@page "/" @using System.ComponentModel.DataAnnotations <EditForm Model="@userModel" OnValidSubmit="@HandleValidSubmit"> <DataAnnotationsValidator/> <ValidationSummary/> <p> <label for="FirstName">FirstName: </label> <InputText id="FirstName" @bind-Value="userModel.FirstName"/> <ValidationMessage For="() => userModel.FirstName"/> </p> <p> <label for="LastName">LastName: </label> <InputText id="LastName" @bind-Value="userModel.LastName"/> <ValidationMessage For="() => userModel.LastName"/> </p> <p> <label for="Phone">Phone Number: </label> <InputText id="Phone" @bind-Value="userModel.Phone"/> <ValidationMessage For="() => userModel.Phone"/> </p> <p> <label for="CanReceiveText">Receive Text Messages: </label> <InputCheckbox id="CanReceiveText" @bind-Value="userModel.CanReceiveText"/> <ValidationMessage For="() => userModel.CanReceiveText"/> </p> <button type="submit">Submit</button> </EditForm> @code { readonly UserModel userModel = new UserModel(); private void HandleValidSubmit() { // Save the data } public class UserModel { public int Id { get; set; } [Required] [Display(Name = "First Name")] public string FirstName { get; set; } [Required] [Display(Name = "Last Name")] public string LastName { get; set; } [RequiredIfAttribute("CanReceiveText", true)] [Display(Name = "Phone Number")] public string Phone { get; set; } [Required] [Display(Name = "Receive Text Messages")] public bool CanReceiveText { get; set; } } public class RequiredIfAttribute : ValidationAttribute { readonly RequiredAttribute _innerAttribute = new RequiredAttribute(); private string _dependentProperty { get; } private object _targetValue { get; } public RequiredIfAttribute(string dependentProperty, object targetValue) { _dependentProperty = dependentProperty; _targetValue = targetValue; } protected override ValidationResult IsValid(object value, ValidationContext validationContext) { var field = validationContext.ObjectType.GetProperty(_dependentProperty); if (field != null) { var dependentValue = field.GetValue(validationContext.ObjectInstance, null); if ((dependentValue == null && _targetValue == null) || dependentValue.Equals(_targetValue)) { if (!_innerAttribute.IsValid(value)) { var name = validationContext.DisplayName; var specificErrorMessage = ErrorMessage; if (string.IsNullOrEmpty(specificErrorMessage)) specificErrorMessage = $"{name} is required."; return new ValidationResult(specificErrorMessage, new[] { validationContext.MemberName }); } } return ValidationResult.Success; } return new ValidationResult(FormatErrorMessage(_dependentProperty)); } } }