Visual Studio 2015 Debug doesn't work in multithread application

Question

In my project I have a heavy part of code that should be executed on a separate thread without blocking UI. When debugger hits the breakpoint inside this code, VS2015 freezes for 5-10 seconds. After that, if I try to continue debug (by pressing Step Over, Step In or Continue), the app goes from paused state to working state, Debugging Tools are ticking, but nothing happens and there's 0% of CPU utilization. If I press Break All then, the "cursor" (don't know the correct term) is shown at Application.Run( new Form1() ); in Program.cs where Main() is.

As I'm pretty new to C#, I thought that there was some problem with my approach to multithreading, but apparently it happens whatever I try - using async/await with Tasks, using BackgroundWorker component, or simple new Thread(myFunc).Start().

Just to be clear.

• The code itself works perfectly fine.
• Debugger itself also works, no freezes and "deadlocks" on breakpoints in my main thread. If I launch the code from main thread - everything is fine.
• I also checked it in a fully new solution on a simple for ( int i = 0; i < Int32.MaxValue; ++i ) function - same problem.
• Also checked on different versions of .NET: 4.6, 4.5, 4.0. Same everywhere.
• The problem doesn't appear neither in VS2010 (which I used before), nor in VS2013 (which I tried just to be sure that it's a VS2015 problem). However, my friend working with the same VS2015 also doesn't have this problem.

Edit: per request, code of test form where I keep getting the problem. Only three buttons, label, and BackgroundWorker. The overall scheme is similar to the code of the main project.

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

    const int period = 10000;
    void FuncAsync(IProgress<int> progress)
    {
        for ( int i = 0; i < Int32.MaxValue; ++i )
        {
            double part = (double)i / Int32.MaxValue;
            int percent = (int)(part * 100.0);

            if ( (i % period) == 0 )
                progress.Report( percent );
        }
    }
    void FuncBW(BackgroundWorker worker)
    {
        for ( int i = 0; i < Int32.MaxValue; ++i )
        {
            double part = (double)i / Int32.MaxValue;
            int percent = (int)(part * 100.0);

            if ( (i % period) == 0 )
                worker.ReportProgress( percent );
        }
    }
    void FuncThread()
    {
        for ( int i = 0; i < Int32.MaxValue; ++i )
        {
            double part = (double)i / Int32.MaxValue;
            int percent = (int)(part * 100.0);

            if ( (i % period) == 0 )
                label1.Text = percent.ToString();
            //yes, this one will cause exception of accessing UI from different thread
            //if i press "Break" in the exception window, i will also get a 10sec freeze
        }
    }



    private async void button1_Click(object sender, EventArgs e)
    {
        var progress = new Progress<int>(i => label1.Text = i.ToString() );
        await Task.Factory.StartNew( () => FuncAsync( progress ),
                                    TaskCreationOptions.LongRunning );
    }

    private void button2_Click(object sender, EventArgs e)
    {
        backgroundWorker1.RunWorkerAsync();
    }

    private void button3_Click(object sender, EventArgs e)
    {
        Thread t = new Thread(FuncThread);
        t.Start();
    }

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
    {
        FuncBW( (BackgroundWorker)sender );
    }

    private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        label1.Text = e.ProgressPercentage.ToString();
    }
}

Answer 1

I've experienced similar issues with VS2015 freezing (indefinitely) whilst debugging a multi thread WinForms application.

I had no problems debugging the same code in VS2013.

The problem appears to go away when I disable the VS hosting process (Project -> Properties -> Debug -> Enable the Visual Studio hosting process).

Hope that works for others.