As you may figured out from 1 of my previous posts, i’m not a big fan of Silverlight Dataform control, although I do like some of the capabilities presented there. In fact, after numerous frustrations with it, i decided to roll my own customization of it, rip out everything i didn’t need and adjust it to act the way i like it. You can find my first stab at it in this post.

That turned out to be one of the better decisions, although fighthing with the code mess inside the datafirn proved to be quite a challenge. Lately i had an issue with validation which i thought was just another dataform bug, but it proved to be a lot harder than it seemed at first. It’s well presented with the following picture i picked up from linkedIn:

Both of the fields are required to be filled out but the latter one is not highlighted. This case is only present if you try to submit unchanged value, this is very important as you’ll see later on.
Lets assume you have the following entities:

    public class Partner
    {
        [Key]
        public int Id { get; set; }
        
        [Required]
        public int? CountryId { get; set; }
		
	public Country Country{ get; set; }

    }

    public class Country
    {
        [Key]
        public int? Id { get; set; }
    }

Now, let’s say that you bound the combobox to Country property of a Partner entity:

<ComboBox Name="Country" SelectedItem="{Binding Country,Mode=TwoWay}" />
 

So you press an ok button and nothing really happens.
The problem
The main problem is not actually the dataform, but the way validation works with binding and the way Ria generates code for you. You see, every time you change value and lose focus, or you submit the data with an OK button, the bindings try to push the value to your entity, which means set the property. And the property in your generated file looks like this:

	public class Partner: Entity
	{ .
	 .
	 .
        /// <summary>
        /// Gets or sets the associated <see cref="Country"/> entity.
        /// </summary>
        [Association("Partner_Country", "CountryId", "Id", IsForeignKey=true)]
        public Country Country
        {
            get
            {
                if ((this._c == null))
                {
                    this._c = new EntityRef<Country>(this, "Country", this.FilterC);
                }
                return this._c.Entity;
            }
            set
            {
                Country previous = this.Country;
                if ((previous != value))
                {
                    this.ValidateProperty("Country", value);
                    if ((value != null))
                    {
                        this.CountryId = value.Id;
                    }
                    else
                    {
                        this.CountryId = default(Nullable<int>);
                    }
                    this._c.Entity = value;
                    this.RaisePropertyChanged("Country");
                }
            }
        }
	}

Pay attention to the part if (previous!=value). What that effectively means is that if your property came in as invalid, and you try to set that same value (for instance null) your property wouldn’t even go through the vaildation (that this.ValidateProperty(“Country”, value) line) !

A second problem alltogether would be that you can’t even put a required attribute on a property which already has the association attribute. You can put a required attibute on a CountryId field:

    public class Partner
    {
        [Key]
        public int Id { get; set; }
        
        [Required]
        public int? CountryId { get; set; }
		
	public Country Country{ get; set; }

    }

    public class Country
    {
        [Key]
        public int? Id { get; set; }
    }

But that wouldn’t then trigger validation for the Country if you defined your combobox like in the upper case.

So how to fix this?

Ok, let’s first define the combobox so that it uses the Required attribute of CountryId. The solution would look something like this:

<ComboBox ItemsSource="{Binding Data, ElementName=DF_CSource}" SelectedValue="{Binding CountryId,Mode=TwoWay}" 
          SelectedItem="{Binding Country,Mode=TwoWay}" SelectedValuePath="Id"/>

We are biniding the selectedValue to CountryId, and selected value is determined from SelectedItem.SelectedValuePath. This way the binding will trigger the validation of CountryId which would then highlight our combobox that there’s something wrong with it.
An alternative approach would be to define a custom rule which would check the value of Country property for null:

    public class Partner
    {
        [Key]
        public int Id { get; set; }
        
        public int? CountryId { get; set; }
		
	[CustomValidation(typeof(Validation),"ValidateAssociation")]
	public Country Country { get; set; }

    }

    public class Country
    {
        [Key]
        public int? Id { get; set; }
    }

This way, you could leave only the SelectedItem inside combobox and have full validation. My approach was the latter one, but i hid the custom validation behind some nice fluent interface so it’s easy to use.

Ok so now to the problem of original values not validating properly….
As i shown before, this is mainly the problem because dataform and controls rely on bindings to update the source and do validation consequently. Validation is not done if value never changes. One of the quick fixes you could introduce is to hook on the DataForm’s ValidatingItem event and do the following:

var entity= currentItem as Entity;
var context = new ValidationContext(entity, null, null);
var validationResults = new List<ValidationResult>();
Validator.TryValidateObject(entity, context, validationResults, validateAllProperties);
entity.ValidationErrors.Clear();
validationResults.ForEach(entity.ValidationErrors.Add);

This will force the validation of entity and it will update the validation bindings. I have this implemented in my SlimForm, i did have to include additional reference to Domainservices.Client assembly, but i have that assembly defined either way in my project, plus i couldn’t find a more elegant solution. In general, there’s no easy way to solve this problem i’m afraid….
Anyway i hope this helps someone, if something’s not clear enough just post a comment. Cheers! The sample project is a bit different than the sample code, but i’ll think you’ll get the picture:)
Combobox sample project with updated SlimForm