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基于方法的查询语法示例：转换运算符 (LINQ to DataSet)

  • 项目

本主题中的示例演示如何使用 ToArray、ToDictionary 和 ToList 方法立即执行查询表达式。

这些示例中使用的 FillDataSet 方法在将数据加载到数据集中中指定。

本主题中的示例使用 AdventureWorks 示例数据库中的 Contact、Address、Product、SalesOrderHeader 和 SalesOrderDetail 表。

本主题中的示例使用下面的 using/Imports 语句：

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Globalization;
using System.Linq;
using System.Windows.Forms;

Option Explicit On

Imports System.Linq
Imports System.Linq.Expressions
Imports System.Collections.Generic
Imports System.Data
Imports System.Data.SqlClient
Imports System.Data.Common
Imports System.Globalization

有关详细信息，请参阅如何：在 Visual Studio 中创建 LINQ to DataSet 项目。

ToArray

示例

此示例使用 ToArray 方法立即将序列计算为数组。

// Fill the DataSet.
DataSet ds = new DataSet();
ds.Locale = CultureInfo.InvariantCulture;
FillDataSet(ds);

DataTable products = ds.Tables["Product"];

IEnumerable<DataRow> productsArray = products.AsEnumerable().ToArray();

IEnumerable<DataRow> query =
    from product in productsArray
    orderby product.Field<Decimal>("ListPrice") descending
    select product;

Console.WriteLine("Every price from highest to lowest:");
foreach (DataRow product in query)
{
    Console.WriteLine(product.Field<Decimal>("ListPrice"));
}

' Fill the DataSet.
Dim ds As New DataSet()
ds.Locale = CultureInfo.InvariantCulture
' See the FillDataSet method in the Loading Data Into a DataSet topic.
FillDataSet(ds)

Dim products As DataTable = ds.Tables("Product")

Dim productsArray = products.AsEnumerable().ToArray()

Dim query = _
        From product In productsArray _
        Select product _
        Order By product.Field(Of Decimal)("ListPrice") Descending

Console.WriteLine("Every price From highest to lowest:")
For Each product In query
    Console.WriteLine(product.Field(Of Decimal)("ListPrice"))
Next

ToDictionary

示例

此示例使用 ToDictionary 方法立即将序列和相关键表达式计算为字典。

// Fill the DataSet.
DataSet ds = new DataSet
{
    Locale = CultureInfo.InvariantCulture
};
FillDataSet(ds);

DataTable products = ds.Tables["Product"];

var scoreRecordsDict =
    products.AsEnumerable()
        .Where(rec=>rec.Field<string>("Name") !=null)
        .ToDictionary(record => record.Field<string>("Name"));
Console.WriteLine("Top Tube's ProductID: {0}",
    scoreRecordsDict["Top Tube"]["ProductID"]);

' Fill the DataSet.
Dim ds As New DataSet()
ds.Locale = CultureInfo.InvariantCulture
' See the FillDataSet method in the Loading Data Into a DataSet topic.
FillDataSet(ds)

Dim products As DataTable = ds.Tables("Product")

Dim scoreRecordsDict = products.AsEnumerable(). _
    ToDictionary(Function(record) record.Field(Of String)("Name"))

Console.WriteLine("Top Tube's ProductID: {0}", _
    scoreRecordsDict("Top Tube")("ProductID"))

ToList

示例

此示例使用 ToList 方法立即将序列计算为 List<T>，其中 T 的类型为 DataRow。

// Fill the DataSet.
DataSet ds = new DataSet();
ds.Locale = CultureInfo.InvariantCulture;
FillDataSet(ds);

DataTable products = ds.Tables["Product"];

IEnumerable<DataRow> productList = products.AsEnumerable().ToList();

IEnumerable<DataRow> query =
    from product in productList
    orderby product.Field<string>("Name")
    select product;

Console.WriteLine("The product list, ordered by product name:");
foreach (DataRow product in query)
{
    Console.WriteLine(product.Field<string>("Name").ToLower(CultureInfo.InvariantCulture));
}

' Fill the DataSet.
Dim ds As New DataSet()
ds.Locale = CultureInfo.InvariantCulture
' See the FillDataSet method in the Loading Data Into a DataSet topic.
FillDataSet(ds)

Dim products As DataTable = ds.Tables("Product")

Dim productList = products.AsEnumerable().ToList()

Dim query = _
    From product In productList _
    Select product _
    Order By product.Field(Of String)("Name")

Console.WriteLine("The sorted name list:")

For Each product In query
    Console.WriteLine(product.Field(Of String)("Name").ToLower(CultureInfo.InvariantCulture))
Next

请参阅