CREATE PROCEDURE dbo.tbh_Articles_InsertCategory
(
   @AddedDate     datetime,
   @AddedBy       nvarchar(256),
   @Title         nvarchar(256),
   @Importance    int,
   @Description   nvarchar(4000),
   @ImageUrl      nvarchar(256),
   @CategoryID    int OUTPUT
)
AS
SET NOCOUNT ON

-- check whether a category with the same name already exists
DECLARE @CurrID int
SELECT @CurrID = CategoryID FROM tbh_Categories WHERE
   LOWER(@Title) = LOWER(Title)

IF @CurrID IS NOT NULL
   BEGIN
   SET @CategoryID = -1
   RETURN
   END

INSERT INTO tbh_Categories 
   (AddedDate, AddedBy, Title, Importance, Description, ImageUrl)
   VALUES (@AddedDate, @AddedBy, @Title, @Importance, @Description, @ImageUrl)

SET @CategoryID = scope_identity()

public class TestControl : UserControl
{
    public string UserName
    {
        get{ return txtUserName.Text;}
        set{ txtUserName.Text=value;}
    }

    protected void btnSumit_Click(object sender, EventArgs e)
    {
        Reponse.Write("Hello,"+UserName);
    }

}

<asp:TextBox runat="server" id="txtUserName" />
<asp:Button runat="server" id="btnSumit" />

But actually the ASP.NET will do this things for us:
it first check whether the ViewState collection of the TestControl 
has some value(if it is post back ,it should have),and load from it,
otherwise it give the default value to its property,which for a TextBox
is null:
public string Username
{
    get
    { 
        return ViewState["txtUserName"]!=null ? 
            (string)ViewState["txtUserName"] : null;
    }
    set
    {
        ViewState["txtUserName"]=value;
    }
}

it also set a flag HasUserNameChanged to indicate whether the property
value is changed during the postback:

public bool HasUserNameChanged;


when the user clicks the Submit button and posts the page back to the server, the page
calls the LoadPostData method of the TestControl control and passes a NameValueCollection
collection into it. This collection contains a name/value pair for each form HTML element where the
name contains the value of the name attributes of the element and value contains the value that the
element has posted back to the server. In other words, this collection contains one name/value pairs
associated with the one form elements that the TestControl contains, the UserName for the txtUserName
text fields.

protected virtual bool LoadPostData(string postDataKey,NameValueCollection values)
{
    string username=values[UserNameTextInputName] ;
    if(username !=UserName)
    {
        UserName=username;
        hasUserNameChanged=true;
    }
    return hasUserNameChanged;
}

where the UserNameTextInputName is something like:
protected virtual string UserNameTextInputName
{
    get { return "UserName"; }
}
Just to identity it from the post back data Name/Value collections

From these code we can see that the LoadPostData method assigns the retrieved values to 
these properties only if the new values are different from the old values of these
properties. This is only possible if these properties can maintain their values across 
page postbacks,which means that they must use the ViewState collection of the TestControls
as their backing stores,just as list above:

public string Username
{
    get
    { 
        return ViewState["txtUserName"]!=null ? 
            (string)ViewState["txtUserName"] : null;
    }
    set
    {
        ViewState["txtUserName"]=value;
    }
}

Then it will come to the Raise Post Data Changed Event (postback only),

and finaly to the Raise Postback Event (postback only):
in our example,the btnSumit_Click event ,whick just print a welcome.

What I want to ask is , What's the different between ViewState and Postback data,
It seems that the Postback data contains all the data the page need to main its 
state,and get the user's input data.
Consider another situation:
I have two TextBox , the user input "Magi" in one of them,and remain the other
unchanged,(let's say,there old value are "aaa" and "Imar" respectively),then Click 
the sumit button.In this way,What's in the postback data's name/value collection,
and what is in the viewstate.
Will it be like this:
the postback data is "Magi",(since it changed)
and the ViewState is "aaa && Imar"?(the old value)
or postback data is "Magi && Imar",(since they in the same form)
and the ViewState is "aaa && Imar"?