Question

I’m trying to understand the c# using directive…Why does this work…

using System;
using System.ComponentModel.DataAnnotations;
namespace BusinessRuleDemo
{
    class MyBusinessClass
    {
        [Required]
        public string SomeRequiredProperty { get; set; }
    }
}

but this does not?

using System;
using System.ComponentModel;
namespace BusinessRuleDemo
{
    class MyBusinessClass
    {
        [DataAnnotations.Required]
        public string SomeRequiredProperty { get; set; }
    }
}

The second results in a compile error “The type or namespace DataAnnotations cannot be found. Are you missing are missing a using directive or an assembly reference?

Answer 1

using directive makes it possible to use class names from the namespace you specified as its argument. Since DataAnnotations is not a class, but a namespace, it’s not accessible in second case. You should either use fully qulified class names NS1.NS2.Class1, or start you program with using NS1.NS2; and then use Class1.