`UserOptions.AllowedUserNameCharacters` should be marked as allowing `null`.

[justindbaur]

Background and Motivation

UserOptions.AllowedUserNameCharacters is marked as not allowing null to be assigned to it but the code consuming this property treats a null or empty as all characters being allowed in the username.

aspnetcore/src/Identity/Extensions.Core/src/UserValidator.cs

Line 61 in b50ab86

else if (!string.IsNullOrEmpty(manager.Options.User.AllowedUserNameCharacters) &&

Proposed API

namespace Microsoft.AspNetCore.Identity;

public class UserOptions
{
+    public string? AllowedUserNameCharacters { get; set; }
-    public string AllowedUserNameCharacters { get; set; }
}

Usage Examples

services.Configure<IdentityOptions>(options =>
{
    // Allow all characters in usernames
    options.User.AllowedUserNameCharacters = null;
});

Alternative Designs

null can still be disallowed and an empty string should be used to denote that all characters are allowed in usernames.

Risks

There should be no risks, the property will still retain its same default set of characters it will just not cause nullable reference types warnings when null is set to it.

There are already tests using null as a value here.