Question

We have legacy character codes that we want to store as numbers in a new system. To increase readibility and general understanding in the code for devs making the migration, I want to do Enums like this…

Public Enum Status As Short
    Open = AscW("O")
    Closed = AscW("C")
    Pending = AscW("P")
    EnRoute = AscW("E")
End Enum

With this setup, the code will be readable (imagine If Record.Status = Status.Open), and yet the values will be stored in the database as small numbers so it will be efficient. However… I am a VB.NET guy, but everybody wants to code in C#, so I need this sort of structure in C#.

After Googling, I discovered the the general .NET equivalent of AscW is Convert.ToInt32("C"). When I try to use that statement in an enum, I get the compiler error “Constant Expression Required”.

How can I do this in C#? Is there a better way?

Answer 1

A method call is not a constant expression. Try this:

public enum Status { 
   Open = 'O',
   Closed = 'C',
   Pending = 'P',
   EnRoute = 'E'
}

The reason AscW works in VB is that it’s an internal thing that VB compiler understands and evaluates at compile time and is considered a constant expression by the compiler. Even in VB, Convert.ToInt32 will not work.

To quote the Visual Basic specification:

11.2 Constant Expressions

A constant expression is an expression whose value can be fully evaluated at compile time. […] The following constructs are permitted in constant expressions:

[…]

• The following run-time functions:

  • Microsoft.VisualBasic.Strings.ChrW
  • Microsoft.VisualBasic.Strings.Chr, if the constant value is between 0 and 128
  • Microsoft.VisualBasic.Strings.AscW, if the constant string is not empty
  • Microsoft.VisualBasic.Strings.Asc, if the constant string is not empty