Proposal: Support using static readonly fields as default values for method parameters

[pikoscielniak]

I would like to propose, or inquire about the official support for, using static readonly fields as default values for method parameters.

I would like to propose that this construct be considered valid language syntax and compilable, but under the current rules, it is not allowed:

public static class Program
{
    public static void Main()
    {
        SomeMethod();
    }

    static void SomeMethod(SomeValueObject value = SomeValueObject.ValueOne)
    {
        Console.WriteLine(value.Value);
    }
}


public record SomeValueObject(string Value)
{
    public static readonly SomeValueObject ValueOne = new("one");
    public static readonly SomeValueObject ValueTwo = new("two");
    public static readonly SomeValueObject ValueThree = new("three");
}

I was wondering if there are any plans to support something like this in the future?

[colejohnson66]

This is not really possible; IL requires default parameters to be constant. It's not a C# language decision. Maybe it could be faked with attributes, but that's not easy either.

[HaloFour]

Not entirely. IL requires that the metadata used to indicate that the parameter is optional is a constant, but only for cross-language scenarios. The compiler is able to use additional metadata to indicate how to populate the default value when calling the method. This is how C# emulates default parameter values with decimal, and VB.NET with DateTime. While those do use a binary representation of the value embedded in an attribute, nothing really prevents said metadata from pointing to a field or whatever as the source of the value.

[colejohnson66]

Ah - I stand corrected.