This documentation is archived and is not being maintained.

Compiler Error CS1638

'identifier' is a reserved identifier and cannot be used when ISO language version mode is used

When the ISO language compatibility option is specified by the /langversion compiler switch, any identifier with double underscores anywhere in the identifier will produce this error. To avoid this error, eliminate any identifiers with double underscores, or do not use the ISO-1 language version option.

The following sample generates CS1638:

// CS1638.cs
// compile with: /langversion:ISO-1
class bad__identifer // CS1638 (double underscores are not ISO compliant)

// Try this instead:
//class GoodIdentifier

class CMain
    public static void Main() { }