Why do I get a "possibly null reference" after checking for null?

Question

Why do I get a "possibly null reference" after checking for null?

Charles Burkitt 30

I am checking a property for null, but Visual Studio then warns me about a possibly null reference.
vs question a

vs question b

I am explicitly checking that the DisplayNames property is not null before enumerating it.

Please enlighten me as to what I am doing wrong, if anything.

Thank you.

David Lowndes 4,726 Reputation points

2023-01-24T17:00:10.2966667+00:00

Have you got a small example that anyone can use to reproduce this?
Karen Payne MVP 35,596 Reputation points Volunteer Moderator

2023-01-24T17:00:22.39+00:00

Try using a null-forgiving operator
Charles Burkitt 30 Reputation points

2023-01-24T20:54:43.8466667+00:00

Thank you, Viorel. You must be right about the analyser, because that works without any warning.

In order for me to mark this as correct, please post it as an answer. Please note that the property in the first line should be DisplayNames (plural).

Thanks for responding.
Charles Burkitt 30 Reputation points

2023-01-25T20:50:56.8+00:00

Karen, I'm not experienced enough to trust myself with this. The last time I used the null-forgiving operator to suppress a warning I got into a right mess. Maybe when I have greater knowledge I will be able to play the analyser.

Answer accepted by question author

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David Lowndes 4,726 Reputation points

2023-01-24T17:00:10.2966667+00:00

Have you got a small example that anyone can use to reproduce this?
Karen Payne MVP 35,596 Reputation points Volunteer Moderator

2023-01-24T17:00:22.39+00:00

Try using a null-forgiving operator
Charles Burkitt 30 Reputation points

2023-01-24T20:54:43.8466667+00:00

Thank you, Viorel. You must be right about the analyser, because that works without any warning.

In order for me to mark this as correct, please post it as an answer. Please note that the property in the first line should be DisplayNames (plural).

Thanks for responding.
Charles Burkitt 30 Reputation points

2023-01-25T20:50:56.8+00:00

Karen, I'm not experienced enough to trust myself with this. The last time I used the null-forgiving operator to suppress a warning I got into a right mess. Maybe when I have greater knowledge I will be able to play the analyser.

Answer 1

Viorel 125.8K

Probably the analyser is not smart enough in this case. Maybe you can help it:

var names = vcards[i].DisplayName;
if( names != null )
{
   foreach( var displayName in names )
   . . .

David Lowndes 4,726 Reputation points

2023-01-25T15:49:08.9566667+00:00

Such subterfuge should not be necessary, if anyone has a simple example that reproduces the issue, it could be reported as a bug - so that hopefully such things get fixed.

Charles Burkitt 30

David, here is a simple example that shows the problem.

internal class Program
{
	static void Main(string[] args)
	{
		People p1 = new People();
		People p2 = new People();
		People p3 = new People();

		List<People> Peoples = new List<People> { p1, p2, p3 };

		for (var i = 0; i < Peoples.Count; i++)
		{
			if (Peoples[i].HumanList != null)
			{
				foreach (var human in Peoples[i].HumanList)
				{
					// Do something...
				}
			}
		}
	}
}

public class People
{
    public List<Human?>? HumanList { get; set; }

	public People()
	{
		HumanList = new List<Human?>
		{
			new Human("Bob"),
			new Human("Mike")
		};
	}
}

public class Human
{
	public Human(string name)
	{
		this.Name = name;
	}

	public string Name { get; set; }
}

This is how Visual Studio takes it:

demo1

David Lowndes 4,726 Reputation points

2023-01-25T21:42:48.05+00:00

That's great Charles. It repros with the latest VS Preview and .NET 7.

Would you report it as a bug using the Visual Studio "Report a Problem" facility and post a link back here to your report so that others can confirm & vote on it please.

Be sure to include the full minimal repro project and clear instructions (give the person processing the report an easy life so it doesn't get discarded), and possibly illustrate the fact that using a local reference does prevent the warning.
Charles Burkitt 30 Reputation points

2023-01-26T14:33:54.5966667+00:00

Reported.

Please see [https://developercommunity.visualstudio.com/t/Analyzer-reports-possibly-null-referenc/10261906
David Lowndes 4,726 Reputation points

2023-01-26T15:53:00.0533333+00:00

Thanks for doing that Charles.
Charles Burkitt 30 Reputation points

2023-02-01T12:09:00.94+00:00
For anyone interested, the C# experts reported that this is not a bug; rather it's the expected behaviour.

Thanks for taking the time to file this feedback issue. Unfortunately this is not a bug. The language is operating as intended here. Array element access, unlike property or field access, does not persist nullable state.

In order to persist the state of this check you can assign it out to another value:

if (array[i] is { } value) { // value is non-null here }
David Lowndes 4,726 Reputation points

2023-02-01T13:42:11.67+00:00

"operating as intended" - just not what anyone would intuitively expect (unless they have some deep internal knowledge).

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Why do I get a "possibly null reference" after checking for null?

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