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title author description ms.author ms.custom ms.date monikerRange uid
Model validation in ASP.NET Core MVC
rick-anderson
Learn about model validation in ASP.NET Core MVC and Razor Pages.
riande
mvc
04/06/2019
>= aspnetcore-2.1
mvc/models/validation

Model validation in ASP.NET Core MVC and Razor Pages

This article explains how to validate user input in an ASP.NET Core MVC or Razor Pages app.

View or download sample code (how to download).

Model state

Model state represents errors that come from two subsystems: model binding and model validation. Errors that originate from model binding are generally data conversion errors (for example, an "x" is entered in a field that expects an integer). Model validation occurs after model binding and reports errors where the data doesn't conform to business rules (for example, a 0 is entered in a field that expects a rating between 1 and 5).

Both model binding and validation occur before the execution of a controller action or a Razor Pages handler method. For web apps, it's the app's responsibility to inspect ModelState.IsValid and react appropriately. Web apps typically redisplay the page with an error message:

[!code-csharp]

Web API controllers don't have to check ModelState.IsValid if they have the [ApiController] attribute. In that case, an automatic HTTP 400 response containing issue details is returned when model state is invalid. For more information, see Automatic HTTP 400 responses.

Rerun validation

Validation is automatic, but you might want to repeat it manually. For example, you might compute a value for a property and want to rerun validation after setting the property to the computed value. To rerun validation, call the TryValidateModel method, as shown here:

[!code-csharp]

Validation attributes

Validation attributes let you specify validation rules for model properties. The following example from the sample app shows a model class that is annotated with validation attributes. The [ClassicMovie] attribute is a custom validation attribute and the others are built-in. (Not shown is [ClassicMovie2], which shows an alternative way to implement a custom attribute.)

[!code-csharp]

Built-in attributes

Here are some of the built-in validation attributes:

  • [CreditCard]: Validates that the property has a credit card format.
  • [Compare]: Validates that two properties in a model match.
  • [EmailAddress]: Validates that the property has an email format.
  • [Phone]: Validates that the property has a telephone number format.
  • [Range]: Validates that the property value falls within a specified range.
  • [RegularExpression]: Validates that the property value matches a specified regular expression.
  • [Required]: Validates that the field is not null. See [Required] attribute for details about this attribute's behavior.
  • [StringLength]: Validates that a string property value doesn't exceed a specified length limit.
  • [Url]: Validates that the property has a URL format.
  • [Remote]: Validates input on the client by calling an action method on the server. See [Remote] attribute for details about this attribute's behavior.

A complete list of validation attributes can be found in the System.ComponentModel.DataAnnotations namespace.

Error messages

Validation attributes let you specify the error message to be displayed for invalid input. For example:

[StringLength(8, ErrorMessage = "Name length can't be more than 8.")]

Internally, the attributes call String.Format with a placeholder for the field name and sometimes additional placeholders. For example:

[StringLength(8, ErrorMessage = "{0} length must be between {2} and {1}.", MinimumLength = 6)]

When applied to a Name property, the error message created by the preceding code would be "Name length must be between 6 and 8.".

To find out which parameters are passed to String.Format for a particular attribute's error message, see the DataAnnotations source code.

[Required] attribute

By default, the validation system treats non-nullable parameters or properties as if they had a [Required] attribute. Value types such as decimal and int are non-nullable.

[Required] validation on the server

On the server, a required value is considered missing if the property is null. A non-nullable field is always valid, and the [Required] attribute's error message is never displayed.

However, model binding for a non-nullable property may fail, resulting in an error message such as The value '' is invalid. To specify a custom error message for server-side validation of non-nullable types, you have the following options:

  • Make the field nullable (for example, decimal? instead of decimal). Nullable<T> value types are treated like standard nullable types.

  • Specify the default error message to be used by model binding, as shown in the following example:

    [!code-csharp]

    For more information about model binding errors that you can set default messages for, see xref:Microsoft.AspNetCore.Mvc.ModelBinding.Metadata.DefaultModelBindingMessageProvider#methods.

[Required] validation on the client

Non-nullable types and strings are handled differently on the client compared to the server. On the client:

  • A value is considered present only if input is entered for it. Therefore, client-side validation handles non-nullable types the same as nullable types.
  • Whitespace in a string field is considered valid input by the jQuery Validation required method. Server-side validation considers a required string field invalid if only whitespace is entered.

As noted earlier, non-nullable types are treated as though they had a [Required] attribute. That means you get client-side validation even if you don't apply the [Required] attribute. But if you don't use the attribute, you get a default error message. To specify a custom error message, use the attribute.

[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:

    [!code-csharp]

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

    [!code-csharp]

    The [Remote] attribute is in the Microsoft.AspNetCore.Mvc namespace. Install the Microsoft.AspNetCore.Mvc.ViewFeatures NuGet package if you're not using the Microsoft.AspNetCore.App or Microsoft.AspNetCore.All metapackage.

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:

[!code-csharp]

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 first name and last name arguments:

[!code-csharp]

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:

[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 xref:System.String.Join* to initialize AdditionalFields.

Alternatives to built-in attributes

If you need validation not provided by built-in attributes, you can:

Custom attributes

For scenarios that the built-in validation attributes don't handle, you can create custom validation attributes. Create a class that inherits from xref:System.ComponentModel.DataAnnotations.ValidationAttribute, and override the xref:System.ComponentModel.DataAnnotations.ValidationAttribute.IsValid* method.

The IsValid method accepts an object named value, which is the input to be validated. An overload also accepts a ValidationContext object, which provides additional information, such as the model instance created by model binding.

The following example validates that the release date for a movie in the Classic genre isn't later than a specified year. The [ClassicMovie2] attribute checks the genre first and continues only if it's Classic. For movies identified as classics, it checks the release date to make sure it's not later than the limit passed to the attribute constructor.)

[!code-csharp]

The movie variable in the preceding example represents a Movie object that contains the data from the form submission. The IsValid method checks the date and genre. Upon successful validation, IsValid returns a ValidationResult.Success code. When validation fails, a ValidationResult with an error message is returned.

IValidatableObject

The preceding example works only with Movie types. Another option for class-level validation is to implement IValidatableObject in the model class, as shown in the following example:

[!code-csharp]

Top-level node validation

Top-level nodes include:

  • Action parameters
  • Controller properties
  • Page handler parameters
  • Page model properties

Model-bound top-level nodes are validated in addition to validating model properties. In the following example from the sample app, the VerifyPhone method uses the xref:System.ComponentModel.DataAnnotations.RegularExpressionAttribute to validate the phone action parameter:

[!code-csharp]

Top-level nodes can use xref:Microsoft.AspNetCore.Mvc.ModelBinding.BindRequiredAttribute with validation attributes. In the following example from the sample app, the CheckAge method specifies that the age parameter must be bound from the query string when the form is submitted:

[!code-csharp]

In the Check Age page (CheckAge.cshtml), there are two forms. The first form submits an Age value of 99 as a query string: https://localhost:5001/Users/CheckAge?Age=99.

When a properly formatted age parameter from the query string is submitted, the form validates.

The second form on the Check Age page submits the Age value in the body of the request, and validation fails. Binding fails because the age parameter must come from a query string.

When running with CompatibilityVersion.Version_2_1 or later, top-level node validation is enabled by default. Otherwise, top-level node validation is disabled. The default option can be overridden by setting the xref:Microsoft.AspNetCore.Mvc.MvcOptions.AllowValidatingTopLevelNodes* property in (Startup.ConfigureServices), as shown here:

[!code-csharp]

Maximum errors

Validation stops when the maximum number of errors is reached (200 by default). You can configure this number with the following code in Startup.ConfigureServices:

[!code-csharp]

Maximum recursion

xref:Microsoft.AspNetCore.Mvc.ModelBinding.Validation.ValidationVisitor traverses the object graph of the model being validated. For models that are very deep or are infinitely recursive, validation may result in stack overflow. MvcOptions.MaxValidationDepth provides a way to stop validation early if the visitor recursion exceeds a configured depth. The default value of MvcOptions.MaxValidationDepth is 200 when running with CompatibilityVersion.Version_2_2 or later. For earlier versions, the value is null, which means no depth constraint.

Automatic short-circuit

Validation is automatically short-circuited (skipped) if the model graph doesn't require validation. Objects that the runtime skips validation for include collections of primitives (such as byte[], string[], Dictionary<string, string>) and complex object graphs that don't have any validators.

Disable validation

To disable validation:

  1. Create an implementation of IObjectModelValidator that doesn't mark any fields as invalid.

    [!code-csharp]

  2. Add the following code to Startup.ConfigureServices to replace the default IObjectModelValidator implementation in the dependency injection container.

    [!code-csharp]

You might still see model state errors that originate from model binding.

Client-side validation

Client-side validation prevents submission until the form is valid. The Submit button runs JavaScript that either submits the form or displays error messages.

Client-side validation avoids an unnecessary round trip to the server when there are input errors on a form. The following script references in _Layout.cshtml and _ValidationScriptsPartial.cshtml support client-side validation:

[!code-cshtml]

[!code-cshtml]

The jQuery Unobtrusive Validation script is a custom Microsoft front-end library that builds on the popular jQuery Validate plugin. Without jQuery Unobtrusive Validation, you would have to code the same validation logic in two places: once in the server-side validation attributes on model properties, and then again in client-side scripts. Instead, Tag Helpers and HTML helpers use the validation attributes and type metadata from model properties to render HTML 5 data- attributes for the form elements that need validation. jQuery Unobtrusive Validation parses the data- attributes and passes the logic to jQuery Validate, effectively "copying" the server-side validation logic to the client. You can display validation errors on the client using tag helpers as shown here:

[!code-cshtml]

The preceding tag helpers render the following HTML.

<form action="/Movies/Create" method="post">
    <div class="form-horizontal">
        <h4>Movie</h4>
        <div class="text-danger"></div>
        <div class="form-group">
            <label class="col-md-2 control-label" for="ReleaseDate">ReleaseDate</label>
            <div class="col-md-10">
                <input class="form-control" type="datetime"
                data-val="true" data-val-required="The ReleaseDate field is required."
                id="ReleaseDate" name="ReleaseDate" value="">
                <span class="text-danger field-validation-valid"
                data-valmsg-for="ReleaseDate" data-valmsg-replace="true"></span>
            </div>
        </div>
    </div>
</form>

Notice that the data- attributes in the HTML output correspond to the validation attributes for the ReleaseDate property. The data-val-required attribute contains an error message to display if the user doesn't fill in the release date field. jQuery Unobtrusive Validation passes this value to the jQuery Validate required() method, which then displays that message in the accompanying <span> element.

Data type validation is based on the .NET type of a property, unless that is overridden by a [DataType] attribute. Browsers have their own default error messages, but the jQuery Validation Unobtrusive Validation package can override those messages. [DataType] attributes and subclasses such as [EmailAddress] let you specify the error message.

Add Validation to Dynamic Forms

jQuery Unobtrusive Validation passes validation logic and parameters to jQuery Validate when the page first loads. Therefore, validation doesn't work automatically on dynamically generated forms. To enable validation, tell jQuery Unobtrusive Validation to parse the dynamic form immediately after you create it. For example, the following code sets up client-side validation on a form added via AJAX.

$.get({
    url: "https://url/that/returns/a/form",
    dataType: "html",
    error: function(jqXHR, textStatus, errorThrown) {
        alert(textStatus + ": Couldn't add form. " + errorThrown);
    },
    success: function(newFormHTML) {
        var container = document.getElementById("form-container");
        container.insertAdjacentHTML("beforeend", newFormHTML);
        var forms = container.getElementsByTagName("form");
        var newForm = forms[forms.length - 1];
        $.validator.unobtrusive.parse(newForm);
    }
})

The $.validator.unobtrusive.parse() method accepts a jQuery selector for its one argument. This method tells jQuery Unobtrusive Validation to parse the data- attributes of forms within that selector. The values of those attributes are then passed to the jQuery Validate plugin.

Add Validation to Dynamic Controls

The $.validator.unobtrusive.parse() method works on an entire form, not on individual dynamically generated controls, such as <input> and <select/>. To reparse the form, remove the validation data that was added when the form was parsed earlier, as shown in the following example:

$.get({
    url: "https://url/that/returns/a/control",
    dataType: "html",
    error: function(jqXHR, textStatus, errorThrown) {
        alert(textStatus + ": Couldn't add control. " + errorThrown);
    },
    success: function(newInputHTML) {
        var form = document.getElementById("my-form");
        form.insertAdjacentHTML("beforeend", newInputHTML);
        $(form).removeData("validator")    // Added by jQuery Validate
               .removeData("unobtrusiveValidation");   // Added by jQuery Unobtrusive Validation
        $.validator.unobtrusive.parse(form);
    }
})

Custom client-side validation

Custom client-side validation is done by generating data- HTML attributes that work with a custom jQuery Validate adapter. The following sample adapter code was written for the ClassicMovie and ClassicMovie2 attributes that were introduced earlier in this article:

[!code-javascript]

For information about how to write adapters, see the jQuery Validate documentation.

The use of an adapter for a given field is triggered by data- attributes that:

  • Flag the field as being subject to validation (data-val="true").
  • Identify a validation rule name and error message text (for example, data-val-rulename="Error message.").
  • Provide any additional parameters the validator needs (for example, data-val-rulename-parm1="value").

The following example shows the data- attributes for the sample app's ClassicMovie attribute:

<input class="form-control" type="datetime"
    data-val="true"
    data-val-classicmovie1="Classic movies must have a release year earlier than 1960."
    data-val-classicmovie1-year="1960"
    data-val-required="The ReleaseDate field is required."
    id="ReleaseDate" name="ReleaseDate" value="">

As noted earlier, Tag Helpers and HTML helpers use information from validation attributes to render data- attributes. There are two options for writing code that results in the creation of custom data- HTML attributes:

  • Create a class that derives from AttributeAdapterBase<TAttribute> and a class that implements IValidationAttributeAdapterProvider, and register your attribute and its adapter in DI. This method follows the single responsibility principal in that server-related and client-related validation code is in separate classes. The adapter also has the advantage that since it is registered in DI, other services in DI are available to it if needed.
  • Implement IClientModelValidator in your ValidationAttribute class. This method might be appropriate if the attribute doesn't do any server-side validation and doesn't need any services from DI.

AttributeAdapter for client-side validation

This method of rendering data- attributes in HTML is used by the ClassicMovie attribute in the sample app. To add client validation by using this method:

  1. Create an attribute adapter class for the custom validation attribute. Derive the class from AttributeAdapterBase<T>. Create an AddValidation method that adds data- attributes to the rendered output, as shown in this example:

    [!code-csharp]

  2. Create an adapter provider class that implements xref:Microsoft.AspNetCore.Mvc.DataAnnotations.IValidationAttributeAdapterProvider. In the GetAttributeAdapter method pass in the custom attribute to the adapter's constructor, as shown in this example:

    [!code-csharp]

  3. Register the adapter provider for DI in Startup.ConfigureServices:

    [!code-csharp]

IClientModelValidator for client-side validation

This method of rendering data- attributes in HTML is used by the ClassicMovie2 attribute in the sample app. To add client validation by using this method:

  • In the custom validation attribute, implement the IClientModelValidator interface and create an AddValidation method. In the AddValidation method, add data- attributes for validation, as shown in the following example:

    [!code-csharp]

Disable client-side validation

The following code disables client validation in MVC views:

[!code-csharp]

And in Razor Pages:

[!code-csharp]

Another option for disabling client validation is to comment out the reference to _ValidationScriptsPartial in your .cshtml file.

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