Procedura: eseguire una query che restituisce una raccolta di tipi primitivi (Entity Framework)
In questo argomento vengono forniti alcuni esempi di come eseguire query che restituiscono una raccolta di tipi primitivi. Per restituire un singolo oggetto, specificare uno dei metodi seguenti nella query: First, FirstOrDefault, Single, SingleOrDefault.
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>
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 .
Dim contactId As Integer = 377
Using context As New AdventureWorksEntities()
' Select a value.
Dim orders As ObjectSet(Of SalesOrderHeader) = context.SalesOrderHeaders
Dim orderQuery As IQueryable(Of Int32) = From order In orders _
Where order.Contact.ContactID = contactId _
Select order.PurchaseOrderNumber.Length
' Iterate through the collection of values.
For Each result As Int32 In orderQuery
Console.WriteLine("{0}", result)
Next
' Use a nullable DateTime value because ShipDate can be null.
Dim shipDateQuery As IQueryable(Of System.Nullable(Of DateTime)) = From order In orders _
Where order.Contact.ContactID = contactId _
Select order.ShipDate
' Iterate through the collection of values.
For Each shipDate As System.Nullable(Of DateTime) In shipDateQuery
Dim shipDateMessage As String = "date not set"
If shipDate IsNot Nothing Then
shipDateMessage = shipDate.ToString()
End If
Console.WriteLine("Ship Date: {0}.", shipDateMessage)
Next
End Using
int contactId = 377;
using (AdventureWorksEntities context
= new AdventureWorksEntities())
{
// Select a value.
ObjectSet<SalesOrderHeader> orders
= context.SalesOrderHeaders;
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);
}
}
Di seguito viene fornito un esempio di utilizzo di Entity SQL .
Dim contactId As Integer = 377
Using context As New AdventureWorksEntities()
Dim orderQueryString As String = "SELECT VALUE Length(order.PurchaseOrderNumber) FROM " & _
" AdventureWorksEntities.SalesOrderHeaders AS order WHERE order.CustomerID = @contactId"
Dim shipDateQueryString As String = "SELECT VALUE order.ShipDate" & _
" FROM AdventureWorksEntities.SalesOrderHeaders AS order WHERE order.CustomerID = @contactId"
' Use the SelectValue method to select a value.
Dim orderQuery As New ObjectQuery(Of Int32)(orderQueryString, context, MergeOption.NoTracking)
orderQuery.Parameters.Add(New ObjectParameter("contactId", contactId))
' Iterate through the collection of values.
For Each result As Int32 In orderQuery
Console.WriteLine("{0}", result)
Next
' Use a nullable DateTime value because ShipDate can be null.
Dim shipDateQuery As New ObjectQuery(Of Nullable(Of DateTime))(shipDateQueryString, context, MergeOption.NoTracking)
shipDateQuery.Parameters.Add(New ObjectParameter("contactId", contactId))
' Iterate through the collection of values.
For Each shipDate As Nullable(Of DateTime) In shipDateQuery
Dim shipDateMessage As String = "date not set"
If shipDate IsNot Nothing Then
shipDateMessage = shipDate.ToString()
End If
Console.WriteLine("Ship Date: {0}.", shipDateMessage)
Next
End Using
int contactId = 377;
using (AdventureWorksEntities context =
new AdventureWorksEntities())
{
string orderQueryString = @"SELECT VALUE Length(order.PurchaseOrderNumber)
FROM AdventureWorksEntities.SalesOrderHeaders AS order
WHERE order.CustomerID = @contactId";
string shipDateQueryString = @"SELECT VALUE order.ShipDate
FROM AdventureWorksEntities.SalesOrderHeaders AS order
WHERE order.CustomerID = @contactId";
// Use the SelectValue method to select a value.
ObjectQuery<Int32> orderQuery =
new ObjectQuery<Int32>(orderQueryString,
context, 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,
context, 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);
}
}
Di seguito viene fornito un esempio del metodo del generatore di query.
Dim contactId As Integer = 377
Using context As New AdventureWorksEntities()
' Use the SelectValue method to select a value.
Dim orderQuery As ObjectQuery(Of Int32) = context.SalesOrderHeaders.Where("it.CustomerID = @contactId", _
New ObjectParameter("contactId", contactId)).SelectValue(Of Int32)("Length(it.PurchaseOrderNumber)")
' Iterate through the collection of values.
For Each result As Int32 In orderQuery
Console.WriteLine("{0}", result)
Next
' Use a nullable DateTime value because ShipDate can be null.
Dim shipDateQuery As ObjectQuery(Of Nullable(Of DateTime)) = _
context.SalesOrderHeaders.Where("it.CustomerID = @contactId", _
New ObjectParameter("contactId", contactId)).SelectValue(Of Nullable(Of DateTime))("it.ShipDate")
' Iterate through the collection of values.
For Each shipDate As Nullable(Of DateTime) In shipDateQuery
Dim shipDateMessage As String = "date not set"
If shipDate IsNot Nothing Then
shipDateMessage = shipDate.ToString()
End If
Console.WriteLine("Ship Date: {0}.", shipDateMessage)
Next
End Using
int contactId = 377;
using (AdventureWorksEntities context =
new AdventureWorksEntities())
{
// Use the SelectValue method to select a value.
ObjectQuery<Int32> orderQuery =
context.SalesOrderHeaders
.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 =
context.SalesOrderHeaders
.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);
}
}
Vedere anche
Attività
Procedura: eseguire una query che restituisce oggetti di tipo entità (Entity Framework)
Procedura: eseguire una query che restituisce una raccolta di tipi anonimi (Entity Framework)
Procedura: eseguire una query con parametri (Entity Framework)