LibB::MyCompany.Logging.Logger();
 
        loggerA.Log("From LibraryA");
        loggerB.Log("From LibraryB");
    }
}

• LibA:: tells the compiler to use the LibA assembly.

• LibB:: tells the compiler to use the LibB assembly.

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📌 4. Key Points

Feature	Notes
extern alias	Lets you differentiate between assemblies with conflicting types
Requires alias setup in project references	Default alias is global
Syntax	extern alias AliasName; + AliasName::FullyQualifiedType
Rarely used	Only when multiple versions of same library are referenced

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📌 5. Analogy

• Imagine two folders with the same file name.

• You give each folder a nickname and then explicitly say which folder’s file you want:

  • LibA::Logger vs LibB::Logger

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✅ Summary:

• Use extern alias when your project references multiple assemblies with conflicting types.

• Define the alias in the reference, then use extern alias in code.

• Syntax: extern alias AliasName; + AliasName::TypeName.

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