I have dilemma on how to actually create proper awaitable function. I don't get the entire concept fully, probably due the language barrier :)
A
public async Task<int> InsertASync(string tableName, Dictionary<string, string> data)
{
int r = 0;
using (SQLiteConnection con = NewConnection())
{
await con.OpenAsync();
using (SQLiteCommand com = new SQLiteCommand(Query_Insert(tableName, data), con))
{
r = await com.ExecuteNonQueryAsync();
}
con.Close();
}
return r;
}
or
B
public Task<int> InsertASync(string tableName, Dictionary<string, string> data)
{
return Task<int>.Run(async () =>
{
int r = 0;
using (SQLiteConnection con = NewConnection())
{
await con.OpenAsync();
using (SQLiteCommand com = new SQLiteCommand(Query_Insert(tableName, data), con))
{
r = await com.ExecuteNonQueryAsync();
}
con.Close();
}
return r;
});
}
Reason i'm wondering about it is because the way i create awaitable methods via cheap way.
Example
public void DoHeavyWork() //this will block UI
{
//Do work
}
public Task DoHeavyWorkASync() //this won't
{
return Task.Run(() =>
{
DoHeavyWork();
});
}
You want to go the A route. The compiler will take care of creating the returned task and returning the actual integer, when available.
You can read Asynchronous Programming - Pause and Play with ... for some good in-depth information on what the compiler does internally (how your method is converted into a state machine)
Related
Attempting to execute a block of code on a button click to retrieve data from a SQL database. During build, I get the error message
ItemsPage.Button_Clicked(): not all code paths return a value
I tried adding a return statement with a value of the int points but it returned the error
Cannot implicitly convert type int to System.EventHandler
I'm a C# beginner so have little idea on what to do even after attempting to find other solutions online.
public async Task<EventHandler> Button_Clicked()
{
using (MySqlConnection connection = new MySqlConnection("server=localhost;user=app;database=travel_logger;port=3306;password=app"))
{
connection.Open();
var cmd = new MySqlCommand("SELECT points FROM * WHERE email= 20Test#test");
var reader = await cmd.ExecuteReaderAsync();
int points = (int)Convert.ToInt64(reader.GetInt64(0));
return points;
}
}
Task<EventHandler> indicates that it expects an object of type EventHandler to be returned. Task<int> indicates that it expects an integer to be returned. Personally I suggest you just use Task (equivalent to void but supports await) or async void and not return (I don't see it necessary in a Button Click event)
I understand that you could opt for a more convenient structure:
private async Task<int> GetPoints()
{
using (var conn = new MySqlConnection("server=localhost;user=app;database=travel_logger;port=3306;password=app"))
{
connection.Open();
var cmd = new MySqlCommand("SELECT points FROM * WHERE email= 20Test#test");
var reader = await cmd.ExecuteReaderAsync();
int points = (int)Convert.ToInt64(reader.GetInt64(0));
return points;
}
}
Then:
public async void Button_Clicked()
{
int points = await GetPoints();
}
I migrated an Access database to SQL using Microsoft SQL Server Migration Assistant.
Now, I am having trouble reading data.
return reader.GetInt32(0); Throws Invalid attempt to read when no data is present exception after 32 rows retrieved. If I add CommandBehavior.SequentialAccess to my command, I am able to read 265 rows, every time.
The data
The query (in SQL Management Studio):
SELECT *
FROM Products2
WHERE Products2.PgId = 337;
There is nothing special about row 32, and if I reverse the order, it is still row 32 that kills it.
Row 265, just for good measure.
The code
The query:
SELECT *
FROM Products2
WHERE Products2.PgId = #productGroupId;
The parameter:
Name = "productGroupId"
Value = 337
The execution:
public async Task ExecuteAsync(string query, Action<IDataReader> onExecute, params DataParameter[] parameters)
{
using(var command = builder.BuildCommand(query))
{
foreach(var parameter in parameters)
command.AddParameter(parameter);
if(!connection.IsOpen)
connection.Open();
await Task.Run(async () =>
{
using(var reader = await command.ExecuteAsync())
if(await reader.ReadAsync())
onExecute(reader);
});
}
}
And reading:
await executor.ExecuteAsync(query, async reader =>
{
do
{
products.Add(GetProduct(reader));
columnValues.Enqueue(GetColumnValues(reader).ToList());
} while(await reader.ReadAsync());
}, parameters.ToArray());
await reader.ReadAsync() returns true, but when GetProduct(reader) calls reader.GetInt32(0); for the 32nd time, it throws the exception.
It works fine if the data is less than 32 rows, or 265 in case of SequentialAccess.
I tried increasing the CommandTimeout, but it didn't help. When I swap the connection to OleDb again, it works just fine.
Thanks in advance.
EDIT
If I replace * in the query with just a few specific columns, it works. When I read ~12 columns, it fails, but later than row 32.
As per request, GetProduct:
private Product GetProduct(IDataReader productReader)
{
return new Product
{
Id = productReader.ReadLong(0),
Number = productReader.ReadString(2),
EanNo = productReader.ReadString(3),
Frequency = productReader.ReadInt(4),
Status = productReader.ReadInt(5),
NameId = productReader.ReadLong(6),
KMPI = productReader.ReadByte(7),
OEM = productReader.ReadString(8),
CurvesetId = productReader.ReadInt(9),
HasServiceInfo = Convert.ToBoolean(productReader.ReadByte(10)),
ColumnData = new List<ColumnData>()
};
}
GetColumnData:
private IEnumerable<long> GetColumnValues(IDataReader productReader)
{
var columnValues = new List<long>();
int columnIndex = 11;
while(!productReader.IsNull(columnIndex))
columnValues.Add(productReader.ReadLong(columnIndex++));
return columnValues;
}
And the adapter:
public long ReadLong(int columnIndex)
{
return reader.GetInt32(columnIndex);
}
Alright, it is getting long. :)
Thanks to #Igor, I tried creating a small working example. This seem to work fine:
private static async Task Run()
{
var result = new List<Product>();
string conString = #" ... ";
var con = new SqlConnection(conString);
con.Open();
using(var command = new SqlCommand("SELECT * FROM Products2 WHERE Products2.PgId = #p;", con))
{
command.Parameters.Add(new SqlParameter("p", 337));
await Task.Run(async () =>
{
using(var productReader = await command.ExecuteReaderAsync())
while(await productReader.ReadAsync())
{
result.Add(new Product
{
Id = productReader.GetInt32(0),
Number = productReader.GetString(2),
EanNo = productReader.GetString(3),
Frequency = productReader.GetInt16(4),
Status = productReader.GetInt16(5),
NameId = productReader.GetInt32(6),
KMPI = productReader.GetByte(7),
OEM = productReader.GetString(8),
CurvesetId = productReader.GetInt16(9),
HasServiceInfo = Convert.ToBoolean(productReader.GetByte(10))
});
GetColumnValues(productReader);
}
});
}
Console.WriteLine("End");
}
private static IEnumerable<long> GetColumnValues(SqlDataReader productReader)
{
var columnValues = new List<long>();
int columnIndex = 11;
while(!productReader.IsDBNull(columnIndex))
columnValues.Add(productReader.GetInt32(columnIndex++));
return columnValues;
}
}
Here's the data in Access, just in case:
I managed to fix the problem.
I still have questions though, since I don't understand why this didn't affect Access.
I changed:
public async Task ExecuteAsync(string query, Action<IDataReader> onExecute, params DataParameter[] parameters)
{
using(var command = builder.BuildCommand(query))
{
foreach(var parameter in parameters)
command.AddParameter(parameter);
if(!connection.IsOpen)
connection.Open();
await Task.Run(async () =>
{
using(var reader = await command.ExecuteAsync())
if(await reader.ReadAsync())
onExecute(reader);
});
}
}
To:
public async Task ExecuteAsync(string query, Func<IDataReader, Task> onExecute, params DataParameter[] parameters)
{
using(var command = builder.BuildCommand(query))
{
foreach(var parameter in parameters)
command.AddParameter(parameter);
if(!connection.IsOpen)
connection.Open();
using(var reader = await command.ExecuteAsync())
await onExecute(reader);
}
}
If someone can explain why this helped, I would really appreciate it.
The below method doesn't compile. Alternatives?
public static async Task<IEnumerable<object[]>> GetRecordsAsync(
this Transaction transaction,
string commandText,
params SqlParameter[] parameters)
{
// Get a SqlDataReader
var reader = await transaction.GetReaderAsync(commandText, parameters);
var fieldCount = -1;
// Begin iterating through records asynchronously
while (await reader.ReadAsync()) // Note we don't loop until .ReadAsync returns a boolean
{
// Grab all the field values out
if (fieldCount < 0)
fieldCount = reader.FieldCount;
var fields = new object[fieldCount];
reader.GetValues(fields);
// Yield return the field values from this record
yield return fields;
}
}
Error message:
The body of 'TransactionExtensions.GetRecordsAsync(Transaction, string, params SqlParameter[])' cannot be an iterator block because 'Task>' is not an iterator interface type
I don't see a way to adapt this answer to a similar sounding (but different) question, because I don't know a priori how many times the loop will go.
Edit: fixed formatting
Based on #SLaks's comment to the question, here's a general alternative using Reactive Extensions:
/// <summary>
/// Turns the given asynchronous functions into an IObservable
/// </summary>
static IObservable<T> ToObservable<T>(
Func<Task<bool>> shouldLoopAsync,
Func<Task<T>> getAsync)
{
return Observable.Create<T>(
observer => Task.Run(async () =>
{
while (await shouldLoopAsync())
{
var value = await getAsync();
observer.OnNext(value);
}
observer.OnCompleted();
}
)
);
}
Example usage, tailored to solve the question's specific case:
/// <summary>
/// Asynchronously processes each record of the given reader using the given handler
/// </summary>
static async Task ProcessResultsAsync(this SqlDataReader reader, Action<object[]> fieldsHandler)
{
// Set up async functions for the reader
var shouldLoopAsync = (Func<Task<bool>>)reader.ReadAsync;
var getAsync = new Func<SqlDataReader, Func<Task<object[]>>>(_reader =>
{
var fieldCount = -1;
return () => Task.Run(() =>
{
Interlocked.CompareExchange(ref fieldCount, _reader.FieldCount, -1);
var fields = new object[fieldCount];
_reader.GetValues(fields);
return fields;
});
})(reader);
// Turn the async functions into an IObservable
var observable = ToObservable(shouldLoopAsync, getAsync);
// Process the fields as they become available
var finished = new ManualResetEventSlim(); // This will be our signal for when the observable completes
using (observable.Subscribe(
onNext: fieldsHandler, // Invoke the handler for each set of fields
onCompleted: finished.Set // Set the gate when the observable completes
)) // Don't forget best practice of disposing IDisposables
// Asynchronously wait for the gate to be set
await Task.Run((Action)finished.Wait);
}
(Note that getAsync could be simplified in the above code block, but I like how explicit it is about the closure that's being created)
...and finally:
// Get a SqlDataReader
var reader = await transaction.GetReaderAsync(commandText, parameters);
// Do something with the records
await reader.ProcessResultsAsync(fields => { /* Code here to process each record */ });
Don't return a Task<IEnumerable<T>> and don't even use Task at all for this; instead, return an IAsyncEnumerable<T>. No need for third-party libraries or other workarounds, no need to even alter the body of your original method.
public static async IAsyncEnumerable<object[]> GetRecordsAsync(
this Transaction transaction,
string commandText,
params SqlParameter[] parameters)
{
// Get a SqlDataReader
var reader = await transaction.GetReaderAsync(commandText, parameters);
var fieldCount = -1;
// Begin iterating through records asynchronously
while (await reader.ReadAsync()) // Note we don't loop until .ReadAsync returns a boolean
{
// Grab all the field values out
if (fieldCount < 0)
fieldCount = reader.FieldCount;
var fields = new object[fieldCount];
reader.GetValues(fields);
// Yield return the field values from this record
yield return fields;
}
}
I solved it without third-party extensions:
public async Task<IEnumerable<Item>> GetAllFromDb()
{
OracleConnection connection = null;
DbDataReader reader = null;
try
{
connection = new OracleConnection(connectionString);
var command = new OracleCommand(queryString, connection);
connection.Open();
reader = await command.ExecuteReaderAsync();
return this.BuildEnumerable(connection, reader);
}
catch (Exception)
{
reader?.Dispose();
connection?.Dispose();
throw;
}
}
private IEnumerable<Item> BuildEnumerable(OracleConnection connection, DbDataReader reader)
{
using (connection)
using (reader)
{
while (reader.Read())
{
var item = new Item()
{
Prop = reader.GetString(0),
};
yield return item;
}
}
}
This example is for Oracle Data Reader but the same approach is applicable to any asynchronous operation combined with yield return
I have a set of async methods I wrote to handle a large amount of database pulls and compiles in a speedy manner. For the most part these work fantastic and have really worked wonders for my software. However, recently I have run into a small problem with the methods: Every now and then the user messes up and the time frames the software is pulling the data between become enormous and the data adapter times out before obtaining the information. Normally in a sync method you would use a try/catch to handle such issues but I have tried this to no avail. Is there a way to asynchronously handle exceptions to simply throw as a sync method would so my catch all try/catch can work properly?
This is an example of the data adapter async method i use:
private async Task<DataTable> WWQuery2Run
(string A, string B, string C, string D)
{
using ( var conn = new System.Data.SqlClient.SqlConnection(ReportResources.ConnString) )
{
var temp = new DataTable();
var DA = new SqlDataAdapter(string.Format(ReportResources.Instance.CureInfoQueries["WWQuery2"], A, B, C, D), conn);
await Task.Run(() => DA.Fill(temp));
return temp;
}
}
EDIT:
I realized after all the trouble of trying to handle the exceptions from the timeout that it is not a good practice to work that way. I went ahead and added a method to calculate the duration before entering the shown async method and warning the user of the length and gave them an option to abort the compile. With that in place I increased the timeout of the query to an amount that should cover all but the worst of the worst scenarios for if the user wishes to continue. I also added to the description on the items within the program a calculated duration so they know that it went long before trying to query and compile.
Thank you #Murad Garibzada for your assistance.
Along with increasing the command timeout, you can use a try / catch block. Since you are awaiting, control will not return to your calling code until WWQuery2Run has completed. If WWQuery2Run throws SomeException, it will be caught and handled by the code.
private async Task<DataTable> WWQuery2Run(string A, string B, string C, string D)
{
try
{
using ( var conn = new System.Data.SqlClient.SqlConnection(ReportResources.ConnString) )
{
var temp = new DataTable();
var DA = new SqlDataAdapter(string.Format(ReportResources.Instance.CureInfoQueries["WWQuery2"], A, B, C, D), conn);
await Task.Run(() => DA.Fill(temp));
return temp;
}
}
catch (SomeException ex)
{
// handle exception
}
}
Because your connection conn is wrapped in a using statement, there is a possibility that it is disposed before your call to Fill is executed.
Probably something like the following would work:
private async Task<DataTable> WWQuery2Run (string A, string B, string C, string D)
{
var temp = new DataTable();
await Task.Run(() => {
using ( var conn = new System.Data.SqlClient.SqlConnection(ReportResources.ConnString) )
{
var DA = new SqlDataAdapter(string.Format(ReportResources.Instance.CureInfoQueries["WWQuery2"], A, B, C, D), conn);
DA.Fill(temp)
}
});
return temp;
}
You can increase adapter command timeout like below:
SqlDataAdapter adapter= new SqlDataAdapter(strSQLString, conUS);
adapter.SelectCommand.CommandTimeout=120;
Handling exception:
private async Task<DataTable> WWQuery2Run
(string A, string B, string C, string D)
{
using ( var conn = new System.Data.SqlClient.SqlConnection(ReportResources.ConnString) )
{
var temp = new DataTable();
var DA = new SqlDataAdapter(string.Format(ReportResources.Instance.CureInfoQueries["WWQuery2"], A, B, C, D), conn);
Task task = new Task(() => DA.Fill(temp));
try
{
task.Wait();
}
catch (Exception ae)
{
}
return temp;
}
}
I'm facing a pretty weird construct. The Foo type returned in an IEnumerable loses its data as soon as the enumeration ends. This means that I can't do a enumeration.First() because the data would be lost right away.
A loop over it works, but since I know it will contain only a single element that would be weird.
int Test(out int something)
IEnumerable<Foo> enumeration = ...
for (var foo in enumeration) {
something = foo.GetSomething ();
return foo.GetAnInt ();
}
something = 42;
return 0;
}
Another way I though of is abusing a Linq Select, but that's just as horrible.
Is there a way to work around this limitation? Fixing the root cause is obviously superior, but difficult in this case.
Edit: It's an IEnumerable<IDataRecord> that is yield returned from a transactioned SQL data reader.
public IEnumerable<IDataRecord> ExecuteReader (SqlCommand cmd)
{
using (var con = GetConnection()) {
con.Open ();
using (var tr = con.BeginTransaction ()) {
cmd.Connection = con;
var reader = cmd.ExecuteReader ();
while (reader.Read ()) {
yield return reader;
}
tr.Commit ();
}
}
}
The problem is that your ExecuteReader method does simply return the SqlDataReader itself (which implements IDataRecord), instead of returning a block of data. So when you do this:
var list = ExecuteReader(...).ToList();
In that case all elements of the list will be the same SqlDataReader instance, but after the ToList has been executed, the reader has been closed. I'm a bit surprised that you don't get an ObjectDisposedException.
For this to work, you need to return a copy of the data in the IDataRecord. You think you can iterate the elements in the data record. An other option is to change the ExecuteReader to the following:
public IEnumerable<T> ExecuteReader<T>(SqlCommand cmd,
Func<IDataRecord, T> recordCreator)
{
using (var con = GetConnection()) {
con.Open ();
using (var tr = con.BeginTransaction()) {
cmd.Connection = con;
var reader = cmd.ExecuteReader();
while (reader.Read()) {
yield return recordCreator(reader);
}
tr.Commit();
}
}
}
This way you can do the following:
var list = ExecuteReader(command, record => new
{
Item1 = record.GetInt("id"),
Item2 = record.GetString("name")
});
Note: I'm not sure why you need a transaction for this anyway.
How about
int Test(out int something)
{
IEnumerable<Foo> enumeration = ...
var values = enumeration
.Select(foo => new
{
something = foo.GetSomething(),
anInt = foo.GetAnInt()
})
.FirstOrDefault();
if (values != null)
{
something = values.something;
return values.anInt;
}
else
{
something = 42;
return 0;
}
}
GetSomething and GetAnInt are called while inside the enumeration.
Another idea could be to convert the result type of the method from IEnumerable to IEnumerator. That way, scope control is much easier, and returning single results does not require any (fake) loop, too.
Edit: I think I found a way to refactor the whole issue. This circumvents the initial problem by using new disposable class that contains the logic formerly found in the method. It's very readable and less code even.
public TransactedConnection GetConnection (string text)
{
return new TransactedConnection (_ConnectionString, text);
}
public class TransactedConnection : IDisposable
{
private readonly SQLiteCommand _Cmd;
private readonly SQLiteConnection _Con;
private readonly SQLiteTransaction _Tr;
public TransactedConnection (string connection, string text)
{
_Cmd = new SQLiteCommand (text);
_Con = new SQLiteConnection (connection);
_Con.Open ();
_Cmd.Connection = _Con;
_Tr = _Con.BeginTransaction ();
}
public void Dispose ()
{
_Tr.Commit ();
_Tr.Dispose ();
_Con.Dispose ();
_Cmd.Dispose ();
}
public SQLiteParameterCollection Parameters
{
get
{
return _Cmd.Parameters;
}
}
public int ExecuteNonQuery ()
{
return _Cmd.ExecuteNonQuery ();
}
public object ExecuteScalar ()
{
return _Cmd.ExecuteScalar ();
}
public SQLiteDataReader ExecuteReader ()
{
return _Cmd.ExecuteReader ();
}
}
public void Test (string match)
{
var text = "SELECT * FROM Data WHERE foo=#bar;";
using (var cmd = GetConnection (text)) {
cmd.Parameters.Add ("#bar", DbType.String).Value = match;
using (var reader = cmd.ExecuteReader ()) {
while (reader.Read ()) {
Console.WriteLine (reader["foo"]);
}
}
}
}