C# translation of https://jackfoxy.github.io/DependentTypes/

dependent.cs

void Main()
{
    var digits = Digits.Create("093884765");
    var digitsOfLength3 = Digits3.Create("007");
    
    Console.WriteLine($"digits: [{digits}] digitsOfLength3: [{digitsOfLength3}]");
}

static class DigitsDef
{
    private static readonly Regex regex = new Regex("^[0-9]+$");
    private static string VerifyDigits(int config, string value)
    {
        if (regex.IsMatch(value.Trim()) &&
            (config == 0 || value.Length == config))
        {
            return value;
        }
        
        return null;
    }
    
    public class DigitsValidator : Cctor<int, string, string>
    { public DigitsValidator(int config) : base(config, VerifyDigits) { } }
    
    public class ValidDigits : DigitsValidator
    { public ValidDigits() : base(0) {} }

    public class ValidDigits2 : DigitsValidator
    { public ValidDigits2() : base(2) { } }

    public class ValidDigits3 : DigitsValidator
    { public ValidDigits3() : base(3) { } }

    public class ValidDigits4 : DigitsValidator
    { public ValidDigits4() : base(4) { } }
}

class Digits : DependentType<DigitsDef.ValidDigits, int, string, string> { private Digits() {} }
class Digits2 : DependentType<DigitsDef.ValidDigits2, int, string, string> { private Digits2() {} }
class Digits3 : DependentType<DigitsDef.ValidDigits3, int, string, string> { private Digits3() {} }
class Digits4 : DependentType<DigitsDef.ValidDigits4, int, string, string> { private Digits4() {} }

class Cctor<Config, T, T2>
{
    private Config _config;
    private Func<Config, T, T2> _vfn;
    public Cctor(Config config, Func<Config, T, T2> vfn)
    {
        _config = config;
        _vfn = vfn;
    }
    
    public T2 TryCreate(T input) => _vfn(_config, input);
}

class DependentType<Cctor, Config, T, T2>
    where Cctor : Cctor<Config, T, T2>, new()
{
    T2 _value;
    
    protected DependentType() => throw new NotSupportedException("Only derive to provide aliases");
    private DependentType(T2 value) => _value = value;
    
    public static DependentType<Cctor, Config, T, T2> Create(T input)
    {
        var result = new Cctor().TryCreate(input);
        if (result != null)
        {
            return new DependentType<Cctor, Config, T, T2>(result);
        }
        
        return null;
    }
    
    public override string ToString() => _value.ToString();
}