Validator.ValidateObject() does not throw an error despite violating the StringLength data annotation

Question

I have a class that with a property annotated with [StringLength(2)] like the class shown below.

public class Foo
{
    public Foo()
    {

    }

    [StringLength(2)]
    public string MyProperty { get; set; }
}

Why doesn't the following code to validate the object not throw an error? Shouldn't ValidationException be thrown?

Foo f = new Foo();
f.MyProperty = "A string longer than 2 characters";
ValidationContext validationContext = new ValidationContext(f);
Validator.ValidateObject(f, validationContext);