



Introduction:

In Blazor WebAssembly(client-side framework) form validation can be done with Data Annotations.

Using Data Annotations we can validate form either by default validation attributes or by creating custom validation attributes. Using this Data Annotation attribute we can dynamically add or remove validation on a specific field in a form.





Create Blazor WebAssembly Project:

To create a Blazor WebAssembly template project need to install the latest version of VisualStudio 2019 for rich intelligence support or we can use VisualStudio code but less intelligence support from the editor. Click here to know about Blazor WebAssembly template creation.

Blazor WebAssembly is in preview mode, not yet ready for production.





Create Razor Component:

Razor Component , name it as "UserForm.razor"

Add Route: After creating a sample project using the Blazor WebAssembly template, in "Pages" folder add new

In Blazor routing can be configured using @page directive, and URL should be enclosed in double-quotes and prefixed with "/". In "UserForm.razor" file add routing as below





@page "/userform"

Create Form Model:

Now we need to create a model that holds data to form or data from a form. Add a folder name it as "Models". Create a class into the "Models" folder and name it as "UserFormModel".cs

public class UserFormModel { public string FirstName { get; set; } public string LastName { get; set; } public string ContactType { get; set; } public string Email { get; set; } public string Phone { get; set; } }

EditForm Razor Component:

"EditForm" Razor component is the default razor component provided by Blazor WebAssembly. It helps to create form and its validations. EditFrom creates an EditContext , that tracks fields modified in the form and tracks validation messages. EditForm built up with default events like OnValidSubmit , OnInvalidSubmit , OnSubmit, etc.

<EditForm OnValidSubmit="@submit"> </EditForm>

DataAnnotationsValidator Razor Component:

DataAnnotationsValidator Razor Component attaches validation support using Data Annotations.

<DataAnnotationsValidator />

ValidationSummary Razor Component:

ValidationSummary Razor Component summarizes validation messages

<ValidationSummary />

ValidationMessage Razor Component:

ValidationMessage Razor Component, displays validation message specific field in the form.

<ValidationMessage For="@(() => model.propertyName)" />

Default Form Field Razor Component:

To apply validation logic to form fields Blazor has implemented default Razor Components:

1. "InputText" component for Html Input filed

2. "InputTextArea" component for Html Textarea field

3. "InputSelect" component for Html Select field

Design UserForm:

Update the "UserForm.razor" as follows

@page "/userform" @using Model <h3>UserForm</h3> <EditForm Model="@user" OnValidSubmit="@Submit"> <DataAnnotationsValidator></DataAnnotationsValidator> <div class="form-group row"> <label for="txtFirstName" class="col-sm-2 col-form-label">First Name</label> <div class="col-sm-10"> <InputText id="txtFirstName" class="form-control" @bind-Value="@user.FirstName"></InputText> <ValidationMessage For="@(() => user.FirstName)"></ValidationMessage> </div> </div> <div class="form-group row"> <label for="txtLastName" class="col-sm-2 col-form-label">Last Name</label> <div class="col-sm-10"> <InputText id="txtLastName" class="form-control" @bind-Value="@user.LastName"></InputText> <ValidationMessage For="@(() => user.LastName)"></ValidationMessage> </div> </div> <div class="form-group row"> <label for="txtContactType" class="col-sm-2 col-form-label">Contact Type</label> <div class="col-sm-10"> <InputSelect id="txtContactType" class="form-control" @bind-Value="@user.ContactType"> <option selected value="-1">-Select-</option> <option value="email">Email</option> <option value="phone">Phone</option> </InputSelect> <ValidationMessage For="@(() => user.ContactType)"></ValidationMessage> </div> </div> <div class="form-group row"> <label for="txtEmail" class="col-sm-2 col-form-label">Email</label> <div class="col-sm-10"> <InputText id="txtEmail" class="form-control" @bind-Value="@user.Email"></InputText> <ValidationMessage For="@(() => user.Email)"></ValidationMessage> </div> </div> <div class="form-group row"> <label for="txtPhone" class="col-sm-2 col-form-label">Phone</label> <div class="col-sm-10"> <InputText id="txtPhone" class="form-control" @bind-Value="@user.Phone"></InputText> <ValidationMessage For="@(() => user.Phone)"></ValidationMessage> </div> </div> <div class="form-group"> <button type="submit" class="btn btn-primary">Save</button> </div> </EditForm> @code { UserFormModel user; protected override void OnInitialized() { user = new UserFormModel(); } public void Submit() { // this method calls on valid form // save form data by posting data to your api } }

user

which will be used for form model binding.

. OnInitialized is Blazor life-cycle method, where we can initiate a code before rendering Html.

. Submit will be triggered from the submit button if the form is valid.

. Model property on the EditForm component to bind the model to the form.

. OnValidSubmit event of EditForm component gets triggered on submit click if the form is valid.

. DataAnnotationsValidator component to activate Data Annotation validation functionality to the form





Now run the application and navigate to "

Required Field Validation:

Now make "First Name" and "Last Name" as mandatory fields in the form. For that, we can apply the Data Annotations attribute [Required] to make those fields mandatory. Update code in "UserFormModel.cs" as below

using System; using System.Collections.Generic; using System.ComponentModel.DataAnnotations; using System.Linq; using System.Threading.Tasks; namespace FormValidation.Model { public class UserFormModel { [Required] public string FirstName { get; set; } [Required] public string LastName { get; set; } public string ContactType { get; set; } public string Email { get; set; } public string Phone { get; set; } } }

Dynamic Custom Validation:

. ContactType is "Email", then we need to validate the email field, no need to validate the phone field.

. ContactType is "Phone", then we need to validate the phone field, no need to validate the email field.

. ContacType is "-Select-" then no validation phone and email fields





Now add a new folder name as "Validations" and add a file name it as "DynamicContactValidationAttribute.cs"

using System; using System.Collections.Generic; using System.ComponentModel.DataAnnotations; using System.Linq; using System.Text.RegularExpressions; using System.Threading.Tasks; namespace FormValidation.Validations { public class DynamicContactValidationAttribute:ValidationAttribute { private readonly string _parentFieldName; private readonly string _fieldType; private readonly string[] _validationTypes; public DynamicContactValidationAttribute( string ParentFieldName, string FieldType, string[] ValidationTypes) { _parentFieldName = ParentFieldName; _fieldType = FieldType; _validationTypes = ValidationTypes; } protected override ValidationResult IsValid(object value, ValidationContext validationContext) { if(validationContext.ObjectInstance != null) { var parentFieldValueObject = validationContext.ObjectInstance.GetType() .GetProperty(_parentFieldName).GetValue(validationContext.ObjectInstance, null); string parentFieldValue = parentFieldValueObject != null ? parentFieldValueObject as string : string.Empty; string currentFieldValue = value != null ? value as string : string.Empty; if (!string.IsNullOrEmpty(parentFieldValue) && parentFieldValue.ToLower() == _fieldType.ToLower()) { if (string.IsNullOrEmpty(currentFieldValue) && _validationTypes.Any(_ => _.ToLower() == "required")) { return new ValidationResult($"{validationContext.DisplayName} is requied", new[] { validationContext.MemberName }); } else if (_validationTypes.Any(_ => _.ToLower() == "email")) { bool isEmail = Regex.IsMatch(currentFieldValue, @"\A(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?)\Z", RegexOptions.IgnoreCase); if (!isEmail) { return new ValidationResult($"{validationContext.DisplayName} is not a valid email", new[] { validationContext.MemberName }); } } else if(_validationTypes.Any(_ => _.ToLower() == "phone")) { bool isPhone = Regex.IsMatch(currentFieldValue, @"\+?[0-9]{10}"); if (!isPhone) { return new ValidationResult($"{validationContext.DisplayName} is not a valid phone", new[] { validationContext.MemberName }); } } } } return ValidationResult.Success; } } }

Update Model To Use Custom Validation:

update the code in file "UserFormModel.cs" as below

public class UserFormModel { [Required] public string FirstName { get; set; } [Required] public string LastName { get; set; } public string ContactType { get; set; } [DynamicContactValidation(ParentFieldName: "ContactType",FieldType:"Email",ValidationTypes:new[] { "required","email" })] public string Email { get; set; } [DynamicContactValidation(ParentFieldName: "ContactType", FieldType: "Phone", ValidationTypes: new[] { "required", "phone" })] public string Phone { get; set; } }

Refer:

Create a UserFormModel variable as ahttp://localhost:57213/userform"Now navigate to the application "http://localhost:57213/userform", click on submit button"ValidationAttribute" inherit to override "IsValid(object value, ValidationContext validationContext)"To this custom validator pass parameters like "ParentFieldName(ContactType field)","FieldType(current filed where this custom validation attribute configured either email field or phone field)","ValidationTypes(like extra validation like email,phone, etc)""ValidationContext" provide all the information about current property and model of current propety, it tracks the validation errors and also tracks property value changes."ValidationResult" takes error mesage and field name to which that error message refer to as input parameters.Now run application and change ContactType to "Email" and leave email field empty then error show email requiredNow select ContactType to "Email" and enter invalid email then the error shows as invalid emailSimilarly test for ContactType as "Phone"