Question

I have a simple test page in my Silverlight 4 application in which I’m trying to get a custom validation rule to fire.

I have a TextBox and a Button, and I am showing the validation results in a TextBlock. My view model has a Name property, which is bound the the Text property of the TextBox. I have two validation attributes on the Name property, [Required] and [CustomValidation].

When I hit the Submit button, the Required validator fires correctly, but the breakpoint inside the validation method of my custom validator never gets hit. I can’t see why this is, as I think I have followed MS’s example very carefully: http://msdn.microsoft.com/en-us/library/system.componentmodel.dataannotations.customvalidationattribute(v=vs.95).aspx

Here is the code for the view model:

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using GalaSoft.MvvmLight.Command;

namespace MyProject
{
    // custom validation class
    public class StartsCapitalValidator
    {
        public static ValidationResult IsValid(string value)
        {
            // this code never gets hit
            if (value.Length > 0)
            {
                var valid = (value[0].ToString() == value[0].ToString().ToUpper());
                if (!valid)
                    return new ValidationResult("Name must start with capital letter");
            }
            return ValidationResult.Success;
        }
    }

    // my view model
    public class ValidationTestViewModel : ViewModelBase
    {
        // the property to be validated
        string _name;
        [Required]
        [CustomValidation(typeof(StartsCapitalValidator), "IsValid")]
        public string Name
        {
            get { return _name; }
            set { SetProperty(ref _name, value, () => Name); }
        }

        string _result;
        public string Result
        {
            get { return _result; }
            private set { SetProperty(ref _result, value, () => Result); }
        }

        public RelayCommand SubmitCommand { get; private set; }

        public ValidationTestViewModel()
        {
            SubmitCommand = new RelayCommand(Submit);
        }

        void Submit()
        {
            // perform validation when the user clicks the Submit button
            var errors = new List<ValidationResult>();
            if (!Validator.TryValidateObject(this, new ValidationContext(this, null, null), errors))
            {
                // we only ever get here from the Required validation, never from the CustomValidator
                Result = String.Format("{0} error(s):\n{1}",
                    errors.Count,
                    String.Join("\n", errors.Select(e => e.ErrorMessage)));
            }
            else
            {
                Result = "Valid";
            }
        }
    }
}

Here is the view:

<navigation:Page x:Class="Data.Byldr.Application.Views.ValidationTest" 
           xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
           xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
           xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
           xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
           xmlns:navigation="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Navigation">
    <Grid Width="400">
    <StackPanel>
        <TextBox Text="{Binding Name, Mode=TwoWay}" />
        <Button Command="{Binding SubmitCommand}" Content="Submit" />
        <TextBlock Text="{Binding Result}" />
    </StackPanel>
    </Grid>
</navigation:Page>

Answer 1

As stated on the MSDN page for that overload of Validator.TryValidateObject ( http://msdn.microsoft.com/en-us/library/dd411803(v=VS.95).aspx ), only the object-level validations are checked with this method, and RequiredAttribute on properties.

To check property-level validations, use the overload that also takes a bool ( http://msdn.microsoft.com/en-us/library/dd411772(v=VS.95).aspx )

So it should be as simple as passing “true” as an extra parameter to TryValidateObject