How can I easily convert DataReader to List? [duplicate]

Answer 1

I know this question is old, and already answered, but...

Since SqlDataReader already implements IEnumerable, why is there a need to create a loop over the records?

I've been using the method below without any issues, nor without any performance issues: So far I have tested with IList, List(Of T), IEnumerable, IEnumerable(Of T), IQueryable, and IQueryable(Of T)

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

Strong Typing Class

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

Usage

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

Answer 2

I have seen systems that use Reflection and attributes on Properties or fields to maps DataReaders to objects. (A bit like what LinqToSql does.) They save a bit of typing and may reduce the number of errors when coding for DBNull etc. Once you cache the generated code they can be faster then most hand written code as well, so do consider the “high road” if you are doing this a lot.

See "A Defense of Reflection in .NET" for one example of this.

You can then write code like

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

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

...

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

(AutoMap(), is an extension method)

---

@Stilgar, thanks for a great comment

If are able to you are likely to be better of using NHibernate, EF or Linq to Sql, etc However on old project (or for other (sometimes valid) reasons, e.g. “not invented here”, “love of stored procs” etc) It is not always possible to use a ORM, so a lighter weight system can be useful to have “up your sleeves”

If you every needed too write lots of IDataReader loops, you will see the benefit of reducing the coding (and errors) without having to change the architecture of the system you are working on. That is not to say it’s a good architecture to start with..

I am assuming that CustomerDTO will not get out of the data access layer and composite objects etc will be built up by the data access layer using the DTO objects.

---

A few years after I wrote this answer Dapper entered the world of .NET, it is likely to be a very good starting point for writing your onw AutoMapper, maybe it will completely remove the need for you to do so.

Answer 3

You cant simply (directly) convert the datareader to list.

You have to loop through all the elements in datareader and insert into list

below the sample code

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 4

I have written the following method using this case.

First, add the namespace: System.Reflection

For Example: T is return type(ClassName) and dr is parameter to mapping DataReader

C#, Call mapping method like the following:

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

This is the mapping method:

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, Call mapping method like the following:

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

This is the mapping method:

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 5

Obviously @Ian Ringrose's central thesis that you should be using a library for this is the best single answer here (hence a +1), but for minimal throwaway or demo code here's a concrete illustration of @SLaks's subtle comment on @Jon Skeet's more granular (+1'd) answer:

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();
    }
}

---

As in @Jon Skeet's answer, the

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

bit can be extracted into a helper (I like to dump them in the query class):

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

and used as:

            .Select( FromDataRecord )

---

UPDATE Mar 9 13: See also Some excellent further subtle coding techniques to split out the boilerplate in this answer

Answer 6

The simplest Solution :

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