Procedura: paging dei risultati delle query (Entity Framework)

Articolo
02/27/2011

In questo argomento viene descritto come eseguire il paging dei risultati di una query. Nell'esempio vengono ottenuti cinque oggetti Product, ordinati in base a Product.ListPrice, dopo avere ignorato i primi tre nel risultato della query. Lo stesso esempio viene fornito 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>

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 illustrato un esempio di LINQ to Entities .

Using context As New AdventureWorksEntities
    'LINQ to Entities only supports Skip on ordered collections.
    Dim products As IOrderedQueryable(Of Product) = _
            context.Products.OrderBy(Function(p) p.ListPrice)

    Dim allButFirst3Products As IQueryable(Of Product) = products.Skip(3)

    Console.WriteLine("All but first 3 products:")
    For Each product As Product In allButFirst3Products
        Console.WriteLine("Name: {0} \t ID: {1}", _
                product.Name, _
                product.ProductID)
    Next
End Using

using (AdventureWorksEntities context = new AdventureWorksEntities())
{
    // LINQ to Entities only supports Skip on ordered collections.
    IOrderedQueryable<Product> products = context.Products
            .OrderBy(p => p.ListPrice);

    IQueryable<Product> allButFirst3Products = products.Skip(3);

    Console.WriteLine("All but first 3 products:");
    foreach (Product product in allButFirst3Products)
    {
        Console.WriteLine("Name: {0} \t ID: {1}",
            product.Name,
            product.ProductID);
    }
}

Di seguito viene illustrato un esempio di Entity SQL .

Using context As New AdventureWorksEntities()
    ' Create a query that takes two parameters. 
    Dim queryString As String = "SELECT VALUE product FROM AdventureWorksEntities.Products AS product " & _
        " order by product.ListPrice SKIP @skip LIMIT @limit"

    Dim productQuery As New ObjectQuery(Of Product)(queryString, context)

    ' Add parameters to the collection. 
    productQuery.Parameters.Add(New ObjectParameter("skip", 3))
    productQuery.Parameters.Add(New ObjectParameter("limit", 5))

    ' Iterate through the collection of Contact items. 
    For Each result As Product In productQuery
        Console.WriteLine("ID: {0}; Name: {1}", result.ProductID, result.Name)
    Next
End Using

using (AdventureWorksEntities context =
    new AdventureWorksEntities())
{
    // Create a query that takes two parameters.
    string queryString =
        @"SELECT VALUE product FROM 
          AdventureWorksEntities.Products AS product 
          order by product.ListPrice SKIP @skip LIMIT @limit";

    ObjectQuery<Product> productQuery =
        new ObjectQuery<Product>(queryString, context);

    // Add parameters to the collection.
    productQuery.Parameters.Add(new ObjectParameter("skip", 3));
    productQuery.Parameters.Add(new ObjectParameter("limit", 5));

    // Iterate through the collection of Contact items.
    foreach (Product result in productQuery)
        Console.WriteLine("ID: {0}; Name: {1}",
        result.ProductID, result.Name);
}

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

Using context As New AdventureWorksEntities()
    ' Define the parameters used to define the "page" of returned data. 
    Dim skipValue As Integer = 3
    Dim limitValue As Integer = 5

    ' Define a query that returns a "page" or the full 
    ' Product data using the Skip and Top methods. 
    ' When Top() follows Skip(), it acts like the LIMIT statement. 
    Dim query As ObjectQuery(Of Product) = _
        context.Products.Skip("it.ListPrice", "@skip", _
            New ObjectParameter("skip", skipValue)).Top("@limit", New ObjectParameter("limit", limitValue))

    ' Iterate through the page of Product items. 
    For Each result As Product In query
        Console.WriteLine("ID: {0}; Name: {1}", result.ProductID, result.Name)
    Next
End Using

using (AdventureWorksEntities context =
    new AdventureWorksEntities())
{
    // Define the parameters used to define the "page" of returned data.
    int skipValue = 3;
    int limitValue = 5;

    // Define a query that returns a "page" or the full 
    // Product data using the Skip and Top methods. 
    // When Top() follows Skip(), it acts like the LIMIT statement.
    ObjectQuery<Product> query = context.Products
        .Skip("it.ListPrice", "@skip",
                new ObjectParameter("skip", skipValue))
        .Top("@limit", new ObjectParameter("limit", limitValue));

    // Iterate through the page of Product items.
    foreach (Product result in query)
        Console.WriteLine("ID: {0}; Name: {1}",
        result.ProductID, result.Name);
}

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Procedura: paging dei risultati delle query (Entity Framework)

Esempio

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