IsolationLevel Výčet

Reference

Definice

Obor názvů:: System.Data

Sestavení:: System.Data.Common.dll

Sestavení:: System.Data.dll

Sestavení:: netstandard.dll

Důležité

Některé informace platí pro předběžně vydaný produkt, který se může zásadně změnit, než ho výrobce nebo autor vydá. Microsoft neposkytuje žádné záruky, výslovné ani předpokládané, týkající se zde uváděných informací.

Určuje chování uzamykání transakce pro připojení.

Tento výčet podporuje bitové kombinace hodnot jeho členů.

public enum class IsolationLevel

public enum IsolationLevel

[System.Flags]
public enum IsolationLevel

type IsolationLevel =

[<System.Flags>]
type IsolationLevel =

Public Enum IsolationLevel

Dědičnost: Object

ValueType

Enum
IsolationLevel

Atributy: FlagsAttribute

Pole

Chaos	16	Čekající změny z více izolovaných transakcí nelze přepsat.
ReadCommitted	4096	Sdílené zámky se zachovají, když se data čtou, aby se zabránilo nečteným čtením, ale data je možné před koncem transakce změnit, což vede k neopakovatelným čtením nebo fiktivním datům.
ReadUncommitted	256	Je možné číst nečistě, což znamená, že se nevystavují žádné sdílené zámky a nejsou respektovány žádné výhradní zámky.
RepeatableRead	65536	Zámky se umisťují na všechna data použitá v dotazu, což ostatním uživatelům brání v aktualizaci dat. Zabraňuje neopakovatelnému čtení, ale fiktivní řádky jsou stále možné.
Serializable	1048576	Zámek rozsahu je umístěn na objektu DataSet, který ostatním uživatelům brání v aktualizaci nebo vkládání řádků do datové sady až do dokončení transakce.
Snapshot	16777216	Snižuje blokování ukládáním verze dat, které jedna aplikace může číst, zatímco jiná upravuje stejná data. Označuje, že z jedné transakce nevidíte změny provedené v jiných transakcích, a to ani v případě, že znovu spustíte dotaz.
Unspecified	-1	Používá se jiná úroveň izolace, než je zadaná, ale úroveň nelze určit.

Příklady

Tato aplikace ukazuje, jak používat IsolationLevel v DbTransaction. Ukázka předvede, které z následujících chování je povoleno na různých úrovních izolace:

Špinavá čtení.
Neopakovatelné čtení.
Přízraky.

Tato aplikace se spustí v následujících úrovních izolace:

Readuncommitted
Readcommitted
Repeatableread
Serializovatelný
Snímek

Třída PhantomReadThreads ukazuje, zda konkrétní transakce umožňuje fiktivní čtení chování. Pokud transakce umožňuje chování, vlákna budou fungovat v následujícím pořadí:

V prvním vlákně vyberte products(All).
Do druhého vlákna vložte nový produkt.
Potvrďte transakci ve druhém vlákně.
Znovu vyberte produkty.
Potvrďte transakci v prvním vlákně.

Pokud transakce umožňuje chování, dvě operace Select získají různé výsledky.

NonrepeatableReadThreads Třída ukazuje, zda konkrétní transakce umožňuje nonrepeatable čtení chování. Pokud transakce umožňuje chování, vlákna budou fungovat v následujícím pořadí:

V prvním vlákně vyberte produkt(ProductId=1).
Ve druhém vlákně aktualizujte hodnotu Quantity(ProductId=1).
Potvrďte transakci ve druhém vlákně.
Znovu vyberte produkt.
Potvrďte transakci v prvním vlákně.

Pokud transakce umožňuje chování, dvě operace Select získají různé výsledky.

ExchangeValuesThreads Třída demonstruje rozdíl mezi Serializable a Snapshot transakce. U serializovatelné transakce budou vlákna fungovat v následujícím pořadí:

V prvním vlákně získejte cenu produktu (ProductId=2) a uložte ji do proměnné .
V prvním vlákně aktualizujte price of product(ProductId=1) cenou produktu(ProductId=2).
Potvrďte transakci v prvním vlákně.
Ve druhém vlákně získejte cenu produktu (ProductId=1) a uložte ji do proměnné .
Ve druhém vlákně aktualizujte cenu produktu(ProductId=2) cenou produktu(ProductId=1).
Potvrďte transakci ve druhém vlákně.

Nyní jsou hodnoty Price(ProductId=1 a ProductId=2) stejné jako původní cena produktu(ProductId=2).

V případě transakce snímku budou vlákna fungovat v následujícím pořadí:

V prvním vlákně získejte cenu produktu (ProductId=2) a uložte ji do proměnné ;
V prvním vlákně aktualizujte price of product(ProductId=1) cenou produktu(ProductId=2).
Ve druhém vlákně získejte hodnotu Price of product(ProductId=1) ze snímku a uložte ji do proměnné .
Ve druhém vlákně aktualizujte cenu produktu(ProductId=2) cenou produktu(ProductId=1).
Potvrďte transakci ve druhém vlákně.
Potvrďte transakci v prvním vlákně.

Nyní vyměňte cenu produktů (ProductId=1 a ProductId=2).

DirtyReadThreads Třída ukazuje, zda konkrétní transakce umožňuje chování dirty čtení. Pokud transakce umožňuje chování, vlákna budou fungovat v následujícím pořadí:

V prvním vlákně začněte transakci a přidejte hodnotu Quantity(ProductId=1).
Ve druhém vlákně načtěte hodnotu Quantity a znovu ji přidejte.
Potvrďte transakci ve druhém vlákně.
Vrácení transakce zpět v prvním vlákně.

Pokud transakce umožňuje chování, bude hodnota Quantity přidána dvakrát.

using System;
using System.Data.SqlClient;
using System.Data;
using System.Threading;
using System.Threading.Tasks;

namespace CSDataIsolationLevel {

   // Use the delegate to call the different threads.
   public delegate void AsyncAccessDatabase(String connString, IsolationLevel level);

   static class DirtyReadThreads {
      public static void DirtyReadFirstThread(String connStrig, IsolationLevel level) {
         Console.WriteLine("Begin the DirtyReadFirstThread.....");

         using (SqlConnection conn = new SqlConnection(connStrig)) {
            String cmdText = @"Use DbDataIsolationLevel;

                    Update dbo.Products set Quantity=Quantity+100 where ProductId=1;
                    WaitFor Delay '00:00:06';";

            conn.Open();

            using (SqlTransaction tran = conn.BeginTransaction(level, "DirtyReadFirst")) {
               using (SqlCommand command = new SqlCommand(cmdText, conn)) {
                  command.Transaction = tran;
                  command.ExecuteNonQuery();
               }

               if (tran != null)
                  tran.Rollback();
            }
         }

         Console.WriteLine("Exit from the DirtyReadFirstThread.....");
      }

      public static void DirtyReadSecondThread(String connStrig, IsolationLevel level) {
         Console.WriteLine("Begin the DirtyReadSecondThread.....");

         using (SqlConnection conn = new SqlConnection(connStrig)) {
            String cmdText = @"Use DbDataIsolationLevel;

                    WaitFor Delay '00:00:03';

                    Declare @qty int;
                    select @qty=Quantity from dbo.Products where ProductId=1;

                    Update dbo.Products set Quantity=@qty+100 where ProductId=1;";

            conn.Open();

            using (SqlTransaction tran = conn.BeginTransaction(level, "DirtyReadSecond")) {
               using (SqlCommand command = new SqlCommand(cmdText, conn)) {
                  command.Transaction = tran;
                  command.ExecuteNonQuery();
               }
               tran.Commit();
            }
         }

         Console.WriteLine("Exit from the DirtyReadSecondThread.....");
      }
   }

   static class NonrepeatableReadThreads {
      public static void NonrepeatableReadFirstThread(String connStrig, IsolationLevel level) {
         Console.WriteLine("Begin the NonrepeatableReadFirstThread.....");

         using (SqlConnection conn = new SqlConnection(connStrig)) {
            String cmdText = @"Use DbDataIsolationLevel;

                    Select ProductId,ProductName,Quantity,Price
                    from dbo.Products
                    where ProductId=1

                    WaitFor Delay '00:00:06';

                    Select ProductId,ProductName,Quantity,Price
                    from dbo.Products
                    where ProductId=1";

            conn.Open();

            using (SqlTransaction tran = conn.BeginTransaction(level, "NonrepeatableReadFirst")) {
               using (SqlCommand command = new SqlCommand(cmdText, conn)) {
                  command.Transaction = tran;

                  using (SqlDataReader reader = command.ExecuteReader()) {
                     Boolean isFirstReader = true;
                     do {
                        Console.WriteLine("It's the result of {0} read:", isFirstReader ? "first" : "second");
                        TransactionIsolationLevels.DisplayData(reader);
                        isFirstReader = !isFirstReader;
                     } while (reader.NextResult() && !isFirstReader);
                  }
               }

               tran.Commit();
            }
         }

         Console.WriteLine("Exit from the NonrepeatableReadFirstThread.....");
      }

      public static void NonrepeatableReadSecondThread(String connStrig, IsolationLevel level) {
         Console.WriteLine("Begin the NonrepeatableReadSecondThread.....");

         using (SqlConnection conn = new SqlConnection(connStrig)) {
            String cmdText = @"Use DbDataIsolationLevel;

                    WaitFor Delay '00:00:03';

                    Update dbo.Products set Quantity=Quantity+100 where ProductId=1;";

            conn.Open();

            using (SqlTransaction tran = conn.BeginTransaction(level, "NonrepeatableReadSecond")) {
               using (SqlCommand command = new SqlCommand(cmdText, conn)) {
                  command.Transaction = tran;
                  command.ExecuteNonQuery();
               }
               tran.Commit();
            }
         }

         Console.WriteLine("Exit from the NonrepeatableReadSecondThread.....");
      }
   }

   static class PhantomReadThreads {
      public static void PhantomReadFirstThread(String connStrig, IsolationLevel level) {
         Console.WriteLine("Begin the PhantomReadFirstThread.....");

         using (SqlConnection conn = new SqlConnection(connStrig)) {
            String cmdText = @"Use DbDataIsolationLevel;

                    Select ProductId,ProductName,Quantity,Price
                    from dbo.Products

                    WaitFor Delay '00:00:06';

                    Select ProductId,ProductName,Quantity,Price
                    from dbo.Products";

            conn.Open();

            using (SqlTransaction tran = conn.BeginTransaction(level, "PhantomReadFirst")) {
               using (SqlCommand command = new SqlCommand(cmdText, conn)) {
                  command.Transaction = tran;

                  using (SqlDataReader reader = command.ExecuteReader()) {
                     Boolean isFirstReader = true;
                     do {
                        Console.WriteLine("It's the result of {0} read:", isFirstReader ? "first" : "second");

                        TransactionIsolationLevels.DisplayData(reader);

                        isFirstReader = !isFirstReader;
                     } while (reader.NextResult() && !isFirstReader);
                  }
               }

               tran.Commit();
            }
         }
         Console.WriteLine("Exit from the PhantomReadFirstThread.....");
      }

      public static void PhantomReadSecondThread(String connStrig, IsolationLevel level) {
         Console.WriteLine("Begin the PhantomReadSecondThread.....");

         using (SqlConnection conn = new SqlConnection(connStrig)) {
            String cmdText = @"Use DbDataIsolationLevel;

                    WaitFor Delay '00:00:03';

                    INSERT [dbo].[Products] ([ProductName], [Quantity], [Price])
                    VALUES (N'White Bike', 843, 1349.00)";

            conn.Open();

            using (SqlTransaction tran = conn.BeginTransaction(level, "PhantomReadSecond")) {
               using (SqlCommand command = new SqlCommand(cmdText, conn)) {
                  command.Transaction = tran;
                  command.ExecuteNonQuery();
               }
               tran.Commit();
            }
         }

         Console.WriteLine("Exit from the PhantomReadSecondThread.....");
      }
   }

   // Demonstrates if the specific transaction allows the following behaviors:
   // 1. Dirty reads;
   // 2. Non-repeatable reads;
   // 3. Phantoms.
   static class TransactionIsolationLevels {
      public static void DemonstrateIsolationLevel(String connString, IsolationLevel level) {
         // Before connect the database, recreate the table.
         OperateDatabase.CreateTable(connString);
         DemonstrateIsolationLevel(connString, level, DirtyReadThreads.DirtyReadFirstThread, DirtyReadThreads.DirtyReadSecondThread);
         DisplayData(connString);
         Console.WriteLine();

         OperateDatabase.CreateTable(connString);
         DemonstrateIsolationLevel(connString, level, NonrepeatableReadThreads.NonrepeatableReadFirstThread, NonrepeatableReadThreads.NonrepeatableReadSecondThread);
         Console.WriteLine();

         OperateDatabase.CreateTable(connString);
         DemonstrateIsolationLevel(connString, level, PhantomReadThreads.PhantomReadFirstThread, PhantomReadThreads.PhantomReadSecondThread);
         Console.WriteLine();
      }

      // Demonstrates if the specific transaction allows the specific behaviors.
      public static void DemonstrateIsolationLevel(String connString, IsolationLevel level,
          AsyncAccessDatabase firstThread, AsyncAccessDatabase secondThread) {
         Task[] tasks ={
                            Task.Factory.StartNew(()=>firstThread(connString, level)),
                            Task.Factory.StartNew(()=>secondThread(connString, level))
                        };

         Task.WaitAll(tasks);
      }

      static class ExchangeValuesThreads {
         public static void ExchangeValuesFirstThread(String connStrig, IsolationLevel level) {
            Console.WriteLine("Begin the ExchangeValuesFirstThread.....");

            using (SqlConnection conn = new SqlConnection(connStrig)) {
               String cmdText = @"Use DbDataIsolationLevel;

                    Declare @price money;
                    select @price=Price from dbo.Products where ProductId=2;

                    Update dbo.Products set Price=@price where ProductId=1;

                    WaitFor Delay '00:00:06'; ";

               conn.Open();
               using (SqlTransaction tran = conn.BeginTransaction(level, "ExchangeValuesFirst")) {

                  using (SqlCommand command = new SqlCommand(cmdText, conn)) {
                     command.Transaction = tran;
                     command.ExecuteNonQuery();
                  }

                  tran.Commit();
               }
            }

            Console.WriteLine("Exit from the ExchangeValuesFirstThread.....");
         }

         public static void ExchangeValuesSecondThread(String connStrig, IsolationLevel level) {
            Console.WriteLine("Begin the ExchangeValuesSecondThread.....");

            using (SqlConnection conn = new SqlConnection(connStrig)) {
               String cmdText = @"Use DbDataIsolationLevel;

                    WaitFor Delay '00:00:03';

                    Declare @price money;
                    select @price=Price from dbo.Products where ProductId=1;

                    Update dbo.Products set Price=@price where ProductId=2;";

               conn.Open();

               using (SqlTransaction tran = conn.BeginTransaction(level, "ExchangeValuesSecond")) {
                  using (SqlCommand command = new SqlCommand(cmdText, conn)) {
                     command.Transaction = tran;
                     command.ExecuteNonQuery();
                  }
                  tran.Commit();
               }
            }

            Console.WriteLine("Exit from the ExchangeValuesSecondThread.....");
         }
      }

      // Demonstrates the difference between the Serializable and Snapshot transaction
      public static void DemonstrateBetweenSnapshotAndSerializable(String connString) {
         OperateDatabase.CreateTable(connString);

         Console.WriteLine("Exchange Vaules in the Snapshot transaction:");
         DemonstrateIsolationLevel(connString, IsolationLevel.Snapshot,
             ExchangeValuesThreads.ExchangeValuesFirstThread,
             ExchangeValuesThreads.ExchangeValuesSecondThread);
         DisplayData(connString);
         Console.WriteLine();

         Console.WriteLine("Cannot Exchange Vaules in the Serializable transaction:");
         OperateDatabase.CreateTable(connString);
         DemonstrateIsolationLevel(connString, IsolationLevel.Serializable,
             ExchangeValuesThreads.ExchangeValuesFirstThread,
             ExchangeValuesThreads.ExchangeValuesSecondThread);
         DisplayData(connString);
      }

      public static void DisplayData(String connString) {
         using (SqlConnection conn = new SqlConnection(connString)) {
            String cmdText = @"Use DbDataIsolationLevel;

                    Select ProductId,ProductName,Quantity,Price
                    from dbo.Products";

            conn.Open();

            using (SqlCommand command = new SqlCommand(cmdText, conn)) {
               using (SqlDataReader reader = command.ExecuteReader()) {
                  DisplayData(reader);
               }
            }
         }
      }

      public static void DisplayData(SqlDataReader reader) {
         Boolean isFirst = true;

         while (reader.Read()) {
            if (isFirst) {
               isFirst = false;

               for (int i = 0; i < reader.FieldCount; i++)
                  Console.Write("{0,-12}   ", reader.GetName(i));
               Console.WriteLine();
            }

            for (int i = 0; i < reader.FieldCount; i++)
               Console.Write("{0,-12}   ", reader[i]);
            Console.WriteLine();
         }
      }
   }

   // This class includes database operations. If there's no database 'DbDataIsolationLevel', create the database.
   static class OperateDatabase {
      public static Boolean CreateDatabase(String connString) {
         using (SqlConnection conn = new SqlConnection(connString)) {
            String cmdText = @"Use Master;

                                     if Db_Id('DbDataIsolationLevel') is null
                                      create Database [DbDataIsolationLevel];";

            using (SqlCommand command = new SqlCommand(cmdText, conn)) {
               conn.Open();
               command.ExecuteNonQuery();
            }

            Console.WriteLine("Create the Database 'DbDataIsolationLevel'");
         }

         return true;
      }

      // If there's no table [dbo].[Products] in DbDataIsolationLevel, create the table; or recreate it.
      public static Boolean CreateTable(String connString) {
         using (SqlConnection conn = new SqlConnection(connString)) {
            String cmdText = @"Use DbDataIsolationLevel

                                    if Object_ID('[dbo].[Products]') is not null
                                    drop table [dbo].[Products]

                                    Create Table [dbo].[Products]
                                    (
                                    [ProductId] int IDENTITY(1,1) primary key,
                                    [ProductName] NVarchar(100) not null,
                                    [Quantity] int null,
                                    [Price] money null
                                    )";

            using (SqlCommand command = new SqlCommand(cmdText, conn)) {
               conn.Open();
               command.ExecuteNonQuery();
            }
         }

         return InsertRows(connString);
      }

      // Insert some rows into [dbo].[Products] table.
      public static Boolean InsertRows(String connString) {
         using (SqlConnection conn = new SqlConnection(connString)) {
            String cmdText = @"Use DbDataIsolationLevel

                    INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Blue Bike', 365,1075.00)
                    INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Red Bike', 159, 1299.00)
                    INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Black Bike', 638, 1159.00)";

            using (SqlCommand command = new SqlCommand(cmdText, conn)) {
               conn.Open();
               command.ExecuteNonQuery();
            }
         }
         return true;
      }

      // Turn on or off 'ALLOW_SNAPSHOT_ISOLATION'
      public static Boolean SetSnapshot(String connString, Boolean isOpen) {
         using (SqlConnection conn = new SqlConnection(connString)) {
            String cmdText = null;

            if (isOpen)
               cmdText = @"ALTER DATABASE DbDataIsolationLevel SET ALLOW_SNAPSHOT_ISOLATION ON";
            else
               cmdText = @"ALTER DATABASE DbDataIsolationLevel SET ALLOW_SNAPSHOT_ISOLATION OFF";

            using (SqlCommand command = new SqlCommand(cmdText, conn)) {
               conn.Open();
               command.ExecuteNonQuery();
            }
         }

         return true;
      }
   }
   class Program {
      static void Main(string[] args) {
         String connString = "Data Source=(local);Initial Catalog=master;Integrated Security=True;Asynchronous Processing=true;";

         OperateDatabase.CreateDatabase(connString);
         Console.WriteLine();

         Console.WriteLine("Demonstrate the ReadUncommitted transaction: ");
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
             System.Data.IsolationLevel.ReadUncommitted);
         Console.WriteLine("-----------------------------------------------");

         Console.WriteLine("Demonstrate the ReadCommitted transaction: ");
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
             System.Data.IsolationLevel.ReadCommitted);
         Console.WriteLine("-----------------------------------------------");

         Console.WriteLine("Demonstrate the RepeatableRead transaction: ");
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
             System.Data.IsolationLevel.RepeatableRead);
         Console.WriteLine("-----------------------------------------------");

         Console.WriteLine("Demonstrate the Serializable transaction: ");
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
             System.Data.IsolationLevel.Serializable);
         Console.WriteLine("-----------------------------------------------");

         Console.WriteLine("Demonstrate the Snapshot transaction: ");
         OperateDatabase.SetSnapshot(connString, true);
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
             System.Data.IsolationLevel.Snapshot);
         Console.WriteLine("-----------------------------------------------");

         Console.WriteLine("Demonstrate the difference between the Snapshot and Serializable transactions:");
         TransactionIsolationLevels.DemonstrateBetweenSnapshotAndSerializable(connString);
         OperateDatabase.SetSnapshot(connString, false);
         Console.WriteLine();
      }
   }
}

Imports System.Data.SqlClient
Imports System.Data
Imports System.Threading
Imports System.Threading.Tasks

Namespace CSDataIsolationLevel

   ' Use the delegate to call the different threads.
   Public Delegate Sub AsyncAccessDatabase(connString As [String], level As IsolationLevel)

   NotInheritable Class DirtyReadThreads
      Private Sub New()
      End Sub
      Public Shared Sub DirtyReadFirstThread(connStrig As [String], level As IsolationLevel)
         Console.WriteLine("Begin the DirtyReadFirstThread.....")

         Using conn As New SqlConnection(connStrig)
            Dim cmdText As [String] = "Use DbDataIsolationLevel; " & vbCr & vbLf & vbCr & vbLf &
                "Update dbo.Products set Quantity=Quantity+100 where ProductId=1;" & vbCr & vbLf &
                "WaitFor Delay '00:00:06';"

            conn.Open()

            Using tran As SqlTransaction = conn.BeginTransaction(level, "DirtyReadFirst")
               Using command As New SqlCommand(cmdText, conn)
                  command.Transaction = tran
                  command.ExecuteNonQuery()
               End Using

               If tran IsNot Nothing Then
                  tran.Rollback()
               End If
            End Using
         End Using

         Console.WriteLine("Exit from the DirtyReadFirstThread.....")
      End Sub

      Public Shared Sub DirtyReadSecondThread(connStrig As [String], level As IsolationLevel)
         Console.WriteLine("Begin the DirtyReadSecondThread.....")

         Using conn As New SqlConnection(connStrig)
            Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
                "WaitFor Delay '00:00:03'; " & vbCr & vbLf & vbCr & vbLf &
                "Declare @qty int;" & vbCr & vbLf &
                "select @qty=Quantity from dbo.Products where ProductId=1;" & vbCr & vbLf & vbCr & vbLf &
                "Update dbo.Products set Quantity=@qty+100 where ProductId=1;"

            conn.Open()

            Using tran As SqlTransaction = conn.BeginTransaction(level, "DirtyReadSecond")
               Using command As New SqlCommand(cmdText, conn)
                  command.Transaction = tran
                  command.ExecuteNonQuery()
               End Using
               tran.Commit()
            End Using
         End Using

         Console.WriteLine("Exit from the DirtyReadSecondThread.....")
      End Sub
   End Class


   NotInheritable Class NonrepeatableReadThreads
      Private Sub New()
      End Sub
      Public Shared Sub NonrepeatableReadFirstThread(connStrig As [String], level As IsolationLevel)
         Console.WriteLine("Begin the NonrepeatableReadFirstThread.....")

         Using conn As New SqlConnection(connStrig)
            Dim cmdText As [String] = "Use DbDataIsolationLevel; " & vbCr & vbLf & vbCr & vbLf &
                "Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
                "from dbo.Products" & vbCr & vbLf &
                "where ProductId=1" & vbCr & vbLf & vbCr & vbLf &
                "WaitFor Delay '00:00:06';" & vbCr & vbLf & vbCr & vbLf &
                "Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
                "from dbo.Products" & vbCr & vbLf &
                "where ProductId=1"

            conn.Open()

            Using tran As SqlTransaction = conn.BeginTransaction(level, "NonrepeatableReadFirst")
               Using command As New SqlCommand(cmdText, conn)
                  command.Transaction = tran

                  Using reader As SqlDataReader = command.ExecuteReader()
                     Dim isFirstReader As [Boolean] = True
                     Do
                        Console.WriteLine("It's the result of {0} read:", If(isFirstReader, "first", "second"))
                        TransactionIsolationLevels.DisplayData(reader)
                        isFirstReader = Not isFirstReader
                     Loop While reader.NextResult() AndAlso Not isFirstReader
                  End Using
               End Using

               tran.Commit()
            End Using
         End Using

         Console.WriteLine("Exit from the NonrepeatableReadFirstThread.....")
      End Sub

      Public Shared Sub NonrepeatableReadSecondThread(connStrig As [String], level As IsolationLevel)
         Console.WriteLine("Begin the NonrepeatableReadSecondThread.....")

         Using conn As New SqlConnection(connStrig)
            Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
                "WaitFor Delay '00:00:03'; " & vbCr & vbLf & vbCr & vbLf &
                "Update dbo.Products set Quantity=Quantity+100 where ProductId=1;"

            conn.Open()

            Using tran As SqlTransaction = conn.BeginTransaction(level, "NonrepeatableReadSecond")
               Using command As New SqlCommand(cmdText, conn)
                  command.Transaction = tran
                  command.ExecuteNonQuery()
               End Using
               tran.Commit()
            End Using
         End Using

         Console.WriteLine("Exit from the NonrepeatableReadSecondThread.....")
      End Sub
   End Class


   NotInheritable Class PhantomReadThreads
      Private Sub New()
      End Sub
      Public Shared Sub PhantomReadFirstThread(connStrig As [String], level As IsolationLevel)
         Console.WriteLine("Begin the PhantomReadFirstThread.....")

         Using conn As New SqlConnection(connStrig)
            Dim cmdText As [String] = "Use DbDataIsolationLevel; " & vbCr & vbLf & vbCr & vbLf &
                "Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
                "from dbo.Products" & vbCr & vbLf & vbCr & vbLf &
                "WaitFor Delay '00:00:06';" & vbCr & vbLf & vbCr & vbLf &
                "Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
                "from dbo.Products"

            conn.Open()

            Using tran As SqlTransaction = conn.BeginTransaction(level, "PhantomReadFirst")
               Using command As New SqlCommand(cmdText, conn)
                  command.Transaction = tran

                  Using reader As SqlDataReader = command.ExecuteReader()
                     Dim isFirstReader As [Boolean] = True
                     Do
                        Console.WriteLine("It's the result of {0} read:", If(isFirstReader, "first", "second"))

                        TransactionIsolationLevels.DisplayData(reader)

                        isFirstReader = Not isFirstReader
                     Loop While reader.NextResult() AndAlso Not isFirstReader
                  End Using
               End Using

               tran.Commit()
            End Using
         End Using
         Console.WriteLine("Exit from the PhantomReadFirstThread.....")
      End Sub

      Public Shared Sub PhantomReadSecondThread(connStrig As [String], level As IsolationLevel)
         Console.WriteLine("Begin the PhantomReadSecondThread.....")

         Using conn As New SqlConnection(connStrig)
            Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
                "WaitFor Delay '00:00:03'; " & vbCr & vbLf & vbCr & vbLf &
                "INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) " & vbCr & vbLf &
                "VALUES (N'White Bike', 843, 1349.00)"

            conn.Open()

            Using tran As SqlTransaction = conn.BeginTransaction(level, "PhantomReadSecond")
               Using command As New SqlCommand(cmdText, conn)
                  command.Transaction = tran
                  command.ExecuteNonQuery()
               End Using
               tran.Commit()
            End Using
         End Using

         Console.WriteLine("Exit from the PhantomReadSecondThread.....")
      End Sub
   End Class


   ' Demonstrates if the specific transaction allows the following behaviors:
   ' 1. Dirty reads;
   ' 2. Non-repeatable reads;
   ' 3. Phantoms.
   NotInheritable Class TransactionIsolationLevels
      Private Sub New()
      End Sub
      Public Shared Sub DemonstrateIsolationLevel(connString As [String], level As IsolationLevel)
         ' Before connect the database, recreate the table.
         OperateDatabase.CreateTable(connString)
         DemonstrateIsolationLevel(connString, level, AddressOf DirtyReadThreads.DirtyReadFirstThread, AddressOf DirtyReadThreads.DirtyReadSecondThread)
         DisplayData(connString)
         Console.WriteLine()

         OperateDatabase.CreateTable(connString)
         DemonstrateIsolationLevel(connString, level, AddressOf NonrepeatableReadThreads.NonrepeatableReadFirstThread, AddressOf NonrepeatableReadThreads.NonrepeatableReadSecondThread)
         Console.WriteLine()

         OperateDatabase.CreateTable(connString)
         DemonstrateIsolationLevel(connString, level, AddressOf PhantomReadThreads.PhantomReadFirstThread, AddressOf PhantomReadThreads.PhantomReadSecondThread)
         Console.WriteLine()
      End Sub

      ' Demonstrates if the specific transaction allows the specific behaviors.
      Public Shared Sub DemonstrateIsolationLevel(connString As [String], level As IsolationLevel, firstThread As AsyncAccessDatabase, secondThread As AsyncAccessDatabase)
         ' Dim tasks As Task() = {Task.Factory.StartNew(Function() firstThread(connString, level)), Task.Factory.StartNew(Function() secondThread(connString, level))}

         Dim tasks() As Task = {
                         Task.Factory.StartNew(Sub() firstThread(connString, level)),
                         Task.Factory.StartNew(Sub() secondThread(connString, level))
                                        }

         Task.WaitAll(tasks)
      End Sub

      Private NotInheritable Class ExchangeValuesThreads
         Private Sub New()
         End Sub
         Public Shared Sub ExchangeValuesFirstThread(connStrig As [String], level As IsolationLevel)
            Console.WriteLine("Begin the ExchangeValuesFirstThread.....")

            Using conn As New SqlConnection(connStrig)
               Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
                   "Declare @price money;" & vbCr & vbLf &
                   "select @price=Price from dbo.Products where ProductId=2;" & vbCr & vbLf & vbCr & vbLf &
                   "Update dbo.Products set Price=@price where ProductId=1;" & vbCr & vbLf & vbCr & vbLf &
                   "WaitFor Delay '00:00:06'; "

               conn.Open()
               Using tran As SqlTransaction = conn.BeginTransaction(level, "ExchangeValuesFirst")

                  Using command As New SqlCommand(cmdText, conn)
                     command.Transaction = tran
                     command.ExecuteNonQuery()
                  End Using

                  tran.Commit()
               End Using
            End Using

            Console.WriteLine("Exit from the ExchangeValuesFirstThread.....")
         End Sub

         Public Shared Sub ExchangeValuesSecondThread(connStrig As [String], level As IsolationLevel)
            Console.WriteLine("Begin the ExchangeValuesSecondThread.....")

            Using conn As New SqlConnection(connStrig)
               Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
                   "WaitFor Delay '00:00:03'; " & vbCr & vbLf & vbCr & vbLf &
                   "Declare @price money;" & vbCr & vbLf &
                   "select @price=Price from dbo.Products where ProductId=1;" & vbCr & vbLf & vbCr & vbLf &
                   "Update dbo.Products set Price=@price where ProductId=2;"

               conn.Open()

               Using tran As SqlTransaction = conn.BeginTransaction(level, "ExchangeValuesSecond")
                  Using command As New SqlCommand(cmdText, conn)
                     command.Transaction = tran
                     command.ExecuteNonQuery()
                  End Using
                  tran.Commit()
               End Using
            End Using

            Console.WriteLine("Exit from the ExchangeValuesSecondThread.....")
         End Sub
      End Class

      ' Demonstrates the difference between the Serializable and Snapshot transaction
      Public Shared Sub DemonstrateBetweenSnapshotAndSerializable(connString As [String])
         OperateDatabase.CreateTable(connString)

         Console.WriteLine("Exchange Vaules in the Snapshot transaction:")
         DemonstrateIsolationLevel(connString, IsolationLevel.Snapshot, AddressOf ExchangeValuesThreads.ExchangeValuesFirstThread, AddressOf ExchangeValuesThreads.ExchangeValuesSecondThread)
         DisplayData(connString)
         Console.WriteLine()

         Console.WriteLine("Cannot Exchange Vaules in the Serializable transaction:")
         OperateDatabase.CreateTable(connString)
         DemonstrateIsolationLevel(connString, IsolationLevel.Serializable, AddressOf ExchangeValuesThreads.ExchangeValuesFirstThread, AddressOf ExchangeValuesThreads.ExchangeValuesSecondThread)
         DisplayData(connString)
      End Sub

      Public Shared Sub DisplayData(connString As [String])
         Using conn As New SqlConnection(connString)
            Dim cmdText As [String] = "Use DbDataIsolationLevel; " & vbCr & vbLf & vbCr & vbLf &
                "Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
                "from dbo.Products"

            conn.Open()

            Using command As New SqlCommand(cmdText, conn)
               Using reader As SqlDataReader = command.ExecuteReader()
                  DisplayData(reader)
               End Using
            End Using
         End Using
      End Sub

      Public Shared Sub DisplayData(reader As SqlDataReader)
         Dim isFirst As [Boolean] = True

         While reader.Read()
            If isFirst Then
               isFirst = False

               For i As Integer = 0 To reader.FieldCount - 1
                  Console.Write("{0,-12}   ", reader.GetName(i))
               Next
               Console.WriteLine()
            End If

            For i As Integer = 0 To reader.FieldCount - 1
               Console.Write("{0,-12}   ", reader(i))
            Next
            Console.WriteLine()
         End While
      End Sub
   End Class

   ' This class includes database operations. If there's no database 'DbDataIsolationLevel', create the database.
   NotInheritable Class OperateDatabase
      Private Sub New()
      End Sub
      Public Shared Function CreateDatabase(connString As [String]) As [Boolean]
         Using conn As New SqlConnection(connString)
            Dim cmdText As [String] = "Use Master;" & vbCr & vbLf & vbCr & vbLf &
                "if Db_Id('DbDataIsolationLevel') is null" & vbCr & vbLf &
                "create Database [DbDataIsolationLevel];"

            Using command As New SqlCommand(cmdText, conn)
               conn.Open()
               command.ExecuteNonQuery()
            End Using

            Console.WriteLine("Create the Database 'DbDataIsolationLevel'")
         End Using

         Return True
      End Function


      ' If there's no table [dbo].[Products] in DbDataIsolationLevel, create the table; or recreate it.
      Public Shared Function CreateTable(connString As [String]) As [Boolean]
         Using conn As New SqlConnection(connString)
            Dim cmdText As [String] = "Use DbDataIsolationLevel" & vbCr & vbLf & vbCr & vbLf &
                "if Object_ID('[dbo].[Products]') is not null" & vbCr & vbLf &
                "drop table [dbo].[Products]" & vbCr & vbLf & vbCr & vbLf &
                "Create Table [dbo].[Products]" & vbCr & vbLf &
                "(" & vbCr & vbLf &
                "[ProductId] int IDENTITY(1,1) primary key," & vbCr & vbLf &
                "[ProductName] NVarchar(100) not null," & vbCr & vbLf &
                "[Quantity] int null," & vbCr & vbLf &
                "[Price] money null" & vbCr & vbLf & "                                    )"

            Using command As New SqlCommand(cmdText, conn)
               conn.Open()
               command.ExecuteNonQuery()
            End Using
         End Using

         Return InsertRows(connString)
      End Function

      ' Insert some rows into [dbo].[Products] table.
      Public Shared Function InsertRows(connString As [String]) As [Boolean]
         Using conn As New SqlConnection(connString)
            Dim cmdText As [String] = "Use DbDataIsolationLevel" & vbCr & vbLf & vbCr & vbLf &
                "INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Blue Bike', 365,1075.00)" & vbCr & vbLf &
                "INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Red Bike', 159, 1299.00)" & vbCr & vbLf &
                "INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Black Bike', 638, 1159.00)"

            Using command As New SqlCommand(cmdText, conn)
               conn.Open()
               command.ExecuteNonQuery()
            End Using
         End Using
         Return True
      End Function

      ' Turn on or off 'ALLOW_SNAPSHOT_ISOLATION'
      Public Shared Function SetSnapshot(connString As [String], isOpen As [Boolean]) As [Boolean]
         Using conn As New SqlConnection(connString)
            Dim cmdText As [String] = Nothing

            If isOpen Then
               cmdText = "ALTER DATABASE DbDataIsolationLevel SET ALLOW_SNAPSHOT_ISOLATION ON"
            Else
               cmdText = "ALTER DATABASE DbDataIsolationLevel SET ALLOW_SNAPSHOT_ISOLATION OFF"
            End If

            Using command As New SqlCommand(cmdText, conn)
               conn.Open()
               command.ExecuteNonQuery()
            End Using
         End Using

         Return True
      End Function
   End Class
   Class Program
      Public Shared Sub Main(args As String())
         Dim connString As [String] = "Data Source=(local);Initial Catalog=master;Integrated Security=True;Asynchronous Processing=true;"

         OperateDatabase.CreateDatabase(connString)
         Console.WriteLine()

         Console.WriteLine("Demonstrate the ReadUncommitted transaction: ")
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.ReadUncommitted)
         Console.WriteLine("-----------------------------------------------")

         Console.WriteLine("Demonstrate the ReadCommitted transaction: ")
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.ReadCommitted)
         Console.WriteLine("-----------------------------------------------")

         Console.WriteLine("Demonstrate the RepeatableRead transaction: ")
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.RepeatableRead)
         Console.WriteLine("-----------------------------------------------")

         Console.WriteLine("Demonstrate the Serializable transaction: ")
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.Serializable)
         Console.WriteLine("-----------------------------------------------")

         Console.WriteLine("Demonstrate the Snapshot transaction: ")
         OperateDatabase.SetSnapshot(connString, True)
         TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.Snapshot)
         Console.WriteLine("-----------------------------------------------")

         Console.WriteLine("Demonstrate the difference between the Snapshot and Serializable transactions:")
         TransactionIsolationLevels.DemonstrateBetweenSnapshotAndSerializable(connString)
         OperateDatabase.SetSnapshot(connString, False)
         Console.WriteLine()
      End Sub
   End Class
End Namespace

Poznámky

Hodnoty IsolationLevel jsou používány zprostředkovatelem dat .NET při provádění transakce.

Zůstane IsolationLevel v platnosti, dokud se explicitně nezmění, ale můžete ho kdykoli změnit. Nová hodnota se používá v době provádění, nikoli v době analýzy. Pokud se změní během transakce, očekávané chování serveru je použít novou úroveň uzamčení na všechny zbývající příkazy.

Pokud použijete OdbcTransaction, pokud nenastavíte OdbcTransaction.IsolationLevel nebo nastavíte na Unspecified, transakce se provede podle úrovně izolace určené ovladačem.

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