Some questions about adding validation to a TextBox

From the bit of research I’ve done on TextBox validation in Avalonia I’ve gotten this far:

View.axaml:

<TextBox Text="{Binding Email}" Margin="5"/>

ViewModel.cs:

private string email;
public string Email
{
    get
    {
        return email;
    }
    set
    {
        if (string.IsNullOrWhiteSpace(value) || !Regex.IsMatch(value, @"^\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*$"))
        {
            throw new DataValidationException("Invalid email");
        }

        email = value;
        FirePropertyChanged();
    }
}

This works, but I’ve got a couple questions:

• First, my TextBox does not show a validation error when the window first opens (not until the value of the textbox is changed back). What is the right way to force a validity check when the window first opens?
• Secondly, I would like to be able to test my validation while debugging but it’s a bit annoying that the VS debugger shows the thrown exception, forcing me to press “Continue” between every character that I type. Is there a way that I can run the program in debug and yet ignore DataValidationExceptions?
• Finally, is there a way that my viewmodel can check for any current validation issues in the form when the “Ok” button is pressed? (eg in my case above refusing to let the user submit the form if the Email textbox is still showing a validation error)

[worldbeater]

@dashenxian you can achieve the described behavior by incorporating WhenAnyValue and a ReactiveCommand probably:

// Each time values of UserName and Password properties change,
// the canExecute observable is signalled and the logic that 
// determines if command execution should be allowed is executed.
var canExecute = this.WhenAnyValue(
    x => x.UserName,
    x => x.Password,
    (userName, password) => 
        !string.IsNullOrEmpty(userName) && 
        !string.IsNullOrEmpty(password));

// The command will be unavailable during execution of the LogOnAsync
// method, or while UserName and Password ViewModel properties are 
// nulls or empty strings. In other words, canExecute supplements 
// the default executability behavior, it doesn't replace it.
var login = ReactiveCommand.CreateFromTask(LogOnAsync, canExecute); // ICommand

Borrowed from https://www.reactiveui.net/docs/handbook/commands/#controlling-executability With ReactiveUI.Validation and their INotifyDataErrorInfo implementation this will look like:

this.ValidationRule(
    x => x.Username,
    name  => !string.IsNullOrWhiteSpace(name),
    "User name shouldn't be null or white space.");

this.ValidationRule(
   x => x.Password,
   pass => !string.IsNullOrWhiteSpace(pass),
   "Password shouldn't be null or white space.");

var canExecute = this.IsValid();
var login = ReactiveCommand.CreateFromTask(LogOnAsync, canExecute); // ICommand

[worldbeater]

We’ve released a new version of ReactiveUI.Validation recently, with plenty of updates and enhancements. Even complex asynchronous validations are supported now (see our README.md file for more info). Also, we’ve added a sample app targeting Avalonia that shows how to use ReactiveUI.Validation with AvaloniaUI. This should simplify the process of building validations. Remember to install Avalonia.ReactiveUI and add a call to .UseReactiveUI() if you decide to try out ReactiveUI.Validation!

Here is a simple login form validation example. In this example, the Login button stays disabled while the form is invalid:

• ViewModel layer (C# code)
• View layer (XAML markup)