Procedura: eseguire una query che restituisce risultati StructuralType

  • Articolo

In questo argomento viene illustrato come eseguire un comando su un modello concettuale usando un oggetto EntityCommand e viene mostrato come recuperare i risultati di StructuralType usando un oggetto EntityDataReader. Le classi EntityType, RowType e ComplexType derivano dalla classe StructuralType.

Per eseguire il codice in questo esempio

  1. Aggiungere il modello Sales di AdventureWorks al progetto e configurare il progetto per l'utilizzo di Entity Framework. Per altre informazioni, vedere Procedura: Usare la procedura guidata di Entity Data Model.

  2. Nella tabella codici per l'applicazione aggiungere le istruzioni using seguenti (Imports in Visual Basic):

    using System;
using System.Collections.Generic;
using System.Collections;
using System.Data.Common;
using System.Data;
using System.IO;
using System.Data.SqlClient;
using System.Data.EntityClient;
using System.Data.Metadata.Edm;

    Imports System.Collections.Generic
Imports System.Collections
Imports System.Data.Common
Imports System.Data
Imports System.IO
Imports System.Data.SqlClient
Imports System.Data.EntityClient
Imports System.Data.Metadata.Edm

Esempio

In questo esempio viene eseguita una query che restituisce risultati dell'oggetto EntityType. Se si passa la query riportata di seguito come argomento alla funzione ExecuteStructuralTypeQuery, quest'ultima visualizza i dettagli relativi a Products:

SELECT VALUE Product FROM AdventureWorksEntities.Products AS Product

Se si passa una query con parametri, come quella riportata di seguito, aggiungere gli oggetti EntityParameter alla proprietà Parameters nell'oggetto EntityCommand.

SELECT VALUE product FROM AdventureWorksEntities.Products
    AS product where product.ListPrice >= @price

static void ExecuteStructuralTypeQuery(string esqlQuery)
{
    if (esqlQuery.Length == 0)
    {
        Console.WriteLine("The query string is empty.");
        return;
    }

    using (EntityConnection conn =
        new EntityConnection("name=AdventureWorksEntities"))
    {
        conn.Open();

        // Create an EntityCommand.
        using (EntityCommand cmd = conn.CreateCommand())
        {
            cmd.CommandText = esqlQuery;
            // Execute the command.
            using (EntityDataReader rdr =
                cmd.ExecuteReader(CommandBehavior.SequentialAccess))
            {
                // Start reading results.
                while (rdr.Read())
                {
                    StructuralTypeVisitRecord(rdr as IExtendedDataRecord);
                }
            }
        }
        conn.Close();
    }
}

static void StructuralTypeVisitRecord(IExtendedDataRecord record)
{
    int fieldCount = record.DataRecordInfo.FieldMetadata.Count;
    for (int fieldIndex = 0; fieldIndex < fieldCount; fieldIndex++)
    {
        Console.Write(record.GetName(fieldIndex) + ": ");

        // If the field is flagged as DbNull, the shape of the value is undetermined.
        // An attempt to get such a value may trigger an exception.
        if (record.IsDBNull(fieldIndex) == false)
        {
            BuiltInTypeKind fieldTypeKind = record.DataRecordInfo.FieldMetadata[fieldIndex].
                FieldType.TypeUsage.EdmType.BuiltInTypeKind;
            // The EntityType, ComplexType and RowType are structural types
            // that have members.
            // Read only the PrimitiveType members of this structural type.
            if (fieldTypeKind == BuiltInTypeKind.PrimitiveType)
            {
                // Primitive types are surfaced as plain objects.
                Console.WriteLine(record.GetValue(fieldIndex).ToString());
            }
        }
    }
}

Private Shared Sub ExecuteStructuralTypeQuery(ByVal esqlQuery As String)
    If esqlQuery.Length = 0 Then
        Console.WriteLine("The query string is empty.")
        Exit Sub
    End If

    Using conn As New EntityConnection("name=AdventureWorksEntities")
        conn.Open()

        ' Create an EntityCommand. 
        Using cmd As EntityCommand = conn.CreateCommand()
            cmd.CommandText = esqlQuery
            ' Execute the command. 
            Using rdr As EntityDataReader = cmd.ExecuteReader(CommandBehavior.SequentialAccess)
                ' Start reading results. 
                While rdr.Read()
                    StructuralTypeVisitRecord(TryCast(rdr, IExtendedDataRecord))
                End While
            End Using
        End Using
        conn.Close()
    End Using
End Sub

Private Shared Sub StructuralTypeVisitRecord(ByVal record As IExtendedDataRecord)
    Dim fieldCount As Integer = record.DataRecordInfo.FieldMetadata.Count
    For fieldIndex As Integer = 0 To fieldCount - 1
        Console.Write(record.GetName(fieldIndex) & ": ")

        ' If the field is flagged as DbNull, the shape of the value is undetermined. 
        ' An attempt to get such a value may trigger an exception. 
        If record.IsDBNull(fieldIndex) = False Then
            Dim fieldTypeKind As BuiltInTypeKind = record.DataRecordInfo.FieldMetadata(fieldIndex).FieldType.TypeUsage.EdmType.BuiltInTypeKind
            ' The EntityType, ComplexType and RowType are structural types 
            ' that have members. 
            ' Read only the PrimitiveType members of this structural type. 
            If fieldTypeKind = BuiltInTypeKind.PrimitiveType Then
                ' Primitive types are surfaced as plain objects. 
                Console.WriteLine(record.GetValue(fieldIndex).ToString())
            End If
        End If
    Next
End Sub

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