60

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.

Improve this question

8 Answers 8

Reset to default
61

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.

Improve this answer
Sign up to request clarification or add additional context in comments.

5 Comments

Thank you! Can you explain me why my expression is not a compile-time constant?
@Alessandro: All const members are implicitly static.
Ok, but If I have readonly ColumnID in the same assembly as GetByCompany than how is it different from const ColumID? I still need to recompile referenced assembly to get the change, right? Also, I can always write ...string sortExpression = null) { sortExpression = sortExpression ?? ColumnID;... The idea of variable default parameter seem to be nice future. Is it considered, a bad practice or is it disallowed just because some technical nuances?
@PawełAudionysos: It's different because the value can't be baked into each use of it, which is true for a const expression.
@PawełAudionysos: Using null as a default value that's then replaced by a potentially non-constant value is a reasonable pattern, yes. But Stack Overflow comments aren't intended to be used for questions like this.
7

You must declare your ColumnID as const.

The static readonly string will be instantiated when the class is first accessed in your code, and you could also initialize it with the return value of a static method, so it's not a compile-time constant for the compiler (even if in this case it obviously is for a person reading the code).

Improve this answer

Comments

5

const is something declared with const keyword.

readonly field can be assigned in constructor and its not compile time constant. the code that you've written right now runs in initializer (before constructor). const fields are 'baked' in as constants.

Improve this answer

Comments

5

change

public static readonly string ColumnID = "ID";

to

public const string ColumnID = "ID";
Improve this answer

Comments

5

readonly

The value of a readonly field can be changed (in a constructor). You need a const.

Improve this answer

Comments

4

Try to change this

public static readonly string ColumnID = "ID";

to

public const string ColumnID = "ID";
Improve this answer

Comments

3

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.
Improve this answer

Comments

2 
public const string ColumnID = "ID";
Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.