Model validation in ASP.NET Core MVC and Razor Pages

[Remote] attribute

The [Remote] attribute implements client-side validation that requires calling a method on the server to determine whether field input is valid. For example, the app may need to verify whether a user name is already in use.

To implement remote validation:

  1. Create an action method for JavaScript to call. The jQuery Validate remote method expects a JSON response:

    • true means the input data is valid.
    • false, undefined, or null means the input is invalid. Display the default error message.
    • Any other string means the input is invalid. Display the string as a custom error message.

    Here's an example of an action method that returns a custom error message:

    C#
  • [AcceptVerbs("GET", "POST")]
public IActionResult VerifyEmail(string email)
{
    if (!_userService.VerifyEmail(email))
    {
        return Json($"Email {email} is already in use.");
    }

    return Json(true);
}

  • In the model class, annotate the property with a [Remote] attribute that points to the validation action method, as shown in the following example:

    C#
[Remote(action: "VerifyEmail", controller: "Users")]
public string Email { get; set; }

The [Remote] attribute is in the Microsoft.AspNetCore.Mvc namespace.

 

Additional fields

The AdditionalFields property of the [Remote] attribute lets you validate combinations of fields against data on the server. For example, if the User model had FirstName and LastName properties, you might want to verify that no existing users already have that pair of names. The following example shows how to use AdditionalFields:

C#
[Remote(action: "VerifyName", controller: "Users", AdditionalFields = nameof(LastName))]
[Display(Name = "First Name")]
public string FirstName { get; set; }

[Remote(action: "VerifyName", controller: "Users", AdditionalFields = nameof(FirstName))]
[Display(Name = "Last Name")]
public string LastName { get; set; }

AdditionalFields could be set explicitly to the strings "FirstName" and "LastName", but using the nameof operator simplifies later refactoring. The action method for this validation must accept both firstName and lastName arguments:

C#
[AcceptVerbs("GET", "POST")]
public IActionResult VerifyName(string firstName, string lastName)
{
    if (!_userService.VerifyName(firstName, lastName))
    {
        return Json($"A user named {firstName} {lastName} already exists.");
    }

    return Json(true);
}

When the user enters a first or last name, JavaScript makes a remote call to see if that pair of names has been taken.

To validate two or more additional fields, provide them as a comma-delimited list. For example, to add a MiddleName property to the model, set the [Remote] attribute as shown in the following example:

C#
[Remote(action: "VerifyName", controller: "Users", AdditionalFields = nameof(FirstName) + "," + nameof(LastName))]
public string MiddleName { get; set; }

AdditionalFields, like all attribute arguments, must be a constant expression. Therefore, don't use an interpolated string or call Join to initialize AdditionalFields.