Question

I have a class like:

class SomeTests {
  private Guid[] someGuids = new Guid[] { ... }

  public void ThoseGuidsShouldAlwaysBeThere() {
    foreach (Guid g in someGuids) { // error appears here
        // ...
    }
  }
}

Semantically, I want someGuids to be const, since they shouldn’t be updated, ever, except before recompiling the code. But adding the const keyword generates error CS0168: null is not valid in this context.

Reading the MSDN page for that error, it seems to me that the compiler thinks I’m doing this:

foreach (Guid g in null) {

I don’t understand how adding const causes this problem here, and how to solve my semantic problem (list is read-only, not writable) — keeping it as an array instead of a List is “almost” good enough.

Answer 1

Trying to make Guid[] a constant should give you an error of “A const field of a reference type other than string can only be initialized with null.”

Make it readonly instead:

private readonly Guid[] someGuids = new Guid[] { Guid.NewGuid() };

When it’s readonly you can also assign the value in the constructor:

public SomeTests()
{
    someGuids = new[] { Guid.NewGuid(), Guid.NewGuid() };
}

As Jeffery mentioned in the comments, this solution prevents someGuids from being reassigned, but the items can still be modified. Jeffery addresses this issue in his answer.