Question: Convert error occurs with null-conditional, null-forgiving, and extension method when update VS to 16.8

[tzengshinfu]

Hi, folks:

I write a extension method that convert null to default string like this:

public static string ToStringOrDefault(this object obj, string defaultValue) {
    //When obj is null then returns defaultValue.
}

And I use the extension like this:

using System.Xml.Linq;

XElement rootNode = XDocument.Parse("some XML text...").Root;
bool someNode = rootNode.Element("some Node name...")?.Value!.ToStringOrDefault("N").Contains("Y");

Everything is ok in VS 16.7 + .NET Core 3.1.
When I update VS to 16.8, I get errors like this:
CS0266 - Cannot implicitly convert type 'bool?' to 'bool'. An explicit conversion exists (are you missing a cast?)

Could someone tell me what happend?

[ufcpp]

#3393

[tzengshinfu]

Hi, Iwanaga san:

Thanks for your reply.
I added parentheses to rootNode.Element("some Node name...")?.Value to resolve these errors like this:

bool someNode = (rootNode.Element("some Node name...")?.Value).ToStringOrDefault("N").Contains("Y");

[RikkiGibson]

If your extension method accepts a null this argument, consider changing the parameter to this object? obj. If you do that, nullability analysis will just do the right thing--it'll see that the this argument could be null, but that the extension method accepts a null this argument, so everything is fine.

[tzengshinfu]

Hi, Riki:

Thanks for your reply.
I tried to change object to object?, but it still doesn't call the extension method and returns null directly.

[Joe4evr]

Because the purpose of ?. is to short-circuit and not call the right-hand side if the left-hand side is null.

[RikkiGibson]

That's right, my suggestion doesn't solve the problem you're experiencing, but it does make the signature of your method more accurate :)

[tzengshinfu]

That's right, my suggestion doesn't solve the problem you're experiencing, but it does make the signature of your method more accurate :)

Thanks, Rikki!

[tzengshinfu]

Because the purpose of ?. is to short-circuit and not call the right-hand side if the left-hand side is null.

Hi, Joe4evr:

Thanks for your explanation.
It's a shame that null-forgiving operator(!) is not working now.
I like

rootNode.Element("some Node name...")?.Value!.ToStringOrDefault("N")

more than

(rootNode.Element("some Node name...")?.Value).ToStringOrDefault("N")