Relax new() and required mutual exclusion

[TonyValenti]

Today when a type has a required property, the type cannot fulfill the new() constraint.

This requirements should be removed when all the following are true:

• Additional constraints specify all required properties for the type.

Example:

public record LegacyRecord {
  public required long Id {get; init; }
}

pubic record D1 : LegacyRecord {
  public string? Name {get; init;}
}

pubic  record D2 : LegacyRecord {
  public required string? Name {get; init;}
}

public static void Test<T> where T : LegacyRecord, new() {
  var ret = new T(){
    Id = 0,
  };
  return ret;
}

Test<LegacyRecord>(); //this should work.
Test<D1>(); //this also should work.
Test<D2>(); //this should not work because D2 introduces required members that are not covered by a constraint..

Thanks!

[TahirAhmadov]

Test<LegacyRecord>(); //this should work.

This cannot work because the compiler does not know that Test sets the required property Id in its body - imagine that it lives in a different DLL and C# does not interpret the contents, and it can be an interface method so there is no body at all.
For this to work, there would need to be something in the signature - the new() constraint would get additional metadata:

public static void Test<T> where T : LegacyRecord, new() sets_required_properties // new keyword here
{
  var ret = new T() { Id = 0, }; // no error
  var x = new T(); // error - Id not set
  return ret;
}

The imaginary new sets_required_properties keyword specifies that 1) a base class constraint is also required, and 2) all of the required properties of said base class are set in the generic method.

[TonyValenti]

Oh! I see. But that would not prevent the scenario I described. It would just result in double initialization for some properties in some cases.

[333fred]

It's not that it cannot work. It's that it would require a much more complex analysis, one we weren't sure we were ready to define. We've also not seen much ask for it since required was introduced; I went looking to see if this was a duplicate and couldn't find any direct duplicate discussions.

[TonyValenti]

Thanks @333fred !
Here are details on my scenario and what triggered this need:
I have classes that have base-class properties that are always set at construction. I wanted to make those properties required so that I could prevent any bugs from cropping up where one of the properties might accidentally not be set. I could not.

[qrli]

This scenario is similar to new() constraint with non-empty parameter list.

[TonyValenti]

@qrli Excellent parallel! You're exactly right!