Validating user-provided values

In this tutorial, we're going to extend the CustomerId type we created in the first tutorial.

When Vogen generates the backing code for your type, it will call your Validate method if it exists.

Let's use that to stop the code creating Customer IDs with values of zero or less.

Add the following Validate method to CustomerId:

private static Validation Validate(int input) => input > 0 
    ? Validation.Ok 
    : Validation.Invalid("Customer IDs must be greater than 0.");

Add the following to try to create one:

var newCustomer = CustomerId.From(0);

You'll get an exception at runtime:

Unhandled exception. Vogen.ValueObjectValidationException: 
    Customer IDs must be greater than 0.

Note that the Validate method is private and static. The method doesn't need to be public because it doesn't make sense for anyone to call it; if something has a CustomerId, then it is valid, so users don't need to check the validity. It is also static because it's not related to any particular instance of this type.

For more advanced scenarios, please see the How-to article on validation

Last modified: 23 June 2025