// Assumes GetConnectionString returns a valid connection string
// where pooling is turned off by setting Pooling=False;.
string connectionString = GetConnectionString();
using (SqlConnection connection1 = new SqlConnection(connectionString))
{// Drop the TestSnapshot table if it existsconnection1.Open();SqlCommand command1 = connection1.CreateCommand();command1.CommandText = "IF EXISTS "+ "(SELECT * FROM sys.tables WHERE name=N'TestSnapshot') "+ "DROP TABLE TestSnapshot";try{command1.ExecuteNonQuery();}catch (Exception ex){Console.WriteLine(ex.Message);}// Enable Snapshot isolationcommand1.CommandText ="ALTER DATABASE AdventureWorks SET ALLOW_SNAPSHOT_ISOLATION ON";command1.ExecuteNonQuery();// Create a table named TestSnapshot and insert one row of datacommand1.CommandText ="CREATE TABLE TestSnapshot (ID int primary key, valueCol int)";command1.ExecuteNonQuery();command1.CommandText ="INSERT INTO TestSnapshot VALUES (1,1)";command1.ExecuteNonQuery();// Begin, but do not complete, a transaction to update the data // with the Serializable isolation level, which locks the table// pending the commit or rollback of the update. The original // value in valueCol was 1, the proposed new value is 22.SqlTransaction transaction1 =connection1.BeginTransaction(IsolationLevel.Serializable);command1.Transaction = transaction1;command1.CommandText ="UPDATE TestSnapshot SET valueCol=22 WHERE ID=1";command1.ExecuteNonQuery();// Open a second connection to AdventureWorksusing (SqlConnection connection2 = new SqlConnection(connectionString)){connection2.Open();// Initiate a second transaction to read from TestSnapshot// using Snapshot isolation. This will read the original // value of 1 since transaction1 has not yet committed.SqlCommand command2 = connection2.CreateCommand();SqlTransaction transaction2 =connection2.BeginTransaction(IsolationLevel.Snapshot);command2.Transaction = transaction2;command2.CommandText ="SELECT ID, valueCol FROM TestSnapshot";SqlDataReader reader2 = command2.ExecuteReader();while (reader2.Read()){Console.WriteLine("Expected 1,1 Actual "+ reader2.GetValue(0).ToString()+ "," + reader2.GetValue(1).ToString());}transaction2.Commit();}// Open a third connection to AdventureWorks and// initiate a third transaction to read from TestSnapshot// using ReadCommitted isolation level. This transaction// will not be able to view the data because of // the locks placed on the table in transaction1// and will time out after 4 seconds.// You would see the same behavior with the// RepeatableRead or Serializable isolation levels.using (SqlConnection connection3 = new SqlConnection(connectionString)){connection3.Open();SqlCommand command3 = connection3.CreateCommand();SqlTransaction transaction3 =connection3.BeginTransaction(IsolationLevel.ReadCommitted);command3.Transaction = transaction3;command3.CommandText ="SELECT ID, valueCol FROM TestSnapshot";command3.CommandTimeout = 4;try{SqlDataReader sqldatareader3 = command3.ExecuteReader();while (sqldatareader3.Read()){Console.WriteLine("You should never hit this.");}transaction3.Commit();}catch (Exception ex){Console.WriteLine("Expected timeout expired exception: "+ ex.Message);transaction3.Rollback();}}// Open a fourth connection to AdventureWorks and// initiate a fourth transaction to read from TestSnapshot// using the ReadUncommitted isolation level. ReadUncommitted// will not hit the table lock, and will allow a dirty read  // of the proposed new value 22 for valueCol. If the first// transaction rolls back, this value will never actually have// existed in the database.using (SqlConnection connection4 = new SqlConnection(connectionString)){connection4.Open();SqlCommand command4 = connection4.CreateCommand();SqlTransaction transaction4 =connection4.BeginTransaction(IsolationLevel.ReadUncommitted);command4.Transaction = transaction4;command4.CommandText ="SELECT ID, valueCol FROM TestSnapshot";SqlDataReader reader4 = command4.ExecuteReader();while (reader4.Read()){Console.WriteLine("Expected 1,22 Actual "+ reader4.GetValue(0).ToString()+ "," + reader4.GetValue(1).ToString());}transaction4.Commit();}// Roll back the first transactiontransaction1.Rollback();
}// CLEANUP
// Delete the TestSnapshot table and set
// ALLOW_SNAPSHOT_ISOLATION OFF
using (SqlConnection connection5 = new SqlConnection(connectionString))
{connection5.Open();SqlCommand command5 = connection5.CreateCommand();command5.CommandText = "DROP TABLE TestSnapshot";SqlCommand command6 = connection5.CreateCommand();command6.CommandText ="ALTER DATABASE AdventureWorks SET ALLOW_SNAPSHOT_ISOLATION OFF";try{command5.ExecuteNonQuery();command6.ExecuteNonQuery();}catch (Exception ex){Console.WriteLine(ex.Message);}
}
Console.WriteLine("Done!");

// Assumes GetConnectionString returns a valid connection string
// where pooling is turned off by setting Pooling=False;.
string connectionString = GetConnectionString();
using (SqlConnection connection1 = new SqlConnection(connectionString))
{connection1.Open();SqlCommand command1 = connection1.CreateCommand();// Enable Snapshot isolation in AdventureWorkscommand1.CommandText ="ALTER DATABASE AdventureWorks SET ALLOW_SNAPSHOT_ISOLATION ON";try{command1.ExecuteNonQuery();Console.WriteLine("Snapshot Isolation turned on in AdventureWorks.");}catch (Exception ex){Console.WriteLine("ALLOW_SNAPSHOT_ISOLATION ON failed: {0}", ex.Message);}// Create a table command1.CommandText ="IF EXISTS "+ "(SELECT * FROM sys.tables "+ "WHERE name=N'TestSnapshotUpdate')"+ " DROP TABLE TestSnapshotUpdate";command1.ExecuteNonQuery();command1.CommandText ="CREATE TABLE TestSnapshotUpdate "+ "(ID int primary key, CharCol nvarchar(100));";try{command1.ExecuteNonQuery();Console.WriteLine("TestSnapshotUpdate table created.");}catch (Exception ex){Console.WriteLine("CREATE TABLE failed: {0}", ex.Message);}// Insert some datacommand1.CommandText ="INSERT INTO TestSnapshotUpdate VALUES (1,N'abcdefg');"+ "INSERT INTO TestSnapshotUpdate VALUES (2,N'hijklmn');"+ "INSERT INTO TestSnapshotUpdate VALUES (3,N'opqrstuv');";try{command1.ExecuteNonQuery();Console.WriteLine("Data inserted TestSnapshotUpdate table.");}catch (Exception ex){Console.WriteLine(ex.Message);}// Begin, but do not complete, a transaction // using the Snapshot isolation level.SqlTransaction transaction1 = null;try{transaction1 = connection1.BeginTransaction(IsolationLevel.Snapshot);command1.CommandText ="SELECT * FROM TestSnapshotUpdate WHERE ID BETWEEN 1 AND 3";command1.Transaction = transaction1;command1.ExecuteNonQuery();Console.WriteLine("Snapshot transaction1 started.");// Open a second Connection/Transaction to update data// using ReadCommitted. This transaction should succeed.using (SqlConnection connection2 = new SqlConnection(connectionString)){connection2.Open();SqlCommand command2 = connection2.CreateCommand();command2.CommandText = "UPDATE TestSnapshotUpdate SET CharCol="+ "N'New value from Connection2' WHERE ID=1";SqlTransaction transaction2 =connection2.BeginTransaction(IsolationLevel.ReadCommitted);command2.Transaction = transaction2;try{command2.ExecuteNonQuery();transaction2.Commit();Console.WriteLine("transaction2 has modified data and committed.");}catch (SqlException ex){Console.WriteLine(ex.Message);transaction2.Rollback();}finally{transaction2.Dispose();}}// Now try to update a row in Connection1/Transaction1.// This transaction should fail because Transaction2// succeeded in modifying the data.command1.CommandText ="UPDATE TestSnapshotUpdate SET CharCol="+ "N'New value from Connection1' WHERE ID=1";command1.Transaction = transaction1;command1.ExecuteNonQuery();transaction1.Commit();Console.WriteLine("You should never see this.");}catch (SqlException ex){Console.WriteLine("Expected failure for transaction1:");Console.WriteLine("  {0}: {1}", ex.Number, ex.Message);}finally{transaction1.Dispose();}
}// CLEANUP:
// Turn off Snapshot isolation and delete the table
using (SqlConnection connection3 = new SqlConnection(connectionString))
{connection3.Open();SqlCommand command3 = connection3.CreateCommand();command3.CommandText ="ALTER DATABASE AdventureWorks SET ALLOW_SNAPSHOT_ISOLATION OFF";try{command3.ExecuteNonQuery();Console.WriteLine("CLEANUP: Snapshot isolation turned off in AdventureWorks.");}catch (Exception ex){Console.WriteLine("CLEANUP FAILED: {0}", ex.Message);}command3.CommandText = "DROP TABLE TestSnapshotUpdate";try{command3.ExecuteNonQuery();Console.WriteLine("CLEANUP: TestSnapshotUpdate table deleted.");}catch (Exception ex){Console.WriteLine("CLEANUP FAILED: {0}", ex.Message);}
}

SELECT * FROM TestSnapshotUpdate WITH (UPDLOCK) WHERE PriKey BETWEEN 1 AND 3