Validation controls

After you defined the validation rules and specified the validation target, you need to specify how the validation errors will be presented to the user.


The easiest way is usually to display a list of all validation errors in the viewmodel. You can use the ValidationSummary control.

<dot:ValidationSummary />

Because of performance reasons, the ValidationSummary control displays only the errors attached to its Validation.Target properties, and doesn't look in its children.

If you want the ValidationSummary to show all the errors from the child objects in the validation target, you can set the IncludeErrorsFromChildren property to true.

Some validation errors have no reasonable property to attach to (for example, we don't know from a generic error reporting that the user account could not be created, which property in the viewmodel caused the issue). In such cases, the ModelState errors are attached to the entire validation target object.

If you want to display the errors attached to the Validation.Target object itself, set IncludeErrorsFromTarget to true.

Validator control

The second options is to use the Validator to display an error attached to a particular viewmodel property.

<dot:TextBox Text="{value: NewTaskTitle}" />
<dot:Validator Value="{value: NewTaskTitle}">*</dot:Validator>

This will display the * character when the property contains invalid value. The Value property contains a value binding to a property which is being validated.

The Validator control has several properties that let you set how the error is reported. You can combine them as you need:

  • HideWhenValid - set to false if you need this control to remain visible even when the field is valid. By default, the control is hidden when the field is valid.

  • InvalidCssClass - the CSS class specified in this property will be set to this control when the field is not valid.

  • ShowErrorMessageText - the text of the error message will be displayed inside this control.

  • SetToolTipText - the text of the error message will be set as the title attribute of the control.

Attached properties

In some cases, you may need to apply the behavior on existing elements in the page.

For example, if a property is not valid, you need to apply a CSS class to a div element which wraps the form field:

<div Validator.InvalidCssClass="has-error" 
     Validator.Value="{value: FirstName}">
    First name:
    <dot:TextBox Text="{value: FirstName}" />

The validation behavior will be applied only on elements which has the Validator.Value property set. Unless you set this property on some element, nothing will happen.

Additionally, you need to use the Validator.HideWhenValid, Validator.InvalidCssClass, Validator.ShowErrorMessageText, or Validator.SetToolTipText properties to define what will happen to the element.

These properties are inherited to the child elements, so you may set them once on the form or page level.

<!-- we specify the behavior of validation globally for the entire form -->
<form Validator.InvalidCssClass="has-error">    

    <div Validator.Value="{value: FirstName}">  <!-- the Validator.Value marks the element which will get the invalid CSS class -->
        First Name: <dot:TextBox Text="{value: FirstName}" />
    <div Validator.Value="{value: LastName}">
        Last Name: <dot:TextBox Text="{value: LastName}" />


If you want to set the Validation.InvalidCssClass property for the entire application, you can apply it on the <body> element in the master page. You can of course override the CSS class on any child element.

See also