How to validate that the entry of a number is not with "0" in C #

Question

I have a control of type <Entry> in which I don't want the user to enter values ​​that start with "0", for example...

0

01

00

00010

But if I want to enter values ​​that contain "0" in the form of natural numbers, for example ...

10

2010

200000

MyView.XAML:

<Entry
      HorizontalOptions="FillAndExpand"
      Placeholder="Cantidad"
      Keyboard="Numeric"
      MaxLength="9"
      Text="{Binding CantidadContenedor}"></Entry>

MyViewModel.CS:

   string cantidadContenedor;

   public string CantidadContenedor
        {
            get
            {
                return  cantidadContenedor;
            }
            set
            {
                if (cantidadContenedor != value)
                {
                    cantidadContenedor = value;
                    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(CantidadContenedor)));
                }
            }
        }

How can I add this validation once I capture the value of the Entry?

Can I occupy the TryParse Property being a Container Quantity of type string?

Any help for me?

Answer 1

I would create a validation rule.

In your XAML

<Entry
  HorizontalOptions="FillAndExpand"
  Placeholder="Cantidad"
  Keyboard="Numeric"
  MaxLength="9">
  <Entry.Text>
     <Binding Path=CantidadContenedor>
         <Binding.ValidationRules>
             <validationRules:StartWithRule Prefix="0"/>
     </Binding>
  </Entry.Text>
</Entry>

Now you crate the validation you see fit. In your case

public class StartWithRule : ValidationRule
{
    public string Prefix { get; set; }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        // Pass the value to string
        var numberToCheck = (string) value;

        if (numberToCheck == null) return ValidationResult.ValidResult;

        // Check if it starts with prefix
        return numberToCheck.StartsWith(Prefix)
            ? new ValidationResult(false, $"Starts with {Prefix}")
            : ValidationResult.ValidResult;
    }
}

And you should be good to go.