Implement a Custom Validation Rule

If the built-in validation rules do not meet your requirements, implement a custom one. These are the steps for you to follow.

In ASP.NET MVC 3, 4, 5, create a class that inherits from the ValidationAttribute and IClientValidatable. In this class, override the IsValid method and implement the GetClientValidationRules method. Note that the latter method returns an object of the ModelClientValidationRule type. The ValidationType property of this object should be set to "custom", and the ValidationParameters collection should be given a new entry called "validationcallback", which binds a JavaScript function to the validation rule. This function will be declared in step 3.

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
namespace ApplicationName.Models
{
    public class VerifyAgeAttribute : ValidationAttribute, IClientValidatable
    {
        public VerifyAgeAttribute()
            : base("The value of the {0} field is not valid") {
        }
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            DateTime? dateToCheck = (DateTime?)value;
            if(dateToCheck <= DateTime.Now.AddYears(-21)) {
                return ValidationResult.Success;
            }
            return new ValidationResult(this.FormatErrorMessage(validationContext.DisplayName));
        }
        public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata,
            ControllerContext context)
        {
            var rule = new ModelClientValidationRule();
            rule.ErrorMessage = FormatErrorMessage(metadata.GetDisplayName());
            // Binds the "verifyAge" JavaScript function to the validation rule
            rule.ValidationParameters.Add("validationcallback", "verifyAge");
            // "ValidationType" should always be "custom"
            rule.ValidationType = "custom";
            yield return rule;
        }
    }
}

Imports System.Collections.Generic
Imports System.ComponentModel.DataAnnotations
Imports System.Web.Mvc

Namespace Models
    Public Class VerifyAgeAttribute
        Inherits ValidationAttribute
        Implements IClientValidatable
        Public Sub New()
            MyBase.New("The value of the {0} field is not valid")
        End Sub
        Protected Overrides Function IsValid(value As Object, validationContext As ValidationContext) As ValidationResult
            Dim DateToCheck As Date? = CType(value, Date?)
            If DateToCheck <= Now.AddYears(-21) Then
                Return ValidationResult.Success
            End If
            Return New ValidationResult(FormatErrorMessage(validationContext.DisplayName))
        End Function
        Public Iterator Function IClientValidatable_GetClientValidationRules(metadata As ModelMetadata, context As ControllerContext)
                        As IEnumerable(Of ModelClientValidationRule) Implements IClientValidatable.GetClientValidationRules
            Dim Rule = New ModelClientValidationRule()
            Rule.ErrorMessage = FormatErrorMessage(metadata.GetDisplayName())
            ' Binds the "verifyAge" JavaScript function to the validation rule
            Rule.ValidationParameters.Add("validationcallback", "verifyAge")
            ' "ValidationType" should always be "custom"
            Rule.ValidationType = "custom"
            Yield Rule
        End Function
    End Class
End Namespace

In ASP.NET Core MVC, create a class that inherits from the ValidationAttribute and IClientModelValidator. In this class, implement the following methods.

AddValidation
Adds the custom validation attribute to the collection of validation attributes.
MergeAttribute
Checks that the key of the custom validation attribute is unique within the collection of validation attributes. Note that the key must begin with the "data-val-custom-" prefix. The key that ends with "validationcallback" must point to a JavaScript function implementing the validation logic. This function will be declared in step 3.
IsValid
Validates data on the server to insure it against invalid values when JavaScript is switched off on the client.

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using Microsoft.AspNetCore.Mvc.ModelBinding.Validation;
namespace ApplicationName.Models
{
    public class VerifyAgeAttribute : ValidationAttribute, IClientModelValidator
    {
        public VerifyAgeAttribute()
            : base("The value of the {0} field is not valid") {
        }
        public void AddValidation(ClientModelValidationContext context) {
            MergeAttribute(context.Attributes, "data-val-custom-verifyage",
                    FormatErrorMessage(context.ModelMetadata.GetDisplayName()));
            MergeAttribute(context.Attributes, "data-val-custom-verifyage-validationcallback", "verifyAge");
        }
        bool MergeAttribute(IDictionary<string, string> attributes, string key, string value) {
            if(attributes.ContainsKey(key)) {
                return false;
            }
            attributes.Add(key, value);
            return true;
        }
        protected override ValidationResult IsValid(object value, ValidationContext validationContext) {
            DateTime? date = (DateTime?)value;
            if(date <= DateTime.Now.AddYears(-21)) {
                return ValidationResult.Success;
            }
            return new ValidationResult(this.FormatErrorMessage(validationContext.DisplayName));
        }
    }
}

Attach the custom attribute to a model property.

using System.ComponentModel.DataAnnotations;
namespace ApplicationName.Models {
    public class Person {
        // ...
        [VerifyAge(ErrorMessage = "Persons under 21 are not allowed")]
        public DateTime? BirthDate { get; set; }
    }
}

Imports System.ComponentModel.DataAnnotations
Namespace Models
    Public Class Person
        ' ...
        <VerifyAge(ErrorMessage:="Persons under 21 are not allowed")>
        Public Property BirthDate() As Date?
    End Class
End Namespace

In the view, declare a JavaScript function that implements all validation logic.

<script>
    function verifyAge(options) {
        return options.value && new Date(Date.now()).getFullYear() - options.value.getFullYear() >= 21;
    }
</script>

To use the custom validation rule, create a DevExtreme editor for the model property from step 2 in the same view where the JavaScript function is declared. The following code shows how to create the DevExtreme NumberBox.

Razor C#

Razor VB

@model ApplicationName.Models.Person

@(Html.DevExtreme().DateBoxFor(model => model.BirthDate))

@ModelType ApplicationName.Models.Person

@(Html.DevExtreme().DateBoxFor(Function(model) model.BirthDate))

View Demo

Implement a Custom Validation Rule

See Also