Abstraction for duplicated code [Answered]

public static class IdentityState
{
    public const string Active = "Active";
    public const string Locked = "Locked";
    public const string Pending = "Pending";
    
    public static readonly string[] All = {Active, Locked, Pending};
    public static bool IsValid(string state) => All.Contains(state);
}

public static class CredentialTypes
{
    public const string Password = "password";
    public const string Oidc = "oidc";
    public const string Totp = "totp";
    public const string LookupSecret = "lookup_secret";
    public const string MagicLink = "magic_link";
    
    public static readonly string[] All = { Password, Oidc, Totp, LookupSecret, MagicLink };
    public static bool IsValid(string type) => All.Contains(type);
}

-public static readonly string[] All = { Password, Oidc, Totp, LookupSecret, MagicLink };
+public string[] All { get; } = { Password, Oidc, Totp, LookupSecret, MagicLink };

RuleFor(x => x.SomeString).OneOf(new IdentityState())

interface IInterface
{
    abstract static string[] All { get; }
    abstract static bool IsValid(string value);
}

class Class : IInterface
{
    public static string[] All { get; } = { "foo", "bar", "baz" };
    public static bool IsValid(string value) => All.Contains(value);
}