Question

I use Data Annotations in my ASP.NET MVC 3 project to validate the model. These are extremely convenient but currently they are magic to me. I read that data annotations do not throw exceptions. How then does MVC know to add validation errors to the model state dictionary? How does the failure to set a property on the model because of model validation bubble up to MVC if no exception is thrown? I always assumed that exceptions were thrown every time a property failed and that MVC model binding caught the exception and added it to the model state dictionary.

To test this I created a console application and added a sample class with a validation annotation to it:

public class MyObject
{
    [StringLength(10, MinimumLength=3)]
    public string Name { get; set; }
}

I then instantiated the object and tried to assign values to the Name property that were less than 3. The property assigned just fine, despite the annotation that says string length of less than 3 is not allowed.

    static void Main(string[] args)
    {
        MyObject mine = new MyObject();
        mine.Name = "hi";
        Console.WriteLine(mine.Name);
        Console.ReadLine();
    }

This little program writes out “hi” to the console. Why? I was expecting it to get angry when trying to set mine.Name to “hi”.

What am I missing?

Thanks in advance.

Answer 1

You never call anything to validate the properties. The validation doesn’t happen magically on its own. from http://msdn.microsoft.com/en-us/library/dd901590%28v=vs.95%29.aspx

Manually Validating Values

When you do not use the DataGrid
control to provide the interface for
editing data, the validation
attributes are not automatically
applied. However, you can manually
apply the validation test by using the
Validator class. You can call the
ValidateProperty method on the set
accessor of a property to check the
value against the validation
attributes for the property. You must
also set bothValidatesOnExceptions
and NotifyOnValidationError properties
to true when data binding to receive
validation exceptions from validation
attributes. For an example of manually
applying validation, see the Data
Binding Example below.