Why does HttpContext.Current need to used within a class, but not a method

For instance if I'm inside the Page_Load method and I want to get query string data I just do this:

  public partial class Product_Detail : System.Web.UI.Page
  {    
        protected void Page_Load(object sender, EventArgs e)
        {
           string id = Request.QueryString["p"];
        }
   }

but if I am inside the class, but not within any method I have to use:

   public partial class Product_Detail : System.Web.UI.Page
   { 
     string id = HttpContext.Current.Request.QueryString["p"];
         protected void Page_Load(ob开发者_Python百科ject sender, EventArgs e)
         {
         }
   }

Why is that?

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Member variable initialisers - which is what your id assignment is -- can't use instance methods or properties. Page.Request is an instance property, and therefore isn't available to member initialisers.

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I would surmise it is because class members are not created until the class is instantated. Because of this you cannot access the Request property except within your class methods.

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Properties of the current class (including this) are not accessible until the constructor. Since field initializers happen before the constructor is executed, properties (and fields, and methods) are not accessible.

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You cannot refer to an instance's properties for a field's initializer—when the field is initialized the instance isn't fully constructed yet (i.e., there's no this pointer).