> For the complete documentation index, see [llms.txt](https://jen-hsuan-hsieh.gitbook.io/asp-net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jen-hsuan-hsieh.gitbook.io/asp-net/chapter1/1.1.the-complete-asp.net-mvc-5-course/15implement-validation/152data-annotation.md).

# 1.5.2.Data annotation

* 1.Add new class inherit ValidationAttribute

```
    public class Min18YearsIfAMember: ValidationAttribute
    {
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            var customer = (Customer)validationContext.ObjectInstance;
            if (customer.membershipTypeId == 1)
            {
                return ValidationResult.Success;
            }
            if (customer.Birthday == null)
            {
                return new ValidationResult("Birthdate is required");
            }
            return ValidationResult.Success;
        }
    }
```

* 2.Add data annotation to the model

```
        [Display(Name = "Date or Birth")]
        [Min18YearsIfAMember]
        public string Birthday { get; set; }
```

* 3.Add warning message to the view

```
    <div class = "form-group">
        @Html.LabelFor(m => m.customer.Name)
        @Html.TextBoxFor(m => m.customer.Name, new { @class = "form-control" })
        @Html.ValidationMessageFor(m => m.customer.Name);
    </div>
```