Question

What I want is being able to read old data from the table, while some transaction is writing to the same table. When such atomic writing is completed, I want to replace the old data with the new one.

Transaction I’m coping with is long and I don’t want enable dirty reads nor block reading possibility.

I’ve turned on snapshot isolation level on my database:

SET ALLOW_SNAPSHOT_ISOLATION ON

and used code like this to write data in transaction:

using (var scope = new TransactionScope(TransactionScopeOption.RequiresNew,
                       new TransactionOptions
                           { IsolationLevel = IsolationLevel.Snapshot }))
{
    update();
    scope.Complete();
}

Unfortunately while I’m trying to read some data using SELECT * FROM [Table] in Microsoft SQL Server Management Studio, query execution is waiting with message Executing query..., and retrieves data after transaction is completed. What I’ve missed ?

EDIT: code for update() – basically simple inserts within a loop, mixed with no db related stuff connected with WCF which slows down the process:

var val = Web.Download();
using (var connection = new SqlConnection(connection))
{
    connection.Open();
    using (var command = connection.CreateCommand())
    {
        command.CommandType = CommandType.Text;
        command.CommandText = "INSERT INTO [table]([value]) VALUES (@val)";
        command.Parameters.Add(CreateParam(command, "@val", val, DbType.String));
        command.ExecuteNonQuery();
    }
}

Answer 1

You need to set the isolation level for your select too.
Before your SELECT * FROM [Table], add SET TRANSACTION ISOLATION LEVEL SNAPSHOT.