Input Validation

Overview

Provide consistent, user-friendly validation across forms and data entry points using MVVM patterns.

Validation Approaches

1. CommunityToolkit.Mvvm Validation

public partial class SettingsViewModel : ObservableValidator
{
    [ObservableProperty]
    [NotifyDataErrorInfo]
    [Required(ErrorMessage = "Name is required")]
    [MinLength(3, ErrorMessage = "Name must be at least 3 characters")]
    private string _name = "";

    [RelayCommand(CanExecute = nameof(CanSave))]
    private void Save()
    {
        ValidateAllProperties();
        if (HasErrors) return;
        
        // Proceed with save
    }

    private bool CanSave() => !HasErrors && !string.IsNullOrEmpty(Name);
}

2. INotifyDataErrorInfo (Built-in)

public partial class MyViewModel : ObservableValidator
{
    public MyViewModel()
    {
        // Validate on property change
        ErrorsChanged += (s, e) => OnPropertyChanged(nameof(CanSave));
    }

    [ObservableProperty]
    [NotifyDataErrorInfo]
    [Range(1, 100, ErrorMessage = "Value must be between 1 and 100")]
    private int _value;
}

3. Custom Validation Logic

[CustomValidation(typeof(MyValidator), nameof(MyValidator.ValidateUrl))]
private string _url = "";

public static class MyValidator
{
    public static ValidationResult ValidateUrl(string url, ValidationContext context)
    {
        if (string.IsNullOrEmpty(url))
            return ValidationResult.Success!;
            
        if (!Uri.TryCreate(url, UriKind.Absolute, out _))
            return new ValidationResult("Invalid URL format");
            
        return ValidationResult.Success!;
    }
}

XAML Error Display

Validation Error Template

<TextBox Text="{Binding Name, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}">
    <Validation.ErrorTemplate>
        <ControlTemplate>
            <StackPanel>
                <AdornedElementPlaceholder/>
                <TextBlock Text="{Binding [0].ErrorContent}"
                           Foreground="{DynamicResource Brushes.Error}"
                           FontSize="11"
                           Margin="4,2,0,0"/>
            </StackPanel>
        </ControlTemplate>
    </Validation.ErrorTemplate>
</TextBox>

Border Highlight on Error

<Style TargetType="TextBox">
    <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="True">
            <Setter Property="BorderBrush" Value="{DynamicResource Brushes.Error}"/>
            <Setter Property="ToolTip" 
                    Value="{Binding (Validation.Errors)[0].ErrorContent, RelativeSource={RelativeSource Self}}"/>
        </Trigger>
    </Style.Triggers>
</Style>

Command Can-Execute

[RelayCommand(CanExecute = nameof(CanSubmit))]
private async Task SubmitAsync()
{
    // ...
}

private bool CanSubmit()
{
    return !string.IsNullOrWhiteSpace(Name) 
        && Email.Contains("@") 
        && !HasErrors
        && !IsLoading;
}

// Notify when dependencies change
partial void OnNameChanged(string value) => SubmitCommand.NotifyCanExecuteChanged();
partial void OnEmailChanged(string value) => SubmitCommand.NotifyCanExecuteChanged();

Definition of Done (DoD)

• All user inputs have appropriate validation
• Validation errors displayed inline near the control
• Commands disabled when validation fails (CanExecute)
• Error messages are user-friendly (not technical)
• Required fields clearly indicated
• Validation runs on property change or blur
• HasErrors property prevents form submission

Common Validation Attributes

Anti-Patterns

• ❌ Validation only on submit (validate on change)
• ❌ Technical error messages ("NullReferenceException")
• ❌ Allowing submit with errors
• ❌ Silent validation failures
• ❌ Validation in code-behind

Integration with Forms

// Full form validation before save
public bool ValidateForm()
{
    ValidateAllProperties();
    return !HasErrors;
}

// Clear validation state
public void ResetValidation()
{
    ClearErrors();
}