#586 – Default Values for Optional Parameters Must Be Constants

When you specify a default value for an optional parameter, that value must be a constant, of the same type as the parameter.  The expression must be able to be evaluted at compile-time.  The parameter’s default value can be one of the following:

  • A constant expression (see below)
  • new S(),  where S is a value-type (parameters not allowed on constructor)

A constant expression is one that can be fully determined at compile-time.  In other words, you can use any expression that you’d use in the initialization of a constant.

For reference types, the expression must be null, with the exception of the string type.

        private const int OFFSET = 1;

        static void Optional1(int x = 5 + OFFSET, double y = 1.0/3.0) { }
        static void Optional2(Point3D fln = new Point3D()) { }
        static void Optional3(Dog d = null) { }
        static void Optional4(string s = "DEFAULT") { }

        static void Main()
        {
            Optional1();
            Optional2();
            Optional3();
        }
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About Sean
Software developer in the Twin Cities area, passionate about .NET technologies. Equally passionate about my own personal projects related to family history and preservation of family stories and photos.

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