Optional parameters "must be a compile-time constant"
.net, c#
Solution
No, the expression `PersonRespository.ColumnID` is not classified as a compile-time constant. The expression "ID" is, but that's not what you're using as the default parameter.
In particular, if `ColumnID` is "just a normal field" then any references to it will be resolved as a field - so if you compile an assembly which refers to the field, then change the value and rebuild the assembly containing `PersonRepository`, the referring assembly will see that change.
If you change your declaration to:
public const string ColumnID = "ID";
then it is a compile-time constant expression. That means in our previous scenario, the value of the constant is baked into any code that refers to it - and changing the value later without recompiling that referring code won't change the value used by that referring code.
See section 7.19 of the C# 4 language specification for more details about what counts as a constant expression.
Problem
I have a class divided in two partial files, like this: ``` public partial class PersonRepository : BaseRepository<Person> { public static readonly string ColumnID = "ID"; ... ``` and ``` public partial class PersonRepository : BaseRepository<Person> { public List<Person> GetByCompany(int companyID, string sortExpression = ColumnID) { ... ``` But the compiler keeps saying that `sortExpression` "must be a compile-time constant". To me it seems a perfect compile-time constant, so I don't understand where the problem is.