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<p class="a4" style="MARGIN-LEFT: 10.7pt"><span style="FONT-FAMILY: 宋体">自从最后一次调用</span><span lang="EN-US">AcceptChanges</span><span style="FONT-FAMILY: 宋体">以来，数据行都没有发生改变</span></p>
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<p class="MsoNormal"><span lang="EN-US"> </span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">行的状态也取决于在其上调用的方法。一般在成功更新数据源</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">即把改变返回数据库后</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">之后调用</span><span lang="EN-US">AcceptChanges</span><span style="FONT-FAMILY: 宋体">方法。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">修改</span><span lang="EN-US">DataRow</span><span style="FONT-FAMILY: 宋体">中数据最常见的方式是使用索引器，但如果对数据进行了许多修改，就需要考虑使用</span><span lang="EN-US">BeginEdit() </span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US"> EndEdit()</span><span style="FONT-FAMILY: 宋体">方法。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">在对</span><span lang="EN-US">DataRow</span><span style="FONT-FAMILY: 宋体">中的行进行了修改后，就会在该行的</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">上引发</span><span lang="EN-US">ColumnChanging</span><span style="FONT-FAMILY: 宋体">事件。可以重写</span><span lang="EN-US">DataColumnChangeEventArgs </span><span style="FONT-FAMILY: 宋体">类的</span><span lang="EN-US">ProposedValue</span><span style="FONT-FAMILY: 宋体">属性，按照需要修改它。这是在列值上进行某些数据有效性验证的一种方式。如果在进行修改前调用</span><span lang="EN-US">BeginEdit()</span><span style="FONT-FAMILY: 宋体">方法，就不会引发</span><span lang="EN-US">ColumnChanging</span><span style="FONT-FAMILY: 宋体">事件，可以进行多次修改，再调用</span><span lang="EN-US">EndEdit()</span><span style="FONT-FAMILY: 宋体">方法，保存这些修改。如果要回退到初值，应调用</span><span lang="EN-US">CancelEdit()</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="MsoNormal"><span lang="EN-US">DataRow</span><span style="FONT-FAMILY: 宋体">可以以某种方式链接到其他数据行上，在数据行之间能够建立可导航的链接，这在主</span><span lang="EN-US">/</span><span style="FONT-FAMILY: 宋体">从数据表中非常常见。</span><span lang="EN-US">DataRow</span><span style="FONT-FAMILY: 宋体">包含一个</span><span lang="EN-US">GetChildRows()</span><span style="FONT-FAMILY: 宋体">方法，该方法可以从同一个</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">DataSet</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">的另一个表中把一组相关行返回为当前行。这些将在本章后面的“数据关系”一节中介绍。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-GB">2. </span><span style="FONT-FAMILY: 黑体">模式的生成</span></h4>
<p class="MsoNormal"><a ftel="DataTable2"></a><a ftel="schemas"><span style="FONT-FAMILY: 宋体">为</span><span lang="EN-US">DataTable</span></a><span style="FONT-FAMILY: 宋体">创建模式有</span><span lang="EN-US">3</span><span style="FONT-FAMILY: 宋体">种方式：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">让运行库来完成</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">编写代码来创建表</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span style="FONT-FAMILY: 宋体">使用</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">模式生成器</span></p>
<p class="MsoNormal"><span lang="EN-GB">(1) </span><span style="FONT-FAMILY: 宋体">运行</span><span style="FONT-FAMILY: 宋体">库</span><span style="FONT-FAMILY: 宋体">生成的模式</span> </p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">前面的</span><span lang="EN-US">DataRow</span><span style="FONT-FAMILY: 宋体">示例用下面的代码从数据库中选择数据，并生成一个</span><span lang="EN-US">DataSet</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">SqlDataAdapter da = new SqlDataAdapter(select , conn);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">DataSet ds = new DataSet();</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">da.Fill(ds , "Customers");</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">这是很容易使用的，但也有几个缺点。例如，必须利用默认的列名来处理<span style="LETTER-SPACING: -1pt">——</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体">这是可以的，但在某些情况下，还要把物理数据库的列</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">如</span><span lang="EN-US">PKID)</span><span style="FONT-FAMILY: 宋体">重新命名为一个用户友好性更高的名称。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">自然，可以在</span><span lang="EN-US">SQL</span><span style="FONT-FAMILY: 宋体">子句中重新给列指定别名，例如</span><span lang="EN-US">SELECT PID AS PersonID FROM PersonTable</span><span style="FONT-FAMILY: 宋体">。最好不要在</span><span lang="EN-US">SQL</span><span style="FONT-FAMILY: 宋体">中重新给列命名，因为列实际上只需要在屏幕上显示一个“比较好”的名称即可。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">自动生成</span><span lang="EN-US">DataTable/DataColumn</span><span style="FONT-FAMILY: 宋体">的另一个潜在的问题是不能控制运行库为列选择的数据类型。运行库可以确定正确的数据类型，但有时需要对此有更多的控制。例如，为给定的列定义枚举类型，以简化类的用户代码。如果接受运行库生成的默认列类型，该列就可能是一个</span><span lang="EN-US">32</span><span style="FONT-FAMILY: 宋体">位的整数，而不是有预定选项的枚举。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">最后，也是最有可能出的问题是，在使用自动生成的表时，不能对</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">中的数据进行类型安全的访问<span style="LETTER-SPACING: -1pt">——</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体">索引器就会返回</span><span lang="EN-US">object</span><span style="FONT-FAMILY: 宋体">的实例，而不是派生的数据类型。如果要用代码对表达式进行类型转换，就可以跳过下面的章节。</span></p>
<p class="MsoNormal"><span lang="EN-US">(2) </span><span style="FONT-FAMILY: 宋体">手工编码的模式</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">用生成代码来创建</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">，</span><span style="FONT-FAMILY: 宋体">再用相关的</span><span lang="EN-US">DataColumns</span><span style="FONT-FAMILY: 宋体">来填充是相当简单的。本节的示例将访问</span><span lang="EN-US">Northwind</span><span style="FONT-FAMILY: 宋体">数据库中的</span><span lang="EN-US">Product</span><span style="FONT-FAMILY: 宋体">表</span><span style="FONT-FAMILY: 宋体">，</span><span style="FONT-FAMILY: 宋体">如图</span><span lang="EN-US">21-6</span><span style="FONT-FAMILY: 宋体">所示。</span></p>
<p align="center"><span lang="EN-US"><img height="144" src="21/image006.jpg" width="224" alt="" /></span></p>
<p style="FTEL: 8.15pt" align="center"><span style="FONT-FAMILY: 宋体">图</span><span lang="EN-US"> 21-6</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面的代码生成了一个</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">，对应于上面的模式</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">但没有包含可为空的列</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">public static void ManufactureProductDataTable(DataSet ds)</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> DataTable products = new DataTable("Products");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("ProductID", typeof(int)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("ProductName", typeof(string)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("SupplierID", typeof(int)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("CategoryID", typeof(int)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("QuantityPerUnit", typeof(string)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("UnitPrice", typeof(decimal)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("UnitsInStock", typeof(short)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("UnitsOnOrder", typeof(short)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("ReorderLevel", typeof(short)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> products.Columns.Add(new DataColumn("Discontinued", typeof(bool)));</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> ds.Tables.Add(products);</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">可以改变</span><span lang="EN-US">DataRow</span><span style="FONT-FAMILY: 宋体">示例中的代码，使用新生成的表定义：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">string source = "server=(local)\\NetSDK;" +</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> "integrated security=sspi;" + </span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> "database=Northwind";</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">string select = "SELECT * FROM Products";</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">SqlConnection conn = new SqlConnection(source);</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">SqlDataAdapter cmd = new SqlDataAdapter(select, conn);</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">DataSet ds = new DataSet();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">ManufactureProductDataTable(ds);</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">cmd.Fill(ds, "Products");</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">foreach(DataRow row in ds.Tables["Products"].Rows)</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US"> Console.WriteLine("'{0}' from {1}", row[0], row[1]);</span></p>
<p class="MsoNormal"><span lang="EN-US">ManufactureProductDataTable</span><span style="FONT-FAMILY: 宋体">方法创建一个新</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">，依次添加每个列，最后把这个表添加到</span><span lang="EN-US">DataSet</span><span style="FONT-FAMILY: 宋体">中表的清单上。</span><span lang="EN-US">DataSet</span><span style="FONT-FAMILY: 宋体">有一个索引器，它的参数是表名，给调用者返回该</span><span lang="EN-US">DataTable</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">上面的示例仍不是类型安全的，因为在列上使用了索引器来检索数据。最好是有一个类</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">或一组类</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">派生于</span><span lang="EN-US">DataSet</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US"> DataTable</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US"> DataRow</span><span style="FONT-FAMILY: 宋体">，为表、行和列定义类型安全的存取器。可以自己生成这段代码，这并不是特别乏味，最终将得到可以进行类型安全访问的类。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">如果不愿意自己生成这些类型安全的类，可以使用帮助。</span><span lang="EN-US">.NET Framework</span><span style="FONT-FAMILY: 宋体">允许使用</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">模式来定义</span><span lang="EN-US">DataSet</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US">DataTables</span><span style="FONT-FAMILY: 宋体">和本节介绍的其他类。本章后面的</span><span lang="EN-US">XML</span><span style="FONT-FAMILY: 宋体">模式一节详细介绍了这个方法。</span></p></div>
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<h1 id="divCurrentNode2" style="color: #b83507; width: 100%; text-align: left; font-size: 12px; padding-left: 2px">当前章节:<a href='100165605.htm'><font color='red'>21.5.2 数据列</font></a></h1>
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