Polly handles a few exceptions like expected first but then System.Private.CoreLib.dll steps in and triggers an unhandled exception

Question

The program below behaves as expected for three retries but then crashes:

$ ./PollyTest/bin/Debug/net8.0/PollyTest.exe
Hello, World!
retry number 0
retry number 1
retry number 2
Unhandled exception. System.ApplicationException: work work
   at Program.<<Main>$>g__WorkAsync|0_1(CancellationToken token) in C:\git\test\pollytest\PollyTest\PollyTest\Program.cs:line 34
   at Program.<>c.<<<Main>$>b__0_0>d.MoveNext() in C:\git\test\pollytest\PollyTest\PollyTest\Program.cs:line 25
--- End of stack trace from previous location ---
   at Polly.ResiliencePipeline.<>c.<<ExecuteAsync>b__3_0>d.MoveNext()
--- End of stack trace from previous location ---
   at Polly.Outcome`1.GetResultOrRethrow()
   at Polly.ResiliencePipeline.ExecuteAsync(Func`2 callback, CancellationToken cancellationToken)
   at Program.<Main>$(String[] args) in C:\git\test\pollytest\PollyTest\PollyTest\Program.cs:line 23
   at Program.<Main>(String[] args)
$

with the following source code:

using Polly.Retry;
using Polly;

// See https://aka.ms/new-console-template for more information
Console.WriteLine("Hello, World!");

var retryOptions = new RetryStrategyOptions
{
    BackoffType = DelayBackoffType.Exponential,
    Delay = TimeSpan.FromMilliseconds(100),
    MaxDelay = TimeSpan.FromSeconds(60),
    OnRetry = arguments =>
    {
        Console.WriteLine($"retry number {arguments.AttemptNumber}");
        return default;
    },
    ShouldHandle = args => PredicateResult.True(),
};
var pipeline = new ResiliencePipelineBuilder()
    .AddRetry(retryOptions)
    .Build();

await pipeline.ExecuteAsync(async token =>
{
    await WorkAsync(token);
}, CancellationToken.None);

return;

///////////////////////////////////

static Task WorkAsync(CancellationToken token)
{
    throw new ApplicationException("work work");
}

When running in debug mode in Visual Studio, the following is printed in the Debug output

...
'PollyTest.exe' (CoreCLR: clrhost): Loaded 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\8.0.2\System.ObjectModel.dll'. Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
'PollyTest.exe' (CoreCLR: clrhost): Loaded 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\8.0.2\System.Collections.Concurrent.dll'. Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
'PollyTest.exe' (CoreCLR: clrhost): Loaded 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\8.0.2\System.Private.Uri.dll'. Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
Exception thrown: 'System.ApplicationException' in PollyTest.dll
Exception thrown: 'System.ApplicationException' in PollyTest.dll
Exception thrown: 'System.ApplicationException' in PollyTest.dll
Exception thrown: 'System.ApplicationException' in PollyTest.dll
Exception thrown: 'System.ApplicationException' in System.Private.CoreLib.dll
An exception of type 'System.ApplicationException' occurred in System.Private.CoreLib.dll but was not handled in user code
work work

so obviously all Exception thrown: ... in PollyTest.dll are captured and handled as expected but Exception thrown: ... in System.Private.CoreLib.dll is not. But why is suddenly System.Private.CoreLib.dll involved? How can I avoid?

This is with Polly version 8.3.0.

Answer 1

Your observed behavior has nothing to do with Polly. If you remove the retry logic from your application and execute directly the WorkAsync

change from

await pipeline.ExecuteAsync(async token =>
{
    await WorkAsync(token);
}, CancellationToken.None);

to

await WorkAsync(CancellationToken.None);

then you will see the exact same behavior.

---

The System.ApplicationException is defined inside the System.Runtime.dll which is a reference assembly. The implementation assembly of this type is System.Private.CoreLib.dll.

So, that's the reason why do you see System.Private.CoreLib.dll in your standard output / error.