Type narrowing for LINQ function Where

[JonasAlmaas]

I find myself writing a lot of code following this pattern:

List<string?> someList = ["foo,bar,baz", null, "foo"];
var res = someList
	.Where(r => r!=null)
	.SelectMany(r => r!.Split(','));

Without LINQ I don't have to do r!. to narrow the type since if already does that for me:

List<string?> someList = ["asda", null, ""];
List<string> res = [];
foreach (var r in someList) {
	if (r!=null) {
		res.AddRange(r.Split(','));
	}
}

This is not only something I encounter with nullability, here is another example:

List<ISomeInterface> someList = [/* Imagine there being something here */];
var res = someList
	.Where(r => r is SomeImplClass)
	.Select(r => (r as SomeImplClass)!.Foo());

Would be nice if the following code worked:

List<ISomeInterface> someList = [/* Imagine there being something here */];
var res = someList
	.Where(r => r is SomeImplClass)
	.Select(r => r.Foo());

I can see how this might not be wanted, maybe where shouldn't change the return type. But at the same time, an if statement already does, so why can't where?

[spydacarnage]

You can use the .OfType<T>() method.

List<ISomeInterface> someList = [/* Imagine there being something here */];
var res = someList
	.OfType<SomeImplClass>()
	.Select(r => r.Foo());

[TahirAhmadov]

public static partial class _IEnumerable_Extensions
{
  public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> This)
    where T : class
    => This.Where(x=>x != null)!;
}