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Compiler Error CS1679

Invalid extern alias for '/reference'; 'identifier' is not a valid identifier

When using the external assembly alias feature of the /reference option, the text that follows /reference: and that precedes the '=' must be a valid C# identifier or keyword according to the C# Language Specification.

To correct this error, change text before the "=" to a valid C# identifier or keyword.

The following example generates CS1679.

// CS1679.cs
// compile with: /reference:123$BadIdentifier%=System.dll
class TestClass {
    static void Main()