C＃对象中[key]的目的是什么？

Question

I'm trying to code a WCF RIA class for the first time in C# (also my first time with the language), just to join some tables for a LightSwitch data source. Following the example code , I created the following class to return my rows of data:

public class StudentTerm
{
    [Key]
    public string TermCode { get; set; }
    public string StudentID { get; set; }
    public decimal? TermGPA { get; set; }
    public decimal? CumulativeGPA { get; set; }
}

But I'm curious -- what is the purpose of the [Key] typedef? Does it apply only to TermCode in my example, or to the entire class?

Answer 1

Key on top of TermCode is called property attribute.
Attributes provide a powerful method of associating declarative information with C# code (types, methods, properties, and so forth). Once associated with a program entity, the attribute can be queried at run time and used in any number of ways.
More on attributes you can find here.

In this speciffic case we are talking about WCF RIA Services attribute.
In WCF RIA Services each entity must have a primary key defined and you designate a property as the primary key with the [Key] attribute. You can decorate multiple properties if the class has a composite primary key.

Answer 2

As @LIUFA says above, Key is an Attribute. It's full name is System.ComponentModel.DataAnnotations.KeyAttribute This is to denote that the Id field is the primary key of the underlying data store.

In a more general sense, an Attribute can decorate a class, method, property, or assembly. They don't do anything on their own, but are used by other code to "mark" things.

Answer 3

You should probably check the msdn libraries for an answer: http://msdn.microsoft.com/en-us/library/system.componentmodel.dataannotations.keyattribute(v=vs.110).aspx

And yes, it only applies to TermCode . Class attributes are placed just above the class definition. Also Key Is a property of field attribute so it can't be applied to a class