通过

Method-Based 查询语法示例：元素运算符（LINQ to DataSet）

本主题中的示例演示如何使用First和ElementAt方法，通过查询表达式语法，从DataRow获取DataSet元素。

FillDataSet这些示例中使用的方法是在将数据加载到数据集中指定的。

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以下 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。

ElementAt

示例：

此示例使用 ElementAt 该方法检索第五个地址，其中 PostalCode == “M4B 1V7”。

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

DataTable addresses = ds.Tables["Address"];

var fifthAddress = (
    from address in addresses.AsEnumerable()
    where address.Field<string>("PostalCode") == "M4B 1V7"
    select address.Field<string>("AddressLine1"))
.ElementAt(5);

Console.WriteLine($"Fifth address where PostalCode = 'M4B 1V7': {fifthAddress}");

' 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 addresses As DataTable = ds.Tables("Address")

Dim fifthAddress = ( _
    From address In addresses.AsEnumerable() _
    Where address.Field(Of String)("PostalCode") = "M4B 1V7" _
    Select address.Field(Of String)("AddressLine1")).ElementAt(5)

Console.WriteLine("Fifth address where PostalCode = 'M4B 1V7': " & _
        fifthAddress)

第一

示例：

此示例使用First方法返回名字为“Brooke”的第一个联系人。

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

DataTable contacts = ds.Tables["Contact"];

DataRow query = (
    from contact in contacts.AsEnumerable()
    where (string)contact["FirstName"] == "Brooke"
    select contact)
    .First();

Console.WriteLine("ContactID: " + query.Field<int>("ContactID"));
Console.WriteLine("FirstName: " + query.Field<string>("FirstName"));
Console.WriteLine("LastName: " + query.Field<string>("LastName"));

' 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 contacts As DataTable = ds.Tables("Contact")

Dim query = ( _
    From contact In contacts.AsEnumerable() _
    Where contact.Field(Of String)("FirstName") = "Brooke" _
    Select contact).First()

Console.WriteLine("ContactID: " & query.Field(Of Integer)("ContactID"))
Console.WriteLine("FirstName: " & query.Field(Of String)("FirstName"))
Console.WriteLine("LastName: " & query.Field(Of String)("LastName"))

另请参阅