How do I convert List<String?> to List<String> in .NET 6 / C# 10?-CodePudding

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?

CodePudding user response：

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>();

CodePudding user response：

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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