Correct way to use PetaPoco in DAL layer (ASP.NET Web Forms VB.NET)

问题

I'm trying to generate my DAL layer for ASP.NET web forms project using PetaPoco.

Namespace Eva.Dal.Polls  
Public Partial Class EVAMOD_PL_CategoryDb
    Private db As Eva.Dal.Core.EvaDb

    Public Function Insert(a As EVAMOD_PL_Category) As Object
        Return db.Insert(a)
    End Function

    Public Sub New()
        db = New Eva.Dal.Core.EvaDb
    End Sub
End Class

Public Partial Class EVAMOD_PL_GL_CategoryDb
    Private db As Eva.Dal.Core.EvaDb

    Public Function Insert(a As EVAMOD_PL_GL_Category) As Object
        Return db.Insert(a)
    End Function

    Public Sub New()
        db = New Eva.Dal.Core.EvaDb
    End Sub
End Class
End Namespace

In particular I'm interested in how to open the DB. In PetaPoco site there is the example

// Create a PetaPoco database 
objectvar db=new PetaPoco.Database("connectionStringName");
// Show all articles    
foreach (var a in db.Query<article>("SELECT * FROM articles")){
    Console.WriteLine("{0} - {1}", a.article_id, a.title);
}

But then in the T4 generator shipped with PetaPoco there is a nice section wich, along with all DTOs, generate something like

Namespace Eva.Dal.Core
Public Partial Class EvaDb
    Inherits Database

    Public Sub New() 
        MyBase.New("ConnectionString")
        CommonConstruct()
    End Sub

    Public Sub New(connectionStringName As String)
        MyBase.New(connectionStringName)
        CommonConstruct()
    End Sub

    Private Partial Sub CommonConstruct()
    End Sub

    Public Interface IFactory
        Function GetInstance() As EvaDb
    End Interface

    Public Shared Property Factory() As IFactory
        Get
            Return mFactory
        End Get
        Set
            mFactory = Value
        End Set
    End Property

    Private Shared mFactory As IFactory

    Public Shared Function GetInstance() As EvaDb
        If istance IsNot Nothing Then
            Return istance
        End If

        If Factory IsNot Nothing Then
            Return Factory.GetInstance()
        Else
            Return New EvaDb
        End If
    End Function

    <ThreadStatic> _
    Shared istance As EvaDb

    Public Overrides Sub OnBeginTransaction()
        If istance Is Nothing Then
            istance = Me
        End If
    End Sub

    Public Overrides Sub OnEndTransaction()
        If istance Is Me Then
            istance = Nothing
        End If
    End Sub

    Public Class Record(Of T As New)
        Public Shared ReadOnly Property Repo() As EvaDb
            Get
                Return EvaDb.GetInstance()
            End Get
        End Property
        Public Function IsNew() As Boolean
            Return Repo.IsNew(Me)
        End Function

        .......

    End Class
End Class
End Namespace 

So wich is the correct way to create my DB object and use it in a DAL layer with PetaPoco?

Also I read there is a way with PetaPoco to keep a connection open and reuse it, I guess this would not be feasible with a BLL/DAL architecture when for example you have 2-3 operation from the BLL accessing the DB? Or if it is, then how should it be handled correctly? Creating a DAL method wich opens the connection and does all 2-3 operations? Since opening the connection in the BLL should not be the case.

Thanks in advance

回答1:

UPDATE: this is how you can use a shared connection:

public class SharedConnection : IDisposable
{
    private Database _db;

    public SharedConnection(Database db)
    {
        _db = db;
        _db.OpenSharedConnection();
    }

    public void Dispose()
    {
        _db.CloseSharedConnection();
    }
}

public class FooBarDao
{
    private Database _db = new Database("conn_str_name");

    public SharedConnection GetSharedConnection()
    {
        return new SharedConnection(_db);
    }

    public Foo GetFoo(string id)
    {
        return db.SingleOrDefault<Foo>(
            "SELECT FooVal FROM FooTbl WHERE Id = @0", id);
    }

    public Bar GetBar(string id)
    {
        return db.SingleOrDefault<Bar>(
            "SELECT BarVal FROM BarTbl WHERE Id = @0", id);
    }
}

public class FooBarManager
{
    private FooBarDao _dao = new FooBarDao;

    public void GetFooAndBar(string fooId, string barId)
    {
        using (_dao.GetSharedConnection())
        {
            _dao.GetFoo(fooId);
            _dao.GetBar(barId);
        }
    }
}

回答2:

I always create a wrapper to instantiate it:

public class DatabaseCreator
   public shared function GetContext() As EvaDb
       return new EvaDb(ConfigurationManager.ConnectionStrings("X").ConnectionString)
   end sub
end class

I would not keep the database connection open longer only for the immediate operations you are performing, but would not leave it in an open state to reuse. With multiple layers, you could conceivably structure the application to share the connection across your business components (like a repository pattern). I've gotten in the habit of opening the connection in each repository method, or passing the reference into the constructor of the repository.

回答3:

Following Brian suggestion and reading another user suggestion here on Stackoverflow, I created a static helper function wich use the current httpcontext to create (or return existing) a single Db istance for each request. This way (vb.net):

    Public NotInheritable Class DbHelper
    Public Shared Function GetEvaDbIstance() As Eva.Dal.Core.EvaDb
        If HttpContext.Current.Items("EvaDb") Is Nothing Then
            Dim retval = New Eva.Dal.Core.EvaDb
            HttpContext.Current.Items("EvaDb") = retval
            Return retval
        End If
        Return DirectCast(HttpContext.Current.Items("EvaDb"), Eva.Dal.Core.EvaDb)
    End Function

    Private Sub New()

    End Sub
    End Class

Then in my Dal object I just

    Private db As New Eva.Dal.Core.EvaDb = DbHelper.GetEvaDbIstance()

来源：https://stackoverflow.com/questions/14161176/correct-way-to-use-petapoco-in-dal-layer-asp-net-web-forms-vb-net