Question

I have the following problem, I’m trying to use FluentValidation in Windows Phone to validate if a string is a valid username.

The problem I have is that the validator value has the type string which has to be tested. In all the examples I can find it’s always a property of the object that is being tested like this RuleFor(customer => customer.Surname).NotEmpty();

When I run this code I get the following error

Code:

public class UsernameValidator : AbstractValidator<string>
{
    public UsernameValidator()
    {
        RuleFor(username => username).NotNull().NotEmpty().WithMessage("Enter a username");
        RuleFor(username => username).Matches("^[a-zA-Z0-9_]*$").WithMessage("Only letters and numbers");
        RuleFor(username => username).Length(3, 30).WithMessage("Minimum length is 3");
    }
}

Error:

FluentValidation for WP7 can only be used with expressions that reference public properties, ie x => x.SomeProperty
   at FluentValidation.Internal.Extensions.CompilePropertyGetterExpression(LambdaExpression expression, Type delegateType)
   at FluentValidation.Internal.Extensions.Compile[TDelegate](Expression`1 expression)
   at FluentValidation.Internal.PropertyRule`1.Create[TProperty](Expression`1 expression, Func`1 cascadeModeThunk)
   at FluentValidation.AbstractValidator`1.RuleFor[TProperty](Expression`1 expression)
   at WorldChat.ViewModels.Validators.UsernameValidator..ctor()
   at WorldChat.ViewModels.RegisterViewModel.get_ErrorUsername()
   at System.Reflection.RuntimeMethodInfo.InternalInvoke(RuntimeMethodInfo rtmi, Object obj, BindingFlags invokeAttr, Binder binder, Object parameters, CultureInfo culture, Boolean isBinderDefault, Assembly caller, Boolean verifyAccess, StackCrawlMark& stackMark)
   at System.Reflection.RuntimeMethodInfo.InternalInvoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, StackCrawlMark& stackMark)
   at System.Reflection.RuntimePropertyInfo.InternalGetValue(PropertyInfo thisProperty, Object obj, Object[] index, StackCrawlMark& stackMark)
   at System.Reflection.RuntimePropertyInfo.GetValue(Object obj, Object[] index)
   at System.Windows.CLRPropertyListener.get_Value()
   at System.Windows.PropertyAccessPathStep.ConnectToPropertyInSource(Boolean isSourceCollectionViewCurrentItem)
   at System.Windows.PropertyAccessPathStep.ConnectToProperty()
   at System.Windows.PropertyAccessPathStep.ReConnect(Object newSource)
   at System.Windows.PropertyPathListener.ReConnect(Object source)
   at System.Windows.Data.BindingExpression.SourceAcquired()
   at System.Windows.Data.BindingExpression.System.Windows.IDataContextChangedListener.OnDataContextChanged(Object sender, DataContextChangedEventArgs e)
   at System.Windows.Data.BindingExpression.DataContextChanged(Object sender, DataContextChangedEventArgs e)
   at System.Windows.FrameworkElement.OnDataContextChanged(DataContextChangedEventArgs e)
   at System.Windows.FrameworkElement.OnAncestorDataContextChanged(DataContextChangedEventArgs e)
   at System.Windows.FrameworkElement.NotifyDataContextChanged(DataContextChangedEventArgs e)
   at System.Windows.FrameworkElement.OnAncestorDataContextChanged(DataContextChangedEventArgs e)
   at System.Windows.FrameworkElement.NotifyDataContextChanged(DataContextChangedEventArgs e)
   at System.Windows.FrameworkElement.OnTreeParentUpdated(DependencyObject newParent, Boolean bIsNewParentAlive)
   at System.Windows.DependencyObject.UpdateTreeParent(IManagedPeer oldParent, IManagedPeer newParent, Boolean bIsNewParentAlive, Boolean keepReferenceToParent)
   at MS.Internal.FrameworkCallbacks.ManagedPeerTreeUpdate(IntPtr oldParentElement, IntPtr parentElement, IntPtr childElement, Byte bIsParentAlive, Byte bKeepReferenceToParent, Byte bCanCreateParent)
   at MS.Internal.XcpImports.MeasureNative(IntPtr element, Single inWidth, Single inHeight)
   at MS.Internal.XcpImports.UIElement_Measure(UIElement element, Size availableSize)
   at System.Windows.UIElement.Measure(Size availableSize)
   at System.Windows.Controls.ScrollViewer.MeasureOverride(Size constraint)
   at System.Windows.FrameworkElement.MeasureOverride(IntPtr nativeTarget, Double inWidth, Double inHeight, Double& outWidth, Double& outHeight)
   at MS.Internal.XcpImports.MeasureOverrideNative(IntPtr element, Single inWidth, Single inHeight, Single& outWidth, Single& outHeight)
   at MS.Internal.XcpImports.FrameworkElement_MeasureOverride(FrameworkElement element, Size availableSize)
   at System.Windows.FrameworkElement.MeasureOverride(Size availableSize)
   at System.Windows.FrameworkElement.MeasureOverride(IntPtr nativeTarget, Double inWidth, Double inHeight, Double& outWidth, Double& outHeight)
   at MS.Internal.XcpImports.MeasureOverrideNative(IntPtr element, Single inWidth, Single inHeight, Single& outWidth, Single& outHeight)
   at MS.Internal.XcpImports.FrameworkElement_MeasureOverride(FrameworkElement element, Size availableSize)
   at System.Windows.FrameworkElement.MeasureOverride(Size availableSize)
   at Microsoft.Phone.Controls.PhoneApplicationFrame.MeasureOverride(Size availableSize)
   at System.Windows.FrameworkElement.MeasureOverride(IntPtr nativeTarget, Double inWidth, Double inHeight, Double& outWidth, Double& outHeight)

Answer 1

I found a workaround, it requires you to have a extra string wrapper class.

ValidationString.cs

public class ValidationString
{
    public string value { get; set; }

    public ValidationString(string value)
    {
        this.value = value;
    }
}

In the username validator class I add a static method IsUsername

public class UsernameValidator : AbstractValidator<ValidationString>
{
    public UsernameValidator()
    {
        RuleFor(username => username.value).NotNull().NotEmpty().WithMessage("Enter a username");
        RuleFor(username => username.value).Matches("^[a-zA-Z0-9_]*$").WithMessage("Only letters and numbers");
        RuleFor(username => username.value).Length(3, 30).WithMessage("Minimum length is 3");
    }

    internal static bool IsUsername(string value)
    {
        return new UsernameValidator().Validate(new ValidationString(value)).IsValid;
    }
}

And now I will be both able to use this UsernameValidator on a string as well as I’m still able to use it another validater, for example a registration validator

public class RegisterValidator : AbstractValidator<User>
{
    public RegisterValidator()
    {
        RuleFor(user => user.Username).Must(UsernameValidator.IsUsername);
        // some more rules
    }
}

If you don’t want to use your UsernameValidator in other validation classes you can leave the IsUsername method out.