如何转换列表<string?>列出<string>在 .NET 6 / C# 10？</string></string?>

Question

Using .NET 6 I have the following:

List<String> values = new List<String?> { null, "", "value" }
  .Where(x => !String.IsNullOrEmpty(x))
  .Select(y => y)
  .ToList();

But I am getting the warning:

Nullability of reference types in value of type 'string?[]' doesn't match target type 'string[]'.

I thought that using

.Where(x => !String.IsNullOrEmpty(x))

would solve the problem but it doesn't. How to fix this?

Answer 1

This is one case where you know better, and can assure the compiler the value is not null via .Select(y => y!)

List<string> values = new List<string?> { null, "", "value" }
   .Where(x => !string.IsNullOrEmpty(x))
   .Select(y => y!)
   .ToList();

_{Note : .Select(y => y.Value) is not going to work, as strings are reference types and string? represents a nullable reference type , not a nullable value type}

As mentioned in the comments by @Patrick Artner . You could also use .Cast<string>() to similar effect, which is essentially just an iterator and regular cast in a generic method, in turn assuring you have the desired result.

List<string> values = new List<string?> { null, "", "value" }
   .Where(x => !string.IsNullOrEmpty(x))
   .Cast<string>()
   .ToList();

And yet another way (albeit it a little harder to reason about) though likely more efficient

List<string> values = new List<string?> { null, "", "value" }
   .Where(x => !string.IsNullOrEmpty(x))!
   .ToList<string>();  

Answer 2

You can fix it like this:

.Where(x => x.HasValue).Select(y => y.Value)

Answer 3

You can use the null-forgiving operator after ToList without extra Select :

List<string> values = new List<string?> { null, "", "value" }
  .Where(x => !string.IsNullOrEmpty(x))
  .ToList()!;

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