The validation engine in DotVVM offers several extensibility points. You can use your own validation attributes, and you can interact with the validation engine on the client-side.

Custom validation attributes

You can build your own validation attributes by implementing the IValidationAttribute class, or inheriting from the ValidationAttribute base class. See Custom attributes page in the official docs.


Each validation view model can implement the IValidatableObject interface, see Validation overview.


DotVVM validates the view model using this interface, and you can replace it by registering your own implementation in DI (as a singleton). This is more advanced way to completely change how automatic server-side validation is performed. It may be useful if you want to use something different from the data annotation attributes to validate your view models. The default implementation is ViewModelValidator.

Client-side validation

Providing a client-side validation for custom attributes is possible by providing a custom implementation of the IValidationRuleTranslator interface, and then by registering a custom rule into dotvvm.validation.rules collection, but the process is quite complicated. We plan to make it easier in the future versions of DotVVM.

It is also possible to add any validation errors client-side. This can be used to implement validation in JS Commands, or to implement custom validation logic before a command is invoked.

The dotvvm.validation.removeErrors("/") function is used to remove existing validation errors. The "/" is the root path of the removal, this argument can be used to clear only certain parts of the view model. Any number of paths can be specified.

The dotvvm.validation.addErrors(errors) adds new errors. The argument should be an array of objects with

  • errorMessage - a string displayed to the user
  • propertyPath - an absolute validation path of the invalid property, e.g. /SomeArray/2/InvalidProperty.
myCommand() {
    const model = dotvvm.state.MyForm
    const errors = []
    if (model.FirstName.length > model.LastName.length)
        errors.push({ errorMessage: "Persons with longer first name than their last name cannot be registered.", propertyPath: "/MyForm/FirstName"})
    if (errors.length > 0)

Validation events

You can subscribe to the client-side events called dotvvm.validation.events.validationErrorsChanged to be notified when the validation error collection is changed. The error collection is in dotvvm.validation.allErrors.

See also