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The API of action filters has been changed in DotVVM 1.1. See the Upgrading from DotVVM 1.0 page for more information.

In large apps and sites, you need to do apply global actions e.g. for each button click on a specific page, section or even on all pages in the app.

You might need to do global exception handling and logging, or to switch the culture based on a value from cookies etc

In DotVVM, we have a concept of filters for this purpose. If you know Action Filters in ASP.NET MVC or ASP.NET Web API, it is the same.

Action Filters

If you want to apply a common logic to one or more viewmodels, viewmodel commands or a whole application, you have to create a class that derives from ActionFilterAttribute.

You can then override any of the method listed below.

  • Page-level Events

    • OnPageLoadingAsync is executed immediately after the URL is mapped to a specific route and the viewmodel instance is created.
    • OnPageLoadedAsync is executed after the response is rendered completely.
    • OnPageExceptionAsync is executed when an unhandled exception occurs during the HTTP request processing.
  • ViewModel-level Events

    • OnViewModelCreatedAsync is executed when the viewmodel instance is created.
    • OnViewModelSerializingAsync is executed before the viemwodel is serialized to JSON and sent to the client.
    • OnViewModelDeserializedAsync is executed on postbacks, after the viewmodel from the client was deserialized.
  • Command-level Events

    • OnCommandExecutingAsync is executed on postbacks, before the command referenced from a command binding is called.
    • OnCommandExecutedAsync is executed on postbacks, after the command referenced from a command binding is called.

There is also a class called ExceptionFilterAttribute which adds another event:

  • OnCommandException is executed on postbacks, when the command referenced from a command binding throws an exception.

If you only need to target specific events, you don't need to inherit from these attributes. You can implement the IPageActionFilter, ICommandActionFilter or IViewModelActionFilter interface instead.

Model Validation using Filters

If you want to perform the model validation before every command, it is not difficult. Actually, DotVVM already include such filter and registers it for all requests by default.

Here is how it's implemented:

using System.Threading.Tasks;
using DotVVM.Framework.Hosting;
using DotVVM.Framework.ViewModel.Validation;
using Microsoft.Extensions.DependencyInjection;

namespace DotVVM.Framework.Runtime.Filters
    /// <summary>
    /// Runs the model validation and returns the errors if the viewModel is not valid.
    /// </summary>
    public class ModelValidationFilterAttribute : ActionFilterAttribute

        protected internal override Task OnCommandExecutingAsync(IDotvvmRequestContext context, ActionInfo actionInfo)
            if (!string.IsNullOrEmpty(context.ModelState.ValidationTargetPath))
                var validator = context.Services.GetService<IViewModelValidator>();

            return Task.FromResult(0);

In the OnCommandExecutingAsync method we have to check whether the ValidationTargetPath is set or not. If not, then the validation was disabled on the control which fired the postback.

We need to perform the validation and call FailOnInvalidModelState, which throws an exception and return the model errors to the client, if there are any. If the viewmodel is valid, it doesn't do anything.

DotVVM serializes the validation errors and displays the validation errors in the page.

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