Database query not respecting transactions - .net-core

Question

I have a bit of code that I'm using to pull items from a database one at a time. On each get, it updates the status of the row to indicate that it has been retrieved so as to not retrieve the same row multiple times.
The below code is run from a backend service serving a Web API. Sometimes when multiple requests come in, it will return the same row (tasks with the same ID).
I was under the impression that having a transaction around it would mean that an update from one of the runs would preclude the row from being returned by a second query.
Any help would be appreciated.
public async Task<TaskDetail> GetTask()
{
using (var db = new SqlConnection(""))
{
using (var tran = db.BeginTransaction())
{
try
{
var sql = $#"
SELECT TOP 1 * FROM
(SELECT TOP 150 t.*
FROM Task t
INNER JOIN TaskStatus ts ON t.Id = ts.TaskId AND ts.Status = #taskStatus) t
ORDER BY NEWID();";
var chosen = await db
.QuerySingleOrDefaultAsync<TaskDetail>(
sql,
param: new
{
taskStatus = TaskStatusEnum.Ready
},
transaction: tran
);
if (chosen == null)
{
throw new InvalidOperationException();
}
var expiry = await db.ExecuteAsync("UPDATE TaskStatus SET Status = #status WHERE TaskId = #taskId", new {status = TaskStatusEnum.Done, taskId = chosen.TaskId}, tran);
tran.Commit();
return chosen;
}
catch
{
tran.Rollback();
throw;
}
}
}
}

Answer 1

A few wrong configurations can be the reason;
1- Check the SQL Server isolation level, be careful about dirty reads.
2- Be sure that you handle the error in web API properly, because especially transaction error is not displayed properly.
3- And, please remove t-sql from code :)

Answer 2

Select query doesn't block rows data from other transactions, two select in different transaction will be performed at the same time. You can try to set a session id on the row and then select it.
EDIT: hope this help, when update is executed it should block the row for others transactions
public async Task<TaskDetail> GetTask()
{
using (var db = new SqlConnection(""))
{
using (var tran = db.BeginTransaction())
{
try
{
var mySessionId = Guid.NewGuid();
var sql = $#"
UPDATE TaskStatus SET Status = #status, SessionId = #mySessionId WHERE TaskId in
(SELECT TOP 1
t.Id
FROM Task t
INNER JOIN TaskStatus ts ON t.Id = ts.TaskId AND ts.Status = #taskStatus ORDER BY NEWID());";
await db
.QuerySingleOrDefaultAsync<TaskDetail>(
sql,
param: new
{
taskStatus = TaskStatusEnum.Ready
status = TaskStatusEnum.InProgress,
mySessionId = mySessionId
},
transaction: tran
);
var sql = $#"
SELECT TOP 150 t.*
FROM Task t
INNER JOIN TaskStatus ts ON t.Id = ts.TaskId AND ts.Status = #taskStatus
WHERE ts.SessionId = #mySessionId;";
var chosen = await db
.QuerySingleOrDefaultAsync<TaskDetail>(
sql,
param: new
{
taskStatus = TaskStatusEnum.InProgress,
mySessionId = mySessionId
},
transaction: tran
);
if (chosen == null)
{
throw new InvalidOperationException();
}
var expiry = await db.ExecuteAsync("UPDATE TaskStatus SET Status = #status WHERE TaskId = #taskId", new {status = TaskStatusEnum.Done, taskId = chosen.TaskId}, tran);
tran.Commit();
return chosen;
}
catch
{
tran.Rollback();
throw;
}
}
}
}