📄 column-constraints.aspx
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<%@Page Language="VB"%>
<%@Import Namespace="System.Data" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html><head>
<title>Adding Expressions, Default Values and Constraints to a Table</title>
<!-- #include file="..\global\style.inc" -->
</head>
<body bgcolor="#ffffff">
<span class="heading">Adding Expressions, Default Values and Constraints to a Table</span><hr />
<!--------------------------------------------------------------------------->
<asp:datagrid id="dgrResult" runat="server" />
<script language="vb" runat="server">
Sub Page_Load()
'create a new empty DataTable object
Dim objTable As New DataTable("NewBooks")
'declare a variable to hold a DataColumn object
Dim objColumn As DataColumn
'define the columns (fields) within the table
'the first is an AutoIncrement column to act as the key
objColumn = objTable.Columns.Add("kBookKey", System.Type.GetType("System.Int32"))
objColumn.AutoIncrement = True
objColumn.AutoIncrementSeed = 1000
objColumn.AutoIncrementStep = 10
'next, the ISBN column which must be unique and not allow Null values
objColumn = objTable.Columns.Add("ISBN", System.Type.GetType("System.String"))
objColumn.AllowDBNull = False
objColumn.Unique = True
objColumn.MaxLength = 10
'now two more columns to hold general information
objTable.Columns.Add("Title", System.Type.GetType("System.String"))
objTable.Columns.Add("PublicationDate", System.Type.GetType("System.DateTime"))
'add two columns to hold stock and order quantities, with default values of zero
objColumn = objTable.Columns.Add("StockQty", System.Type.GetType("System.Int32"))
objColumn.DefaultValue = 0
objColumn = objTable.Columns.Add("OrderedQty", System.Type.GetType("System.Int32"))
objColumn.DefaultValue = 0
'add a column containing an expression showing the quantity availability
objColumn = objTable.Columns.Add("AvailableQty", System.Type.GetType("System.Int32"))
objColumn.Expression = "[StockQty] - [OrderedQty]"
'declare a variable to hold a DataRow object
Dim objDataRow As DataRow
'create a new DataRow object instance in this table
objDataRow = objTable.NewRow()
'and fill in the values
objDataRow("ISBN") = "1234567800"
objDataRow("Title") = "Professional Video Recorder Programming"
objDataRow("PublicationDate") = "2001-03-01"
objDataRow("StockQty") = 3956
objDataRow("OrderedQty") = 450
objTable.Rows.Add(objDataRow)
'repeat to add two more rows
objDataRow = objTable.NewRow()
objDataRow("ISBN") = "1234567801"
objDataRow("Title") = "Professional WAP Phone Programming"
objDataRow("PublicationDate") = "2001-06-01"
objDataRow("StockQty") = 329
'note - no "OrderedQty" provided so default value used
objTable.Rows.Add(objDataRow)
objDataRow = objTable.NewRow()
objDataRow("ISBN") = "1234567802"
objDataRow("Title") = "Professional Radio Station Programming"
objDataRow("PublicationDate") = "2001-04-01"
'note - no "StockQty" provided so default value used
objDataRow("OrderedQty") = 1200
objTable.Rows.Add(objDataRow)
'assign the DataTable's DefaultView object to the DataGrid control
dgrResult.DataSource = objTable.DefaultView
dgrResult.DataBind() 'and bind (display) the data
End Sub
</script>
<!--------------------------------------------------------------------------->
<!-- #include file="..\global\foot.inc" -->
</body>
</html>
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