Procedura: eseguire una query che restituisce un tipo primitivo (Entity Framework)

In questo argomento vengono forniti alcuni esempi di come eseguire query che restituiscono un tipo primitivo. Lo stesso esempio viene illustrato utilizzando ognuna delle tecnologie di query Entity Framework seguenti:

• LINQ to Entities

• Entity SQL con ObjectQuery<T>

• Metodi del generatore di query di ObjectQuery<T>

Gli esempi inclusi in questo argomento sono basati sul modello Sales di AdventureWorks. Per eseguire il codice incluso in questo esempio, è necessario avere già aggiunto il modello Sales di AdventureWorks al progetto e avere configurato il progetto per l'utilizzo di Entity Framework. A tale scopo, completare le procedure descritte in Procedura: configurare manualmente un progetto di Entity Framework e Procedura: definire manualmente un modello EDM (Entity Framework). È inoltre possibile utilizzare la procedura guidata Entity Data Model per definire il modello Sales di AdventureWorks. Per ulteriori informazioni, vedere Procedura: utilizzare la procedura guidata Entity Data Model (Entity Framework).

Esempio

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

Dim contactId As Integer = 377

Using advWorksContext As New AdventureWorksEntities
    Try
        'Select a value.
        Dim orders As ObjectQuery(Of SalesOrderHeader) _
            = advWorksContext.SalesOrderHeader

        Dim orderQuery = _
        From order In orders _
        Where order.Contact.ContactID = contactId _
        Select order.PurchaseOrderNumber.Length

        ' Iterate through the collection of values.
        For Each result In orderQuery
            Console.WriteLine("{0}", result)
        Next

        ' Use a nullable DateTime value because ShipDate can be null.
        Dim shipDateQuery As IQueryable(Of Date?) = _
        From order In orders _
        Where order.Contact.ContactID = contactId _
        Select order.ShipDate

        ' Iterate through the collection of values.
        For Each shipDate In shipDateQuery
            Dim shipDateMessage As String = "date not set"

            If Not shipDate = Nothing Then
                shipDateMessage = shipDate.ToString()
            End If
            Console.WriteLine("Ship Date: {0}.", shipDateMessage)
        Next
    Catch ex As EntitySqlException
        Console.WriteLine(ex.ToString())
    Catch ex As InvalidOperationException
        Console.WriteLine(ex.ToString())
    End Try
End Using

int contactId = 377;

using (AdventureWorksEntities advWorksContext 
    = new AdventureWorksEntities())
{
    try
    {
        // Select a value.
        ObjectQuery<SalesOrderHeader> orders 
            = advWorksContext.SalesOrderHeader;
        
        IQueryable<Int32> orderQuery = 
            from order in orders                   
            where order.Contact.ContactID == contactId 
            select order.PurchaseOrderNumber.Length;

        // Iterate through the collection of values.
        foreach (Int32 result in orderQuery)
        {
            Console.WriteLine("{0}", result);
        }
        
        // Use a nullable DateTime value because ShipDate can be null.
        IQueryable<DateTime?> shipDateQuery =
            from order in orders
            where order.Contact.ContactID == contactId
            select order.ShipDate;

        // Iterate through the collection of values.
        foreach (DateTime? shipDate in shipDateQuery)
        {
            string shipDateMessage = "date not set";

            if (shipDate != null)
            {
                shipDateMessage = shipDate.ToString();
            }
            Console.WriteLine("Ship Date: {0}.", shipDateMessage);
        }
    }
    catch (EntitySqlException ex)
    {
        Console.WriteLine(ex.ToString());
    }
    catch (InvalidOperationException ex)
    {
        Console.WriteLine(ex.ToString());
    }
}

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

Dim contactId As Integer = 377

Using advWorksContext As New AdventureWorksEntities 
    Dim orderQueryString As String = "SELECT VALUE Length(order.PurchaseOrderNumber) " & _
        "FROM AdventureWorksEntities.SalesOrderHeader AS order " & _
        "WHERE order.CustomerID = @contactId"
    Dim shipDateQueryString As String = "SELECT VALUE order.ShipDate " & _
        "FROM AdventureWorksEntities.SalesOrderHeader AS order " & _
        "WHERE order.CustomerID = @contactId"

    Try
        ' Use the SelectValue method to select a value.
        Dim orderQuery As ObjectQuery(Of Int32) = _
            New ObjectQuery(Of Int32)(orderQueryString, _
                advWorksContext, MergeOption.NoTracking)
        orderQuery.Parameters.Add( _
            New ObjectParameter("contactId", contactId))

        ' Iterate through the collection of values.
        For Each result In orderQuery
            Console.WriteLine("{0}", result)
        Next

        ' Use a nullable DateTime value because ShipDate can be null.
        Dim shipDateQuery As ObjectQuery(Of Nullable(Of Date)) = _
            New ObjectQuery(Of Nullable(Of Date))(shipDateQueryString, _
        advWorksContext, MergeOption.NoTracking)
        shipDateQuery.Parameters.Add( _
            New ObjectParameter("contactId", contactId))

        ' Iterate through the collection of values.
        For Each shipDate In shipDateQuery
            Dim shipDateMessage As String = "date not set"

            If Not shipDate = Nothing Then
                shipDateMessage = shipDate.ToString()
            End If
            Console.WriteLine("Ship Date: {0}.", shipDateMessage)
        Next
    Catch ex As EntityException
        Console.WriteLine(ex.ToString())
    Catch ex As InvalidOperationException
        Console.WriteLine(ex.ToString())
    End Try
End Using

int contactId = 377;

using (AdventureWorksEntities advWorksContext =
    new AdventureWorksEntities())
{
    string orderQueryString = @"SELECT VALUE Length(order.PurchaseOrderNumber)
        FROM AdventureWorksEntities.SalesOrderHeader AS order
        WHERE order.CustomerID = @contactId";
    string shipDateQueryString = @"SELECT VALUE order.ShipDate
        FROM AdventureWorksEntities.SalesOrderHeader AS order
        WHERE order.CustomerID = @contactId";

    try
    {
        // Use the SelectValue method to select a value.
        ObjectQuery<Int32> orderQuery =
            new ObjectQuery<Int32>(orderQueryString, 
                advWorksContext, MergeOption.NoTracking);
        orderQuery.Parameters.Add(
            new ObjectParameter("contactId", contactId));

        // Iterate through the collection of values.
        foreach (Int32 result in orderQuery)
        {
            Console.WriteLine("{0}", result);
        }

        // Use a nullable DateTime value because ShipDate can be null.
        ObjectQuery<Nullable<DateTime>> shipDateQuery =
            new ObjectQuery<Nullable<DateTime>>(shipDateQueryString,
        advWorksContext, MergeOption.NoTracking);
        shipDateQuery.Parameters.Add(                    
            new ObjectParameter("contactId", contactId));

        // Iterate through the collection of values.
        foreach (Nullable<DateTime> shipDate in shipDateQuery)
        {
            string shipDateMessage = "date not set";

            if (shipDate != null)
            {
                shipDateMessage = shipDate.ToString();
            }
            Console.WriteLine("Ship Date: {0}.", shipDateMessage);
        }
    }
    catch (EntityException ex)
    {
        Console.WriteLine(ex.ToString());
    }
    catch (InvalidOperationException ex)
    {
        Console.WriteLine(ex.ToString());
    }
}

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

Dim contactId As Integer = 377

Using advWorksContext As New AdventureWorksEntities
    Try
        ' Use the SelectValue method to select a value.
        Dim orderQuery As ObjectQuery(Of Int32) = _
        advWorksContext.SalesOrderHeader _
            .Where("it.CustomerID = @contactId", _
                New ObjectParameter("contactId", contactId)) _
            .SelectValue(Of Int32)("Length(it.PurchaseOrderNumber)")

        ' Iterate through the collection of values.
        For Each result In orderQuery
            Console.WriteLine("{0}", result)
        Next

        ' Use a nullable DateTime value because ShipDate can be null.
        Dim shipDateQuery As ObjectQuery(Of Nullable(Of Date)) = _
        advWorksContext.SalesOrderHeader _
            .Where("it.CustomerID = @contactId", _
                New ObjectParameter("contactId", contactId)) _
            .SelectValue(Of Nullable(Of Date))("it.ShipDate")

        ' Iterate through the collection of values.
        For Each shipDate In shipDateQuery
            Dim shipDateMessage As String = "date not set"
            If Not shipDate = Nothing Then
                shipDateMessage = shipDate.ToString()
            End If
            Console.WriteLine("Ship Date: {0}.", shipDateMessage)
        Next
    Catch ex As EntitySqlException
        Console.WriteLine(ex.ToString())
    Catch ex As InvalidOperationException
        Console.WriteLine(ex.ToString())
    End Try
End Using

int contactId = 377;

using (AdventureWorksEntities advWorksContext =
    new AdventureWorksEntities())
{
    try
    {
        // Use the SelectValue method to select a value.
        ObjectQuery<Int32> orderQuery =
            advWorksContext.SalesOrderHeader 
            .Where("it.CustomerID = @contactId", 
            new ObjectParameter("contactId", contactId))
            .SelectValue<Int32>("Length(it.PurchaseOrderNumber)");

        // Iterate through the collection of values.
        foreach (Int32 result in orderQuery)
        {
            Console.WriteLine("{0}", result);
        }

        // Use a nullable DateTime value because ShipDate can be null.
        ObjectQuery<Nullable<DateTime>> shipDateQuery =
            advWorksContext.SalesOrderHeader
            .Where("it.CustomerID = @contactId",
                new ObjectParameter("contactId", contactId))
            .SelectValue<Nullable<DateTime>>("it.ShipDate");
        
        // Iterate through the collection of values.
        foreach (Nullable<DateTime> shipDate in shipDateQuery)
        {
            string shipDateMessage = "date not set";

            if (shipDate != null)
            {
                shipDateMessage = shipDate.ToString();
            }
            Console.WriteLine("Ship Date: {0}.", shipDateMessage);
        }
    }
    catch (EntitySqlException ex)
    {
        Console.WriteLine(ex.ToString());
    }
    catch (InvalidOperationException ex)
    {
        Console.WriteLine(ex.ToString());
    }
}

Vedere anche

Attività

Procedura: eseguire una query che restituisce un tipo di entità (Entity Framework)
Procedura: eseguire una query che restituisce un tipo anonimo (Entity Framework)
Procedura: eseguire una query con parametri (Entity Framework)

Concetti

Metodi del generatore di query (Entity Framework)

Altre risorse

Esecuzione di query su Entity Data Model (attività di Entity Framework)