Proper way to cancel task

Details
Written by: Stanko Milosev
Category: C#
Hits: 233
cancellationToken
   CancellationToken
   A cancellation token that can be used to cancel the work if it has not yet started. Run(Func, CancellationToken) does not pass cancellationToken to action.
From here.

If you have an async method that you want to cancel, you need to provide a CancellationTokenSource to that method. Example: 

public void ExecuteWithCancellationTokenSource(string path, CancellationTokenSource cancellationTokenSource)
{
	foreach (var fileName in Directory.EnumerateFiles(path, "*.*", SearchOption.AllDirectories))
	{
		fileNameProgress.Report(fileName);
		Thread.Sleep(1000);
		if (cancellationTokenSource.IsCancellationRequested) break;
	}
}
Otherwise, you can't cancel it if you have a method like this: 
public void ExecuteWithoutCancellationTokenSource(string path)
{
	foreach (var fileName in Directory.EnumerateFiles(path, "*.*", SearchOption.AllDirectories))
	{
		fileNameProgress.Report(fileName);
		Thread.Sleep(1000);
	}
}
and you start execution something like this: 
Task task = Task.Run(() => { readFromHdd.ExecuteWithoutCancellationTokenSource(_path); }, _cancellationTokenSource.Token);

try
{
	await Task.WhenAll(task);
}
catch (Exception ex)
{
	tbFileNames.AppendText(ex.Message + Environment.NewLine);
}
finally
{
	tbFileNames.AppendText("Done" + Environment.NewLine);
}
it will never stop. Or even worse, if you start like this: 
Task task = Task.Run(() => { readFromHdd.ExecuteWithoutCancellationTokenSource(_path); }, _cancellationTokenSource.Token);

Task groupOfAllTasks = Task.WhenAll(task).ContinueWith(t =>
{
	if (t.IsFaulted)
	{
		throw new Exception("rException!");
	}
}, _cancellationTokenSource.Token);

List<Task> allTasks =
[
	groupOfAllTasks
];

try
{
	await Task.WhenAll(allTasks);
}
catch (Exception ex)
{
	tbFileNames.AppendText(ex.Message + Environment.NewLine);
}
finally
{
	tbFileNames.AppendText("Done" + Environment.NewLine);
}
It will raise a TaskCanceledException, but the operation will continue executing in the background.

Example download from here.

How to Load a Large File from MS SQL

Details
Written by: Stanko Milosev
Category: C#
Hits: 222
I have table like this: 
CREATE TABLE [dbo].[BigFiles](
	[ID] [uniqueidentifier] NOT NULL,
	[Name] [nvarchar](255) NOT NULL,
	[Size] [bigint] NOT NULL,
	[Data] [image] NULL,
 CONSTRAINT [PK_File] PRIMARY KEY CLUSTERED 
(
	[ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
and I inserted some file approx big 700mb: 
INSERT INTO [BigFiles] (ID, Name, Size, Data)
SELECT
    NEWID(),
    'bigFile.zip',
    DATALENGTH(BulkColumn),
    BulkColumn
FROM OPENROWSET(BULK N'bigFile.zip', SINGLE_BLOB) AS FileData;
Then I have created 32bit project in Visual Studio, where my csproj looks like this: 
<Project Sdk="Microsoft.NET.Sdk">

	<PropertyGroup>
		<OutputType>Exe</OutputType>
		<TargetFramework>net9.0</TargetFramework>
		<ImplicitUsings>enable</ImplicitUsings>
		<Nullable>enable</Nullable>
		<PlatformTarget>x86</PlatformTarget>
		<Prefer32Bit>true</Prefer32Bit>
	</PropertyGroup>

	<ItemGroup>
		<PackageReference Include="Microsoft.Data.SqlClient" Version="6.0.2" />
	</ItemGroup>

</Project>
Notice: 
<PlatformTarget>x86</PlatformTarget>
<Prefer32Bit>true</Prefer32Bit>
Standard way with Ado.Net will not work: 
using Microsoft.Data.SqlClient;

string connectionString = "Server=localhost;Database=saveBigData;User Id=myUserId;Password=myPass;TrustServerCertificate=True;Encrypt=False;";
using SqlConnection sqlConnection = new SqlConnection();
using SqlCommand sqlCommand = new SqlCommand();
sqlConnection.ConnectionString = connectionString;
sqlConnection.Open();
sqlCommand.Connection = sqlConnection;
sqlCommand.CommandText = "SELECT * FROM [dbo].[BigFiles]";
sqlCommand.CommandTimeout = 0;
using SqlDataReader sqlDataReader = sqlCommand.ExecuteReader();

while (sqlDataReader.Read())
{
    object[] values = new object[4];
    sqlDataReader.GetValues(values);
}
In the line: 
sqlDataReader.GetValues(values);
will be System.OutOfMemoryException raised. Instead the one should use SequentialAccess: 
string fileName = Path.Combine(AppContext.BaseDirectory, $"{Guid.NewGuid()}.bin");

using SqlConnection sqlConnection = new SqlConnection();
using SqlCommand sqlCommand = new SqlCommand();
sqlConnection.ConnectionString = connectionString;
sqlConnection.Open();
sqlCommand.Connection = sqlConnection;
sqlCommand.CommandText = "SELECT * FROM [dbo].[BigFiles]";
sqlCommand.CommandTimeout = 0;
using SqlDataReader reader = sqlCommand.ExecuteReader(CommandBehavior.SequentialAccess);
while (reader.Read())
{
	using Stream sqlStream = reader.GetStream(3);

	using FileStream fileStream = new FileStream(fileName, FileMode.Create, FileAccess.Write);
	byte[] buffer = new byte[81920];
	int bytesRead;
	while ((bytesRead = sqlStream.Read(buffer, 0, buffer.Length)) > 0)
	{
		fileStream.Write(buffer, 0, bytesRead);
	}
}
Example download from here.

One consumer, multiple tasks

Details
Written by: Stanko Milosev
Category: C#
Hits: 490
In the consumer-producer pattern, I want to have a single consumer under which I can define multiple optional tasks. I will store these tasks in a dictionary and execute them later within the consumer. To implement this example I will use the architecture from Steven van Deursen and channels.

In short, my consumer looks like this: 

public class ReadAllHandlerAsync(
    Dictionary<IListOfTasksToExecuteInReader, IListOfTasksToExecuteInReaderCommand> listOfTasksToExecuteInReadAllAsync)
    : IListOfTasksToExecute
{
    public async Task Execute(IListOfTasksToExecuteCommand command)
    {
        var reader = command.MyChannelReader;
        if (reader is not null)
        {
            await foreach (var latLngFileName in reader.ReadAllAsync())
            {
                foreach (var taskToExecuteCommand in listOfTasksToExecuteInReadAllAsync)
                {
                    try
                    {
                        taskToExecuteCommand.Value.LatLngFileName = latLngFileName;
                        await taskToExecuteCommand.Key.Execute(taskToExecuteCommand.Value);
                    }
                    catch (Exception ex)
                    {
                        ((ReadAllAsyncHandlerCommand)command).Exceptions.Enqueue(ex);
                    }
                }
            }
        }
    }
}
Notice method signature: 
Dictionary<IListOfTasksToExecuteInReader, IListOfTasksToExecuteInReaderCommand> listOfTasksToExecuteInReadAllAsync
Then I have created wrapper around my Parallel.ForEachAsync, since therotically speaking I could have also more consumers, I will have also list of taks in my wrapper which I can execute before starting Parallel.ForEachAsync, and where I will hand over the channel: 
public class MyParallelForEachAsyncWrapper(MyParallelForEachAsync myParallelForEachAsync
    , Dictionary<IListOfTasksToExecute, IListOfTasksToExecuteCommand> listOfTasksToExecuteBeforeStartForEach) : ICommandHandlerAsync<MyParallelForEachAsyncWrapperCommand>
{
    public async Task Execute(MyParallelForEachAsyncWrapperCommand command)
    {
        var tasksToExecuteBeforeStartForEach = new List<Task>();
        try
        {
            foreach (var taskToExecuteBeforeStartForEach in listOfTasksToExecuteBeforeStartForEach)
            {
                if (command.MyInfoChannel != null)
                {
                    taskToExecuteBeforeStartForEach.Value.MyInfoChannelReader = command.MyInfoChannel;
                }
 
                tasksToExecuteBeforeStartForEach.Add(
                    taskToExecuteBeforeStartForEach.Key.Execute(taskToExecuteBeforeStartForEach.Value));
            }
 
            var myParallelForEachAsyncCommand = new MyParallelForEachAsyncCommand
            {
                FolderName = command.FolderName
                , MyChannel = command.MyChannel
            };
            await myParallelForEachAsync.Execute(myParallelForEachAsyncCommand);
        }
        catch (Exception e)
        {
            command.Exceptions.Enqueue(e);
        }
        finally
        {
            await Task.WhenAll(tasksToExecuteBeforeStartForEach);
        }
    }
}
Notice how I am handing over the channel: 
if (command.MyInfoChannel != null)
{
	taskToExecuteBeforeStartForEach.Value.MyInfoChannelReader = command.MyInfoChannel;
}
and 
var myParallelForEachAsyncCommand = new MyParallelForEachAsyncCommand
{
	FolderName = command.FolderName
	, MyChannel = command.MyChannel
};
await myParallelForEachAsync.Execute(myParallelForEachAsyncCommand);
Thats why my interface looks like: 
public interface IListOfTasksToExecuteCommand
{
    ChannelReader<LatLngFileNameModel>? GpsInfoChannelReader { get; set; }
}
At the end, my Parallel.ForEachAsync method will look like this: 
public class MyParallelForEachAsync : ICommandHandlerAsync<MyParallelForEachAsyncCommand>
{
    private readonly ConcurrentQueue<Exception> _exceptions = new();

    public async Task Execute(MyParallelForEachAsyncCommand command)
    {
        if (Directory.Exists(command.FolderName))
        {
            var imageExtensions = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
            {
                ".jpg", ".jpeg", ".png", ".gif", ".bmp", ".tiff", ".webp"
            };

            await Parallel.ForEachAsync(
                EnumerateFilesSafe(command.FolderName), async (imageFileName, ct) =>
                {
                    if (imageExtensions.Contains(Path.GetExtension(imageFileName).ToLower()))
                    {
                        var extractGpsInfoFromImageCommand = new ExtractGpsInfoFromImageCommand
                        {
                            ImageFileNameToReadGpsFrom = imageFileName
                        };

                        if (command.GpsInfoChannel != null)
                            await command.GpsInfoChannel.Writer.WriteAsync(
                                ExtractGpsInfoFromImage(extractGpsInfoFromImageCommand), ct);
                    }
                });

            command.GpsInfoChannel?.Writer.Complete();

            if (!_exceptions.IsEmpty)
            {
                throw new AggregateException("Error in der Parallel.ForEachAsync", _exceptions);
            }
        }
        else
        {
            throw new DirectoryNotFoundException($"Directory {command.FolderName} not found.");
        }
    }
}

Get types and size from database

Details
Written by: Stanko Milosev
Category: C#
Hits: 305
Here is a small example of how to display the type and size of fields from a schema table. This example works for MS SQL; I haven't tested it on other databases. 
using System;
using System.Data.SqlClient;
using System.Windows.Forms;

namespace GetDbDataTypes
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnStart_Click(object sender, EventArgs e)
        {
            string connectionString = "Server=myServer;Database=myDb;Integrated Security=True";
            string query = "SELECT top 1 * FROM [myTable]";
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                connection.Open();

                using (SqlCommand command = new SqlCommand(query, connection))
                {
                    command.CommandTimeout = 3600;
                    using (SqlDataReader reader = command.ExecuteReader())
                    {
                        dataGridView1.DataSource = reader.GetSchemaTable();
                    }
                }
            }
        }
    }
}
Example download from here
  1. Task.Run cannot be canceled immediately
  2. Add record in Db from Parallel.ForEachAsync and Ef Core
  3. Exceptions in Parallel.ForEach, Parallel.ForEachAsync, Task.Run and Task.Run.ContinueWith
  4. Update UI from Parallel.ForEach

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