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<h1>Data Access: Retrieve and Process Data Using a DataReader</h1>
<p>This sample application demonstrates the various ways to use a DataReader to
retrieve data and use it to fill a ComboBox, a TextBox, and other controls. A
DataReader is also used behind the scenes by the DataAdapter object (to fill a
DataSet) and the Command object (to execute SQL statements, such as the
ones used in this application to add several stored procedures to the Northwind
database). The focus of the application is, however, on data retrieval, which
is the only time when you work directly with the DataReader object.</p>
<h2>Featured Highlights</h2>
<UL>
<LI>
Programmatically create stored procedures in the SQL Server database
"Northwind" by executing ad hoc SQL statements using classes from the
System.Data.SqlClient namespace.
<LI>
Execute a query (stored procedure) that ships with the Microsoft Access version
of the Northwind database. This example uses classes that reside in the
System.Data.OleDb namespace.
<LI>
Execute a stored procedure that requires an input parameter and fill a
DataGrid with the results.
<LI>
Execute a stored procedure that requires both input and output parameters, and
that makes use of a return value.
<LI>
See how the use of a DataSet compares with a DataReader for filling a ComboBox.
Uncomment the code under the "USING A DATASET:" comments.</LI></UL>
<h2>Requirements</h2>
<P>This application requires the release version of Visual Studio .NET Professional
(or greater) and access to the Northwind database residing in SQL Server or the
Microsoft Data Engine (MSDE). To install MSDE, do the following:</P>
<P><STRONG>1. </STRONG>Open the Start menu, click Programs, click Microsoft .NET
Framework SDK, and then click Samples and Quickstart Tutorials.
</P>
<P><STRONG>2. </STRONG>Click "Step 1: Install the .NET Framework Samples Database".
</P>
<P><STRONG>3. </STRONG>Open a command window, and CD to
<FRAMEWORKSDK>\Samples\Setup This is typically : C:\Program Files\Microsoft
Visual Studio .NET\FrameworkSDK\Samples\Setup
</P>
<P><STRONG>4. </STRONG>Type: osql -E -S (local)\NetSDK -i InstNwnd.sql
<h2>Running the Sample</h2>
<p>Simply press <b>F5</b>.</p>
<P><STRONG>1. </STRONG>A DataReader will tie up the existing connection because the
data is streamed instead of retrieved and cached, like with a DataSet.
Therefore, you will want to close a connection used by a DataReader as soon as
possible. The best way to do this is to pass the
CommandBehavior.CloseConnection enum to the Execute<EM>XXXX</EM> command.</P>
<P><STRONG>2. </STRONG>Although a DataReader is faster and requires less overhead
than a DataSet, a DataReader has the following limitations (among others):</P>
<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
<P>a) It has no inherent support for the serialization of relational data as XML
with a corresponding schema. You can call ExecuteXMLReader and retrieve an XML
stream, but this is only if the T-SQL "FOR XML" clause is used in the query
(SQL Server 2000 only). If you want to retrieve relational data that can be
readily serialized as XML then you should use a DataSet.</P>
<P>b) It cannot be instantiated directly. You must go through the Command object to
get a DataReader object.</P>
<P>c) It cannot be used to create a DataView. If you need to filter and/or sort the
data then you will need a DataSet.</P>
<P>d) It does not permit disconnected access to data.</P>
<P>
e) It cannot be used for databinding (i.e., it does not support the IList or
IListSource interface). You can only "databind" by iterating through its
Items collection, retrieving the values, and adding them to the Items
collection of a control or another object that does implement one of those
interfaces. This object can then be used for true databinding. Examples of both
are demonstrated with the two ComboBox controls (see Note 4, below).</P>
</BLOCKQUOTE>
<P><STRONG>3. </STRONG>The data is retrieved forward-only. Once it is streamed in,
you cannot go back through the rows in the DataReader unless you have stored
the data in an object that supports this, such as an ArrayList or a DataTable.</P>
<P><STRONG>4. </STRONG>One of the best uses for a DataReader is for filling user
interface controls like a ComboBox. "Filling" is not the same as
databinding. You cannot databind a ComboBox to a DataReader like you can
databind the equivalent ASP .NET control--the DropDownList. The ComboBox
requires a DataSource that implements the IList or IListSource interfaces.
Therefore, you must use iterate through the DataReader and use
ComboBox.Items.Add to fill the ComboBox. With a DropDownList you can set the
DataSource property directly to the DataReader. Furthermore, with a ComboBox
you cannot assign values to the DisplayMember and ValueMember properties when
using a DataReader directly. With a DropDownList, however, you can do this.
Following is how you would use a DataReader to databind to a DropDownList in
ASP.NET:</P>
<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
<P>With DropDownList1<BR>
.DataSource = sdr<br>
.DataTextField = "CategoryName"<br>
.DataValueField = "CategoryID"< BR >
.DataBind()<br>
End With</P>
</BLOCKQUOTE>
<P dir="ltr">In the code you will notice that only the "CategoryName" field is used
to fill the ComboBox. There is no value for the ComboBox items.</P>
<P><STRONG>5. </STRONG>If the SQL statement contains output parameters or return
values, these will not be available until after the DataReader is closed.</P>
<H3>See Also:</H3>
<P><A href='http://support.microsoft.com/default.aspx?scid=kb;EN-US;q308049"'>HOW TO:
Call a Parameterized Stored Procedure by Using ADO.NET and Visual Basic .NET
(Q308049)</A></P>
<P><A href="http://support.microsoft.com/default.aspx?scid=kb;EN-US;q308051">PRB:
Output Parameters Are Not Returned When You Run an ADO.NET Command in Visual
Basic .NET (Q308051)</A></P>
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Cdatabase.rar_C#数据库_CDatabase_c# 实例_visual c （164个子文件） 
frmMain.cs 104KB frmMain.cs 103KB frmMain.cs 84KB frmMain.cs 84KB frmMain.cs 81KB frmMain.cs 70KB frmMain.cs 66KB frmMain.cs 65KB frmMain.cs 64KB frmMain.cs 35KB frmMain.cs 34KB CustomersDataSet.cs 25KB frmMain.cs 25KB Northwind.cs 25KB frmAbout.cs 19KB frmAbout.cs 19KB frmAbout.cs 19KB frmAbout.cs 19KB frmMain.cs 18KB DataAccess.cs 9KB frmAbout.cs 7KB frmAbout.cs 7KB frmAbout.cs 7KB frmAbout.cs 7KB frmAbout.cs 7KB frmAbout.cs 7KB frmAbout.cs 7KB frmAbout.cs 7KB frmAbout.cs 7KB frmStatus.cs 6KB frmStatus.cs 5KB frmStatus.cs 5KB frmStatus.cs 5KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB AssemblyInfo.cs 3KB frmStatus.cs 3KB frmStatus.cs 2KB frmStatus.cs 2KB frmStatus.cs 2KB frmStatus.cs 2KB frmStatus.cs 2KB frmStatus.cs 2KB frmStatus.cs 2KB frmStatus.cs 2KB ListItem.cs 1KB AssemblyInfo.cs 1KB Category.cs 964B How-To Create An Offline Application.csproj 6KB How-To Typed Dataset.csproj 6KB How-To Read-Write Images to Database.csproj 5KB How-To Sort and Filter with a DataView.csproj 5KB How-To Sample N-Tier Data Form App.csproj 5KB How-To Retrieve and Process Data Using a DataReader.csproj 5KB How-To ADO 2.6.csproj 5KB How-To Create a Data Entry Form.csproj 5KB How-To Use Stored Procedures.csproj 5KB How-To Build a Master-Details Windows Forms App.csproj 5KB How-To Use Custom DataBinding Format Handlers.csproj 5KB How-To Bind Data to a ComboBox.csproj 5KB How-To Create Database.csproj 5KB DataAccessLayer.csproj 4KB ReadMe.htm 7KB ReadMe.htm 4KB ReadMe.htm 4KB ReadMe.htm 3KB ReadMe.htm 3KB ReadMe.htm 3KB ReadMe.htm 3KB ReadMe.htm 2KB ReadMe.htm 2KB ReadMe.htm 2KB ReadMe.htm 2KB ReadMe.htm 2KB ReadMe.htm 2KB Northwind.mdb 2.02MB Northwind.mdb 2.02MB frmMain.resx 156KB frmMain.resx 140KB frmMain.resx 132KB frmMain.resx 119KB frmMain.resx 111KB frmMain.resx 98KB frmMain.resx 91KB frmMain.resx 90KB frmMain.resx 79KB frmMain.resx 48KB frmAbout.resx 36KB frmAbout.resx 36KB frmAbout.resx 36KB共 164 条
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