Why is a Warning about a nullable type issued, even though there can be no null value?

Question

Why do I get a Warning in the line of list2? I filtered out all null values here. The warning states, that in the select method a null value might be dereferenced.

#nullable enable

using System.Collections.Generic;
using System.Linq;

namespace TestNamespace
{
    public class Test
    {
        public Test()
        {
            List<string?> testList = new List<string?>()
            {
                "hallo",
                null
            };

            IEnumerable<string> list2 = testList.Where(x => x != null).Select(x => x.Replace("A", "")); // warning
            IEnumerable<string> list3 = testList.Where(x => x != null).Select(x => x != null ? x.Replace("A", "") : ""); // no warning
        }
    }
}

This is the warning I get in the line of list2:

In the line of list3 no warning is issued, but the check in the Select-Statement will always be pointless.

Answer 1

You get a warning because the type in the enumerable is still the nullable string?.

I suppose it's possible another thread changes the hallo entry to null during the enumeration, but more likely the compiler is just not sophisticated enough to understand you have filtered out any possible null values ahead of time.

You can use .Cast<string>() after filtering out the null values to change the type in the enumerable, or you can use a null-forgiving operator to remove the warning.

---

For fun, I also wrote this:

public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> items)
{
    foreach(var item in items) 
    {
        if (item is object)
        {
           yield return item;
        }
    }
}

See it work here:

https://dotnetfiddle.net/F8iQ0I