Func<> as optional parameter of a function has to be a compile time constant?

Question

So I have the following function. I would like it to have a standard parameter value but I can't get it to work as it needs to be a compile time constant. I know I can set it to null but I want it to be a specific function.

    private void func(Func<List<int>, bool> eval = _ => true)
    {
        var b = eval();
    }

Is it even possible to something like this?

Nope, lambdas cannot be constant, that's just a CLR limitation, see https://stackoverflow.com/questions/19198037/declare-lambda-expression-as-a-class-constant-field for details. A workaround would be to have your optional parameter default to `null` and just do `if (eval == null) eval = ...` first thing in your code. — Heinzi, Aug 07 '19 at 14:22

Jonathon Chase · Accepted Answer · 2021-08-31T09:27:22.777

6

Since default parameters must be compile time constants, you can't provide an easy default function quite like you want. You can work around this, though, using null as you pointed out.

private void func(Func<List<int>, bool> eval = null)
{
    eval = eval ?? (_ => true);
    // Do things.
}

This will assign your default implementation if null is passed, or the function is called without any parameters.

In modern versions of C#, you can use the null assignment operator, ??=.

This would look something like this:

eval ??= _ => true;

edited Aug 31 '21 at 09:27

answered Aug 07 '19 at 14:23

Jonathon Chase

9,396
21
39

Ok thank you very much! Very disappointed that this is not possible though – krjw Aug 07 '19 at 14:25

score 2 · Answer 2 · answered Aug 07 '19 at 14:26

2

An alternative to the answer from @JonathonChase is to use an overload:

private void func() => func(_ => true);

answered Aug 07 '19 at 14:26

Bill Tür stands with Ukraine

3,425
30
38
48

I like that approach! Thanks for pointing it out – krjw Aug 07 '19 at 14:27

score 1 · Answer 3 · answered Aug 07 '19 at 14:25

You can't assign anything other than null as the default parameter to any reference type other than string. Func<List<int>, bool> is a reference type.

So just do what you want to do as a default in the function:

private void func(Func<List<int>, bool> eval = null)
{
    if (eval == null)
    {
        //do some default thing
    }
    else 
         eval.Invoke(new List<int>());
}

Func<> as optional parameter of a function has to be a compile time constant?

3 Answers3