Procedura: filtrare dati (Entity Framework)

In questo argomento viene illustrato come filtrare i risultati di una query. Nell'esempio viene restituito un insieme di oggetti SalesOrderHeader che rappresentano tutti gli ordini effettuati in linea con un valore superiore a 2.500 dollari. Lo stesso esempio viene illustrato utilizzando le tecnologie di query Entity Framework seguenti:

• LINQ to Entities

• Entity SQL con ObjectQuery<T>

• Metodi del generatore di query di ObjectQuery<T>

L'esempio incluso in questo argomento è basato sul modello Sales di AdventureWorks. Per eseguire il codice incluso in questo argomento, è necessario avere già aggiunto il modello Sales di AdventureWorks al progetto e avere configurato il progetto per l'utilizzo di Entity Framework. Per ulteriori informazioni, vedere Procedura: utilizzare la Procedura guidata Entity Data Model (Entity Framework) o Procedura: configurare manualmente un progetto di Entity Framework e Procedura: definire manualmente un modello EDM (Entity Framework).

Esempio

Di seguito viene fornito un esempio di utilizzo di LINQ to Entities .

Using context As New AdventureWorksEntities()
    ' Specify the order amount. 
    Dim orderCost As Integer = 2500

    ' Define a LINQ query that returns only online orders 
    ' more than the specified amount. 
    Dim onlineOrders = From order In context.SalesOrderHeaders _
        Where order.OnlineOrderFlag = True AndAlso order.TotalDue > orderCost _
        Select order

    ' Print order information. 
    For Each onlineOrder In onlineOrders
        Console.WriteLine("Order ID: {0} Order date: ", onlineOrder.SalesOrderID)
        Console.WriteLine("Order date: {0:d}", onlineOrder.OrderDate)
        Console.WriteLine("Order number: {0}", onlineOrder.SalesOrderNumber)
    Next
End Using

using (AdventureWorksEntities context = new AdventureWorksEntities())
{
    // Specify the order amount.
    int orderCost = 2500;

    // Define a LINQ query that returns only online orders
    // more than the specified amount.
    var onlineOrders =
        from order in context.SalesOrderHeaders
        where order.OnlineOrderFlag == true &&
        order.TotalDue > orderCost
        select order;

    // Print order information.
    foreach (var onlineOrder in onlineOrders)
    {
        Console.WriteLine("Order ID: {0} Order date: "
            + "{1:d} Order number: {2}",
            onlineOrder.SalesOrderID,
            onlineOrder.OrderDate,
            onlineOrder.SalesOrderNumber);
    }
}

Di seguito viene fornito un esempio di utilizzo di Entity SQL .

Using context As New AdventureWorksEntities()
    ' Specify the order amount. 
    Dim orderCost As Decimal = 2500

    ' Specify the Entity SQL query that returns only online orders 
    ' more than the specified amount. 
    Dim queryString As String = "SELECT VALUE o FROM AdventureWorksEntities.SalesOrderHeaders AS o " & _
                                " WHERE o.OnlineOrderFlag = TRUE AND o.TotalDue > @ordercost"

    ' Define an ObjectQuery and pass the maxOrderCost parameter. 
    Dim onlineOrders As New ObjectQuery(Of SalesOrderHeader)(queryString, context)
    onlineOrders.Parameters.Add(New ObjectParameter("ordercost", orderCost))

    ' Print order information. 
    For Each onlineOrder In onlineOrders
        Console.WriteLine("Order ID: {0} Order date: {1:d} Order number: {2}", _
                          onlineOrder.SalesOrderID, onlineOrder.OrderDate, onlineOrder.SalesOrderNumber)
    Next
End Using

using (AdventureWorksEntities context = new AdventureWorksEntities())
{
    // Specify the order amount.
    decimal orderCost = 2500;

    // Specify the Entity SQL query that returns only online orders
    // more than the specified amount.
    string queryString = @"SELECT VALUE o FROM AdventureWorksEntities.SalesOrderHeaders AS o 
        WHERE o.OnlineOrderFlag = TRUE AND o.TotalDue > @ordercost";

    // Define an ObjectQuery and pass the maxOrderCost parameter.
    ObjectQuery<SalesOrderHeader> onlineOrders =
        new ObjectQuery<SalesOrderHeader>(queryString, context);
    onlineOrders.Parameters.Add(
        new ObjectParameter("ordercost", orderCost));

    // Print order information.
    foreach (var onlineOrder in onlineOrders)
    {
        Console.WriteLine("Order ID: {0} Order date: "
            + "{1:d} Order number: {2}",
            onlineOrder.SalesOrderID,
            onlineOrder.OrderDate,
            onlineOrder.SalesOrderNumber);
    }
}

Di seguito viene fornito un esempio del metodo del generatore di query.

Using context As New AdventureWorksEntities()
    ' Specify the order amount. 
    Dim orderCost As Integer = 2500

    ' Define an ObjectQuery that returns only online orders 
    ' more than the specified amount. 
    Dim onlineOrders As ObjectQuery(Of SalesOrderHeader) = _
        context.SalesOrderHeaders.Where("it.OnlineOrderFlag = TRUE AND it.TotalDue > @ordercost", _
                                        New ObjectParameter("ordercost", orderCost))

    ' Print order information. 
    For Each onlineOrder In onlineOrders
        Console.WriteLine("Order ID: {0} Order date: {1:d} Order number: {2}", _
                          onlineOrder.SalesOrderID, onlineOrder.OrderDate, onlineOrder.SalesOrderNumber)
    Next
End Using

using (AdventureWorksEntities context = new AdventureWorksEntities())
{
    // Specify the order amount.
    int orderCost = 2500;

    // Define an ObjectQuery that returns only online orders 
    // more than the specified amount.
    ObjectQuery<SalesOrderHeader> onlineOrders =
        context.SalesOrderHeaders
        .Where("it.OnlineOrderFlag = TRUE AND it.TotalDue > @ordercost",
        new ObjectParameter("ordercost", orderCost));

    // Print order information.
    foreach (var onlineOrder in onlineOrders)
    {
        Console.WriteLine("Order ID: {0} Order date: "
           + "{1:d} Order number: {2}",
           onlineOrder.SalesOrderID,
           onlineOrder.OrderDate,
           onlineOrder.SalesOrderNumber);
    }
}