如何轻松地将datareader转换为list< t>?

vojdkbi0  于 2021-06-20  发布在  Mysql
关注(0)|答案(9)|浏览(356)

这个问题在这里已经有答案了

改进数据访问层选择方法模式(7个答案)
将数据读取器中的行转换为键入的结果(10个答案)
7年前关门了。
我有一个数据库里的数据 DataReader 我想把它转换成 List<T> . 有什么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> .

ax6ht2ek

ax6ht2ek1#

我在一个宠物项目中谈到了这一点。。你想用什么就用什么。
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");

或者像下面的例子一样使用对象Map。

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");

看一看http://caprisoft.codeplex.com

xsuvu9jc

xsuvu9jc2#

您不能简单地(直接地)将datareader转换为list。
您必须遍历datareader中的所有元素并插入到列表中
在示例代码下面

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);
        }

}
daolsyd0

daolsyd03#

我知道这个问题很老,而且已经回答了,但是。。。
既然sqldatareader已经实现了ienumerable,为什么需要在记录上创建一个循环?
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

强类型类

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

使用

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
llmtgqce

llmtgqce4#

我用这个例子写了下面的方法。
首先,添加命名空间: System.Reflection 例如: T 是返回类型(classname)和 dr 是Map的参数 DataReader c#,调用如下Map方法:

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

这是Map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Map方法如下:

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

这是Map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
nr7wwzry

nr7wwzry5#

我见过使用反射和属性或字段属性将数据读取器Map到对象的系统(有点像linqtosql。)它们节省了一点键入时间,并且在为dbnull等编码时可能减少错误的数量。一旦缓存生成的代码,它们也可以比大多数手写代码更快,因此如果您经常这样做,请考虑“高速路”。
请参阅“在.net中对反射的防御”以获取此示例。
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(),是一个扩展方法)
@斯蒂尔加,谢谢你的评论
如果你有能力,你可能会更好地使用nhibernate,ef或linqtosql等,但是在旧的项目上(或由于其他(有时是有效的)原因,例如“不是在这里发明的”,“对存储过程的热爱”等),它并不总是可能使用orm,所以一个更轻的系统可以有助于“袖手旁观”
如果您需要编写大量的idatareader循环,那么您将看到减少编码(和错误)的好处,而不必更改正在处理的系统的体系结构。这并不是说这是一个好的架构开始。。
我假设customerdto不会离开数据访问层,复合对象等将由数据访问层使用dto对象构建。
几年后,我写了这个答案潇洒进入世界的.net,它很可能是一个非常好的起点,为编写你的onw汽车,也许它会完全消除你这样做的需要。

plupiseo

plupiseo6#

最简单的解决方案:

var dt=new DataTable();
dt.Load(myDataReader);
list<DataRow> dr=dt.AsEnumerable().ToList();
lh80um4z

lh80um4z7#

我会(而且已经)开始使用整洁。使用您的示例将类似于(从内存中编写):

public List<CustomerEntity> GetCustomerList()
{
    using (DbConnection connection = CreateConnection())
    {
        return connection.Query<CustomerEntity>("procToReturnCustomers", commandType: CommandType.StoredProcedure).ToList();
    }
}
``` `CreateConnection()` 将处理访问数据库并返回连接。
dapper自动处理数据字段到属性的Map。它还支持多种类型和结果集,速度非常快。
查询返回 `IEnumerable` 因此 `ToList()` .
62o28rlo

62o28rlo8#

很明显 @Ian Ringrose 的中心论点,你应该使用一个库,这是最好的单一答案在这里(因此a+1),但对于最小的一次性或演示代码,这里是一个具体的例子 @SLaks 的微妙评论 @Jon Skeet 更精细的(+1'd)答案:

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();
    }
}

如在 @Jon Skeet 答案是

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

位可以提取到助手中(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 ) );
    }

用作:

.Select( FromDataRecord )

更新3月9日至13日:还请参阅一些优秀的更精细的编码技术,以分割出这个答案中的样板文件

igetnqfo

igetnqfo9#

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);
    }
}

你就可以用林克的 ToList() 方法将其转换为 List<T> 如果你愿意,像这样:

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();
}

我真的建议 FromDataReader 中的方法 Customer (或其他地方):

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

这将留下:

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,但我可能错了……)

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