Skip to main content

Custom DataAnnotation validation on multiple properties

There are a few cases where you want to be able to validate a property value based on the value of another property. As always there are a bunch of different solutions to this problem, but the cleanest way of doing so is to create a custom DataAnnotaion validation attribute.

It is as easy as creating a class derived from ValidationAttribute and overriding the IsValid-method, but instead of the usual IsValid(object value) we will be overriding the IsValid(object value, ValidationContext validationContext) method.

The ValidationContext contains information about the current object beeing validated and makes it possible to get the values of related properties. By including the related properties and values in our validation class constructor. All in all, it could look like this, even though the validation logic does not really make sense in a real world context it explains the idea pretty good.

[AttributeUsage(AttributeTargets.Property)]
public class CustomValidationAttribute : ValidationAttribute
{
    public string PropertyName { get; private set; }

    public CustomValidationAttribute(string propertyName)
    {
        PropertyName = propertyName;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if(validationContext == null)
            return null;

        var property = validationContext.ObjectType.GetProperty(PropertyName);
        if (property == null)
            return new ValidationResult(string.Format(CultureInfo.CurrentCulture, "Unknown property " + PropertyName));
            
        var propertyValue = property.GetValue(validationContext.ObjectInstance, null);
        if(string.IsNullOrWhiteSpace((string)propertyValue))
            return new ValidationResult(string.Format(CultureInfo.CurrentCulture, PropertyName + " value is null"));

        if (string.IsNullOrWhiteSpace((string)value))
            return new ValidationResult(ErrorMessage);

        return null;
    }
}

When using the attribute we will simply atach it to the property with the related property name as an argument.

[CustomValidation("FirstName", ErrorMessage = "FirstName must not be null or empty")]
public string LastName { get; set; }

Using this technique it is easy to create extremely flexible and advanced validation based on several property values on any object.

Comments

Popular posts from this blog

Binding Enum with DescriptionAttribute in WPF

Binding an enumeration to a ComboBox can be done in several ways. In most cases you don't want to display the value itself, but a more user friendly description. One common approach is to use the DescriptionAttribute on the Enum values to supply a description for each value.  This is all possible in a very MVVM friendly way. First step is to add the  DescriptionAttribute  to the values of the enumeration. public enum MyValues { [Description("First value")] First, [Description("Second value")] Second } To retrieve the description from the enum we use a simple extension method. This method returns the value of the DescriptionAttribute if it exists, otherwise the string representation of the enum value is returned. public static string GetDescription(this Enum value) { var fieldInfo = value.GetType().GetField(value.ToString()); var attribute = fieldInfo.GetCustomAttributes(typeof(DescriptionAttribute), false).FirstOrDefault() as

User.Identity returns old login name after name change

When a person gets married or makes a name change for some other reason this usually means that the login name for the Active Directory-account changes as well. This is rarely a problem, but it turned out to cause some issues on our web server, where the  User.Identity  property was still returning the old login name. The user logged on with the new login name, but was identified by the web application as the old login name. The reason this occurs is because the  User.Identity  property relies on the  LsaLookupSids  method to convert the user SID to a login name. The method first calls the local  LSA-cache , which is not synchronized with the Active Directory. For this purpose a simple reboot of the web server to clear the  LSA-cache  propably would have sufficed. But since we didn't want to take the application offline rebooting was not an option. Instead, it is possible to set the registry value  LsaLookupCacheMaxSize in HKLM\SYSTEM\CurrentControlSet\Control\Lsa. If this val

Programming AD with C#.NET – part 4

Our transition to the  System.DirectoryServices.Protocols  has in the whole gone very smooth, but there have been some issues with one environment that contains subdomains. Most things are working fine, but writing to a subdomain does not work in the same way as it did before. What is generally bad with the  System.DirectoryServices.Protocols is the documentation, which is practically non-existent. But most things can  be figured out anyway since most classes just are wrappers for the wldap32.dll, which in turn is way better documented. I would like to have as little bindings to a specific server as possible but still be able to access the domain. In the  LdapConnection  it is possible to set the identifier to null and use the executing computer as a starting point to find a domain controller. But sometimes I must know that I am using a Global Catalog, and with more and more RODC in the environment I sometimes must know that I am working against a writeable domain controller.