Web Forms Data Annotation Validation – Part 3

This is the third part in my look at Data Annotation validation in Web Forms in the Visual Studio 2012 Release Candidate. In the first part, I showed the server-side support for Data Annotation validation and model binding. In the second, I proposed one approach to providing client-side validation using jQuery plugins and a custom validator control that injects validation rules into the input element’s class attribute. The problem with that approach was an external dependency on the metadata plugin and the need to inject JavaScript into the page. So I got to thinking about how I might piggy-back on the unobtrusive validation support to produce a custom validator that worked in exactly the same way as the existing validators, but where the rules and messages were all derived from Data Annotations.

So the first thing to do was see how the existing validators work. I added various validators to a sample form and took a look at the underlying HTML. This is what I found:

client side unobtrusive attributes

There’s lots of interesting stuff here. The Text becomes the content of the span; the ErrorMessage is in data-val-errormessage; the validation method is in data-val-evaluation function; etc. If I can get my validator to read the Data Annotations and output the appropriate spans, that should give me the client-side validation I want.

I began in the same way as last time, by creating a new ASP.NET Web Control library and inheriting from the base validator. That code is in the previous posting, so I’m not going to repeat it here. (If you want to see the full code, I’ll put a link at the bottom of this post).

The reflection code is also the same, but one thing that is different is that this time I need to render HTML directly from the control, so I have to override the render method:

render method

The first thing I need is template for my span(s). I am going to put in {0} placeholders for the bits that need to change, and reuse the same basic string every time. Here is my template:

private static string template = “<span id=”{0}” data-val-evaluationfunction=”{1}” data-val=”true” data-val-errormessage=”{2}” data-val-controltovalidate=”{3}” {4} {5}>{6}</span>”;

Now, as I loop through the Data Annotations I can use string.Format() to fill in the blanks (note the += on the local spanString variable: that means I can add more spans if there are multiple annotations).

building the string

The arguments to the method are as follows:

  • this.ClientID is my current validator’s Id.
  • vat.ErrorMessage is the ErrorMessage from the Data Annotation.
  • c.ClientID is the control I am validating.
  • The string literals set up the validation for a required field
  • So where do display and message come from?

Display maps to the typical Validator display choices – static, dynamic and none. It’s initialized as style=”display: none;” data-val-display=”None” and overridden as appropriate. The message is the ErrorMessage to display in the control – as potentially overridden by the Text attribute.

message and display variables

So let’s add a reference inside our test web project and then add the new DataAnnotationValidator to the toolbox. Then we need to add a Required attribute to the Email class. (I added “DA” at the end of the message to confirm where the message was coming from).

required attribute

Then we need to set up the properties on our DataAnnotationValidator:

DataAnnotationValidator in aspx source

Now we can test it… and see that it works! We are now running client-side validation from Data Annotations, and it works and displays in exactly the same way as the other validators. And we have no additional dependencies and JavaScript injected into the page. Hurray.

working validation

Of course, we want to be able to validate other attributes as well. I went ahead and implemented several of them using the underlying RegularExpressionValidator. First I created a method to build all the regular expression spans:

regular expression builder

Then I called it from the various case statements for validation attributes:

switching attributes

Not forgetting the easiest one of all: the RegularExpressionAttribute itself:


Now I could play around with different values and messages in my Model….

Data Annotations

….and see my working Web Forms client-side Data Attribute validation:

validation working

I’ll probably refactor for robustness at some point, as well as adding support for additional attributes, but if you’re interested in the code for this version of the DataAnnotation validator, it’s online here: DataAnnotation2.txt.

Kevin Rattan

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