Tài liệu Replacing Null Values in a Strongly Typed DataSet ppt

[ Team LiB ]Recipe 2.19 Replacing Null Values in a Strongly Typed DataSet Problem When a column in a database has a null value, you want the value in the DataSet to be a string indicat

[ Team LiB ]

Recipe 2.19 Replacing Null Values in a Strongly Typed DataSet

Problem

When a column in a database has a null value, you want the value in the DataSet to be a string indicating that no value is available

Solution

Use annotations in the XML schema to control the handling of null values

The sample uses one XSD file:

CategoriesDS_AnnotatedNull.xsd

The schema used to generate the strongly typed DataSet The schema is annotated

so the null Description values are replaced with the string "- no description

available -" The annotations are marked in bold in Example 2-25

Example 2-25 File: TypedDataSets\CategoriesDS_AnnotatedNull.xsd

<?xml version="1.0" standalone="yes" ?>

<xs:schema id="CategoriesDS_AnnotatedNull"

targetNamespace=

"http://www.tempuri.org/CategoriesDS_AnnotatedNull.xsd"

xmlns:mstns="http://www.tempuri.org/CategoriesDS_AnnotatedNull.xsd"

xmlns="http://www.tempuri.org/CategoriesDS_AnnotatedNull.xsd"

xmlns:xs="http://www.w3.org/2001/XMLSchema"

xmlns:msdata="urn:schemas-microsoft-com:xml-msdata"

xmlns:codegen="urn:schemas-microsoft-com:xml-msprop"

attributeFormDefault="qualified" elementFormDefault="qualified">

<xs:element name="CategoriesDS_AnnotatedNull" msdata:IsDataSet="true">

<xs:complexType>

<xs:choice maxOccurs="unbounded">

<xs:element name="Categories">

<xs:complexType>

<xs:sequence>

<xs:element

name="CategoryID"

msdata:ReadOnly="true"

msdata:AutoIncrement="true"

type="xs:int" />

<xs:element

name="CategoryName"

type="xs:string" />

<xs:element

name="Description"

type="xs:string"

minOccurs="0"

codegen:nullValue=

"- no description available -" />

</xs:sequence>

</xs:complexType>

</xs:element>

</xs:choice>

</xs:complexType>

<xs:unique name="Constraint1" msdata:PrimaryKey="true">

<xs:selector xpath=".//mstns:Categories" />

<xs:field xpath="mstns:CategoryID" />

</xs:unique>

</xs:element>

</xs:schema>

The sample code creates a strongly typed DataSet based on the Categories table in Northwind The user specifies whether the one based on the default or annotated schema file is used In either case, data is loaded into the DataSet A row is added to the

Categories table with a Description value of null The data in the table is written to the text box on the form to demonstrate the effect of the schema annotation on null column values

The C# code is shown in Example 2-26

Example 2-26 File: TypedDataSetNullsForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Text;

using System.Data;

using System.Data.SqlClient;

// Table name constants

private const String CATEGORIES_TABLE = "Categories";

//

StringBuilder result = new StringBuilder( );

// Create the DataAdapter

SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Categories", ConfigurationSettings.AppSettings["Sql_ConnectString"]);

if (annotatedRadioButton.Checked)

{

// Create the typed DataSet without null annotation

CategoriesDS_AnnotatedNull ds = new CategoriesDS_AnnotatedNull( ); // Fill the Categories table within DataSet

da.Fill(ds, CATEGORIES_TABLE);

// Add a row with a null Description

ds.Categories.AddCategoriesRow("New Category", null);

result.Append("Annotated Nulls" + Environment.NewLine +

Environment.NewLine);

// Iterate over the rows collection and display columns

foreach(CategoriesDS_AnnotatedNull.CategoriesRow row in ds.Categories) {

// Get the Description

String description = row.Description;

// Note that the null Description is replaced

result.Append(row.CategoryID + "\t" + row.CategoryName + "\t" +

description + Environment.NewLine);

}

}

else

{

// Create the typed DataSet annotated for nulls

CategoriesDS ds = new CategoriesDS( );

da.Fill(ds, CATEGORIES_TABLE);

// Add a row with a null Description

ds.Categories.AddCategoriesRow("New Category", null);

result.Append("Default" + Environment.NewLine + Environment.NewLine); // Iterate over the rows collection and display columns

foreach(CategoriesDS.CategoriesRow row in ds.Categories)

{

// Use Is<field>Null method or StrongTypingException will

// result when null row.Description is accessed

String description = row.IsDescriptionNull( ) ?

"NULL" : row.Description;

// Note that the null Description is not replaced

result.Append(row.CategoryID + "\t" + row.CategoryName + "\t" +

description + Environment.NewLine);

}

}

resultTextBox.Text = result.ToString( );

Discussion

Annotations to XSD schemas used to generate strongly typed DataSet objects are discussed in detail in Recipe 2.18

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