Multi-property Custom Validation Attributes

I am going to expand on last week’s Simple Custom Validation Attributes with custom validation attributes that need to look at more than a single property of a model. Again using the contact application example, lets say that if a contact has a country of USA then state and zip code are required.

As before we have class which inherits from ValidationAttribute. IsValid is still overridden, but it is a different overload than the previous example that provides access to the ValidationContext, which in our case is the contact model that is being validated.

For this example I have added a constructor that takes the name of the country property in the model being validated and a string array of the countries that need to make sure the state and zip code are filled in. Both parameters are stored in class level variables for use in the IsValid function.

The IsValid function is still where all the work takes place. The overload of IsValid we are working with provides the value of the property being validated and the validation context that validation is running for. The validation context will allow us to access other properties on the model being validated.

The first step to checking a different property than the one being validated is to get the property info from the validation context by using context.ObjectType.GetProperty and passing the property name that was passed in via the constructor. If GetProperty returns null then the context did not contain the property that was expected. In this case a new ValidationResult is returned with a message that the expected country property was not found.

Using the property info for the country property we can call GetValue with context.ObjectInstance to get the contact’s country. If that country is found in the string array of countries that require the property being validated then a check is run to make sure the property has a value and is not blank.

If the property does not have a value then a new ValidationResult is returned with an error message. If the property does have a value then null is returned indicating that the property is valid.

Example usage from my contact model:

One other thing to note is the usage of context.DisplayName when creating the ValidationResult. By using display name in the message the ZipCode property will show as “Zip Code” in the message returned.

Simple Custom Validation Attributes

Last week I did an overview of  Data Annotations. This week I am going to expand on data annotations by giving an example of a very simple custom validation attribute.

Sticking with my contact application theme lets say that I only wanted to allow contacts from a very small list of countries. The .Net framework does not have a built in validation attribute to cover this scenario which means I would need to create my own custom validation attribute to fill this need.

To create a custom validation attribute all you need is a class that inherits from ValidationAttribute and overrides the IsValid function.

When validation for a property is performed the IsValid function of all the property’s validation attributes are run. The IsValid function is called with the value of the property being validated which will have to be cast to the type needed for validation. In my country example value is being cast to string and used see if the value is found in a list of valid countries.

Any property with the ValidCountry attribute would not validate unless the property was set to USA, Canada or Mexico. This is a contrived example, but it does give you an idea of how easy it is to add your own custom validation attributes.

Data Annotations

Validation via data annotations in .NET is a great feature that is simple to implement. Here is an example model class that is using data annotations.

First step in using data annotations is to add a using statement for the System.ComponentModel.DataAnnotations namespace. This namespace allows you to add validation attributes to the properties that require validation and contains helper classes to run validate on attributed models.

In the example above Required, StringLength and Display are all validation attributes. For the most part the attributes are self-explanatory, but be aware most attributes also have options to give you more control of the validation. For example required attribute has an AllowEmptyStrings property that if set to true will allow an empty string to pass validation. String length is another example which has options for minimum and maximum lengths for the property being validated.

This is just a small sample of the built-in attributes that exist. Other built-in attributes range  from credit card and email address validation to file extension and range validation. If a built-in validation does not meet your needs then a custom attribute a can be created by deriving from ValidationAttribue.

MVC has a lot of great functionality built around data annotations. For instance as long as a client has JavaScript turned a form will not allow submission until the model passes all the validation conditions. On the server-side inside of a controller to run validation all it takes is to run ModelState.IsValid.

With winforms the story is not quite as simple, but still pretty straight forward. The following is a simple example of calling validation on a model.

Validator.TryValidateObject takes the model to be validated, a validation context (created with the model to validate) and an empty list of type ValidationResult. The function call returns false if any properties fail validation. In the example above the validation would fail since name only contains three characters the minimum length is set to four characters.