c# - 如何轻松将 DataReader 转换为 List?

Question

我在 a 中有数据DataReader，我想将其转换为List<T>. 什么是可能的简单解决方案？

例如，在 CustomerEntity 类中，我有 CustomerId 和 CustomerName 属性。如果我的 DataReader 将这两列作为数据返回，那么如何将其转换为List<CustomerEntity>.

Answer 1

我建议为此编写一个扩展方法：

public static IEnumerable<T> Select<T>(this IDataReader reader,
                                       Func<IDataReader, T> projection)
{
    while (reader.Read())
    {
        yield return projection(reader);
    }
}

然后，您可以根据需要使用 LINQ 的ToList()方法将其转换为 a List<T>，如下所示：

using (IDataReader reader = ...)
{
    List<Customer> customers = reader.Select(r => new Customer {
        CustomerId = r["id"] is DBNull ? null : r["id"].ToString(),
        CustomerName = r["name"] is DBNull ? null : r["name"].ToString() 
    }).ToList();
}

我实际上建议在（或其他地方）放置一个FromDataReader方法：Customer

public static Customer FromDataReader(IDataReader reader) { ... }

那将离开：

using (IDataReader reader = ...)
{
    List<Customer> customers = reader.Select<Customer>(Customer.FromDataReader)
                                     .ToList();
}

（我认为类型推断在这种情况下不起作用，但我可能是错的......）

Answer 2

我使用这种情况编写了以下方法。

首先，添加命名空间：System.Reflection

例如：T是返回类型（类名）并且dr是映射的参数DataReader

C#，调用映射方法如下：

List<Person> personList = new List<Person>();
personList = DataReaderMapToList<Person>(dataReaderForPerson);

这是映射方法：

public static List<T> DataReaderMapToList<T>(IDataReader dr)
{
    List<T> list = new List<T>();
    T obj = default(T);
    while (dr.Read()) {
        obj = Activator.CreateInstance<T>();
        foreach (PropertyInfo prop in obj.GetType().GetProperties()) {
            if (!object.Equals(dr[prop.Name], DBNull.Value)) {
                prop.SetValue(obj, dr[prop.Name], null);
            }
        }
        list.Add(obj);
    }
    return list;
}

VB.NET，调用映射方法如下：

Dim personList As New List(Of Person)
personList = DataReaderMapToList(Of Person)(dataReaderForPerson)

这是映射方法：

Public Shared Function DataReaderMapToList(Of T)(ByVal dr As IDataReader) As List(Of T)
        Dim list As New List(Of T)
        Dim obj As T
        While dr.Read()
            obj = Activator.CreateInstance(Of T)()
            For Each prop As PropertyInfo In obj.GetType().GetProperties()
                If Not Object.Equals(dr(prop.Name), DBNull.Value) Then
                    prop.SetValue(obj, dr(prop.Name), Nothing)
                End If
            Next
            list.Add(obj)
        End While
        Return list
    End Function

Answer 3

我见过使用反射和属性或字段上的属性将 DataReader 映射到对象的系统。（有点像 LinqToSql 所做的。）它们节省了一些输入，并且在为 DBNull 等编码时可以减少错误的数量。一旦你缓存了生成的代码，它们也可以比大多数手写代码更快，所以请考虑如果你经常这样做，那就是“高路”。

有关这方面的一个示例，请参见“.NET 中的反射防御” 。

然后，您可以编写如下代码

class CustomerDTO  
{
    [Field("id")]
    public int? CustomerId;

    [Field("name")]
    public string CustomerName;
}

...

using (DataReader reader = ...)
{    
   List<CustomerDTO> customers = reader.AutoMap<CustomerDTO>()
                                    .ToList();
}

（AutoMap()，是一种扩展方法）

---

@Stilgar，感谢您的精彩评论

如果能够，您可能会更好地使用 NHibernate、EF 或 Linq to Sql 等 但是在旧项目中（或出于其他（有时有效）原因，例如“不是在这里发明”、“对存储过程的热爱”等）并非总是可以使用 ORM，因此轻量级的系统可能很有用，“袖手旁观”</p>

如果您每个人都需要编写大量 IDataReader 循环，您将看到减少编码（和错误）的好处，而无需更改您正在处理的系统的体系结构。这并不是说它是一个很好的架构。

我假设 CustomerDTO 不会脱离数据访问层，复合对象等将由数据访问层使用 DTO 对象构建。

---

在我写完这个答案几年后，Dapper进入了 .NET 的世界，这可能是编写你的 onw AutoMapper 的一个很好的起点，也许它会完全消除你这样做的需要。

Answer 4

最简单的解决方案：

var dt = new DataTable();
dt.Load(myDataReader);
List<DataRow> rows = dt.AsEnumerable();
var customers = rows.Select(dr=>new Customer(...)).ToList();

Answer 5

我会（并且已经）开始使用Dapper。使用你的例子就像（从内存中写的）：

public List<CustomerEntity> GetCustomerList()
{
    using (DbConnection connection = CreateConnection())
    {
        return connection.Query<CustomerEntity>("procToReturnCustomers", commandType: CommandType.StoredProcedure).ToList();
    }
}

CreateConnection()将处理访问您的数据库并返回连接。

Dapper 自动处理将数据字段映射到属性。它还支持多种类型和结果集，而且速度非常快。

查询IEnumerable因此返回ToList().

Answer 6

显然@Ian Ringrose，您应该为此使用库的中心论点是这里最好的单一答案（因此是+1），但对于最小的一次性或演示代码，这里是@SLaks对@Jon Skeet更细粒度（+ 1'd）回答：

public List<XXX> Load( <<args>> )
{
    using ( var connection = CreateConnection() )
    using ( var command = Create<<ListXXX>>Command( <<args>>, connection ) )
    {
        connection.Open();
        using ( var reader = command.ExecuteReader() )
            return reader.Cast<IDataRecord>()
                .Select( x => new XXX( x.GetString( 0 ), x.GetString( 1 ) ) )
                .ToList();
    }
}

---

正如在@Jon Skeet的回答中，

            .Select( x => new XXX( x.GetString( 0 ), x.GetString( 1 ) ) )

位可以提取到一个助手中（我喜欢将它们转储到查询类中）：

    public static XXX FromDataRecord( this IDataRecord record)
    {
        return new XXX( record.GetString( 0 ), record.GetString( 1 ) );
    }

并用作：

            .Select( FromDataRecord )

---

2013 年 3 月 9 日更新：另请参阅一些出色的更微妙的编码技术，以在此答案中拆分样板文件

Answer 7

您不能简单地（直接）将数据读取器转换为列表。

您必须遍历 datareader 中的所有元素并插入列表

在示例代码下方

using (drOutput)   
{
            System.Collections.Generic.List<CustomerEntity > arrObjects = new System.Collections.Generic.List<CustomerEntity >();        
            int customerId = drOutput.GetOrdinal("customerId ");
            int CustomerName = drOutput.GetOrdinal("CustomerName ");

        while (drOutput.Read())        
        {
            CustomerEntity obj=new CustomerEntity ();
            obj.customerId = (drOutput[customerId ] != Convert.DBNull) ? drOutput[customerId ].ToString() : null;
            obj.CustomerName = (drOutput[CustomerName ] != Convert.DBNull) ? drOutput[CustomerName ].ToString() : null;
            arrObjects .Add(obj);
        }

}

Answer 8

我已经在一个宠物项目中介绍了这一点。使用你想要的。

请注意，ListEx 实现了 IDataReader 接口。

people = new ListExCommand(command)
.Map(p=> new ContactPerson()
{
  Age = p.GetInt32(p.GetOrdinal("Age")),
  FirstName = p.GetString(p.GetOrdinal("FirstName")),
  IdNumber = p.GetInt64(p.GetOrdinal("IdNumber")),
  Surname = p.GetString(p.GetOrdinal("Surname")),
  Email = "z.evans@caprisoft.co.za"
})
.ToListEx()
.Where("FirstName", "Peter");

或者使用如下示例中的对象映射。

people = new ListExAutoMap(personList)
.Map(p => new ContactPerson()
{
    Age = p.Age,
    FirstName = p.FirstName,
    IdNumber = p.IdNumber,
    Surname = p.Surname,
    Email = "z.evans@caprisoft.co.za"
})
.ToListEx()
.Where(contactPerson => contactPerson.FirstName == "Zack");

看看http://caprisoft.codeplex.com

Answer 9

我知道这个问题很老，并且已经回答了，但是......

既然 SqlDataReader 已经实现了 IEnumerable，为什么还需要在记录上创建一个循环呢？

我一直在使用下面的方法，没有任何问题，也没有任何性能问题：到目前为止，我已经使用 IList、List(Of T)、IEnumerable、IEnumerable(Of T)、IQueryable 和 IQueryable(Of T) 进行了测试

Imports System.Data.SqlClient
Imports System.Data
Imports System.Threading.Tasks

Public Class DataAccess
Implements IDisposable

#Region "   Properties  "

''' <summary>
''' Set the Query Type
''' </summary>
''' <value></value>
''' <remarks></remarks>
Public WriteOnly Property QueryType() As CmdType
    Set(ByVal value As CmdType)
        _QT = value
    End Set
End Property
Private _QT As CmdType

''' <summary>
''' Set the query to run
''' </summary>
''' <value></value>
''' <remarks></remarks>
Public WriteOnly Property Query() As String
    Set(ByVal value As String)
        _Qry = value
    End Set
End Property
Private _Qry As String

''' <summary>
''' Set the parameter names
''' </summary>
''' <value></value>
''' <remarks></remarks>
Public WriteOnly Property ParameterNames() As Object
    Set(ByVal value As Object)
        _PNs = value
    End Set
End Property
Private _PNs As Object

''' <summary>
''' Set the parameter values
''' </summary>
''' <value></value>
''' <remarks></remarks>
Public WriteOnly Property ParameterValues() As Object
    Set(ByVal value As Object)
        _PVs = value
    End Set
End Property
Private _PVs As Object

''' <summary>
''' Set the parameter data type
''' </summary>
''' <value></value>
''' <remarks></remarks>
Public WriteOnly Property ParameterDataTypes() As DataType()
    Set(ByVal value As DataType())
        _DTs = value
    End Set
End Property
Private _DTs As DataType()

''' <summary>
''' Check if there are parameters, before setting them
''' </summary>
''' <value></value>
''' <returns></returns>
''' <remarks></remarks>
Private ReadOnly Property AreParams() As Boolean
    Get
        If (IsArray(_PVs) And IsArray(_PNs)) Then
            If (_PVs.GetUpperBound(0) = _PNs.GetUpperBound(0)) Then
                Return True
            Else
                Return False
            End If
        Else
            Return False
        End If
    End Get
End Property

''' <summary>
''' Set our dynamic connection string
''' </summary>
''' <value></value>
''' <returns></returns>
''' <remarks></remarks>
Private ReadOnly Property _ConnString() As String
    Get
        If System.Diagnostics.Debugger.IsAttached OrElse My.Settings.AttachToBeta OrElse Not (Common.CheckPaid) Then
            Return My.Settings.DevConnString
        Else
            Return My.Settings.TurboKitsv2ConnectionString
        End If
    End Get
End Property

Private _Rdr As SqlDataReader
Private _Conn As SqlConnection
Private _Cmd As SqlCommand

#End Region

#Region "   Methods "

''' <summary>
''' Fire us up!
''' </summary>
''' <remarks></remarks>
Public Sub New()
    Parallel.Invoke(Sub()
                        _Conn = New SqlConnection(_ConnString)
                    End Sub,
                    Sub()
                        _Cmd = New SqlCommand
                    End Sub)
End Sub

''' <summary>
''' Get our results
''' </summary>
''' <returns></returns>
''' <remarks></remarks>
Public Function GetResults() As SqlDataReader
    Try
        Parallel.Invoke(Sub()
                            If AreParams Then
                                PrepareParams(_Cmd)
                            End If
                            _Cmd.Connection = _Conn
                            _Cmd.CommandType = _QT
                            _Cmd.CommandText = _Qry
                            _Cmd.Connection.Open()
                            _Rdr = _Cmd.ExecuteReader(CommandBehavior.CloseConnection)
                        End Sub)
        If _Rdr.HasRows Then
            Return _Rdr
        Else
            Return Nothing
        End If
    Catch sEx As SqlException
        Return Nothing
    Catch ex As Exception
        Return Nothing
    End Try
End Function

''' <summary>
''' Prepare our parameters
''' </summary>
''' <param name="objCmd"></param>
''' <remarks></remarks>
Private Sub PrepareParams(ByVal objCmd As Object)
    Try
        Dim _DataSize As Long
        Dim _PCt As Integer = _PVs.GetUpperBound(0)

        For i As Long = 0 To _PCt
            If IsArray(_DTs) Then
                Select Case _DTs(i)
                    Case 0, 33, 6, 9, 13, 19
                        _DataSize = 8
                    Case 1, 3, 7, 10, 12, 21, 22, 23, 25
                        _DataSize = Len(_PVs(i))
                    Case 2, 20
                        _DataSize = 1
                    Case 5
                        _DataSize = 17
                    Case 8, 17, 15
                        _DataSize = 4
                    Case 14
                        _DataSize = 16
                    Case 31
                        _DataSize = 3
                    Case 32
                        _DataSize = 5
                    Case 16
                        _DataSize = 2
                    Case 15
                End Select
                objCmd.Parameters.Add(_PNs(i), _DTs(i), _DataSize).Value = _PVs(i)
            Else
                objCmd.Parameters.AddWithValue(_PNs(i), _PVs(i))
            End If
        Next
    Catch ex As Exception
    End Try
End Sub

#End Region

#Region "IDisposable Support"

Private disposedValue As Boolean ' To detect redundant calls

' IDisposable
Protected Overridable Sub Dispose(ByVal disposing As Boolean)
    If Not Me.disposedValue Then
        If disposing Then
        End If
        Try
            Erase _PNs : Erase _PVs : Erase _DTs
            _Qry = String.Empty
            _Rdr.Close()
            _Rdr.Dispose()
            _Cmd.Parameters.Clear()
            _Cmd.Connection.Close()
            _Conn.Close()
            _Cmd.Dispose()
            _Conn.Dispose()
        Catch ex As Exception

        End Try
    End If
    Me.disposedValue = True
End Sub

' TODO: override Finalize() only if Dispose(ByVal disposing As Boolean) above has code to free unmanaged resources.
Protected Overrides Sub Finalize()
    ' Do not change this code.  Put cleanup code in Dispose(ByVal disposing As Boolean) above.
    Dispose(False)
    MyBase.Finalize()
End Sub

' This code added by Visual Basic to correctly implement the disposable pattern.
Public Sub Dispose() Implements IDisposable.Dispose
    ' Do not change this code.  Put cleanup code in Dispose(ByVal disposing As Boolean) above.
    Dispose(True)
    GC.SuppressFinalize(Me)
End Sub

#End Region

End Class

强类型

Public Class OrderDCTyping
    Public Property OrderID As Long = 0
    Public Property OrderTrackingNumber As String = String.Empty
    Public Property OrderShipped As Boolean = False
    Public Property OrderShippedOn As Date = Nothing
    Public Property OrderPaid As Boolean = False
    Public Property OrderPaidOn As Date = Nothing
    Public Property TransactionID As String
End Class

用法

Public Function GetCurrentOrders() As IEnumerable(Of OrderDCTyping)
    Try
        Using db As New DataAccess
            With db
                .QueryType = CmdType.StoredProcedure
                .Query = "[Desktop].[CurrentOrders]"
                Using _Results = .GetResults()
                    If _Results IsNot Nothing Then
                        _Qry = (From row In _Results.Cast(Of DbDataRecord)()
                                    Select New OrderDCTyping() With {
                                        .OrderID = Common.IsNull(Of Long)(row, 0, 0),
                                        .OrderTrackingNumber = Common.IsNull(Of String)(row, 1, String.Empty),
                                        .OrderShipped = Common.IsNull(Of Boolean)(row, 2, False),
                                        .OrderShippedOn = Common.IsNull(Of Date)(row, 3, Nothing),
                                        .OrderPaid = Common.IsNull(Of Boolean)(row, 4, False),
                                        .OrderPaidOn = Common.IsNull(Of Date)(row, 5, Nothing),
                                        .TransactionID = Common.IsNull(Of String)(row, 6, String.Empty)
                                    }).ToList()
                    Else
                        _Qry = Nothing
                    End If
                End Using
                Return _Qry
            End With
        End Using
    Catch ex As Exception
        Return Nothing
    End Try
End Function