Optional parameters “must be a compile-time constant”

Question

I have a class divided in two partial files, like this:

public partial class PersonRepository : BaseRepository
{
    public static readonly str         


        

Answer 1

change

public static readonly string ColumnID = "ID";

to

public const string ColumnID = "ID";

Answer 2

Just for completeness, here are the three valid default values for an optional argument: ( from: MSDN: Named and Optional Arguments)

1. a constant expression
2. an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct; (note: only the parameterless constructor can be used)
3. an expression of the form default(ValType), where ValType is a value type.