async try catch - exception variable is null

Question

This code is merely for learning/test purpose:

I am deliberately throwing an Exception inside a Task.Run(...) and calling it from a WindowsForm ButtonClickEventHandler:

async void button_Click(object sender, EventArgs e)
{
    try
    {
        await Task.Run(
            () =>
            {
                throw new Exception("Foo");
            }
        );
    }
    catch (Exception ex) { }
}

When I debug the application and put a breakpoint to the catch statement, I see that the variable "ex" has the value null. Why is that and how can I capture the exception content? (If I use Task.Wait() or ContinueWith() afterwards it is captured correctly, but I want to know how to catch it when using await)

Here is the strange part: If I insert a "Debug.WriteLine(ex.Message);" statement inside the catch, ex is not null anymore. And if I remove the "Debug.WriteLine(ex.Message);" statement again and run the code ex is not null. Apparently on some runs ex has the value, sometimes it's just null.

Is that a Thread issue or is it just a Visual Studio bug?

Environment: Windows 10, VS 2019, .NET 5.0, Windows Forms Application

Does this help you https://stackoverflow.com/questions/32067034/how-to-handle-task-run-exception? — ggeorge, Mar 18 '21 at 19:54
Try putting a `Console.WriteLine(ex);` inside the catch-block. I'm sure it's not null if it actually enters the catch-block — Xerillio, Mar 18 '21 at 19:54
@Xerillio: If I put a Debug.WriteLine(ex.Message) statement inside the catch, the message is displayed and variable "ex" is not null. But I still would like to know why in the first case VS is showing ex as null, despite the exception is caught properly. Is this a bug? — A.B., Mar 18 '21 at 20:03
@A.B. If I remember correctly, because there's no statements in your catch block, the debugger never really enters the scope of that code-block, so no variables for that scope are expanded. This is a good reminder to avoid swallowing exceptions with empty code-blocks - it's bad practice. — Xerillio, Mar 18 '21 at 20:16

score 4 · Accepted Answer · answered Mar 18 '21 at 20:23

The precise position of your breakpoint matters. If you just breakpoint the line, ex will be null. If you breakpoint the opening brace of the catch block, ex will have a value. You can also step past the line breakpoint onto the brace to get the value.

This is a breakpoint on the opening brace

This is a breakpoint on the 'line'

async try catch - exception variable is null

1 Answers1