Is there an analog of ExceptionDispatchInfo in Microsoft.Bcl.Async?

Question

Is there an analog of ExceptionDispatchInfo in Microsoft.Bcl.Async? I cannot find anything similar.

This question was triggered by another question of mine. When the exception's parent task is available, I can use task.GetAwaiter().GetResult() to rethrow, as suggested by @StephenCleary.

What are my options for when it is not available?

Answer 1

Here's the implementation of ExceptionDispatchInfo from Mono. It seems to be compatible with Microsoft .NET 4.0, as far as I have tested it.

public sealed class ExceptionDispatchInfo
{
    readonly Exception _exception;
    readonly object _source;
    readonly string _stackTrace;

    const BindingFlags PrivateInstance = BindingFlags.Instance | BindingFlags.NonPublic;
    static readonly FieldInfo RemoteStackTrace = typeof(Exception).GetField("_remoteStackTraceString", PrivateInstance);
    static readonly FieldInfo Source = typeof(Exception).GetField("_source", PrivateInstance);
    static readonly MethodInfo InternalPreserveStackTrace = typeof(Exception).GetMethod("InternalPreserveStackTrace", PrivateInstance);

    private ExceptionDispatchInfo(Exception source)
    {
        _exception = source;
        _stackTrace = _exception.StackTrace + Environment.NewLine;
        _source = Source.GetValue(_exception);
    }

    public Exception SourceException { get { return _exception; } }

    public static ExceptionDispatchInfo Capture(Exception source)
    {
        if (source == null)
            throw new ArgumentNullException("source");

        return new ExceptionDispatchInfo(source);
    }

    public void Throw()
    {
        try
        {
            throw _exception;
        }
        catch
        {
            InternalPreserveStackTrace.Invoke(_exception, new object[0]);
            RemoteStackTrace.SetValue(_exception, _stackTrace);
            Source.SetValue(_exception, _source);
            throw;
        }
    }
}

Answer 2

I have experimented with the implementation provided by avo - yet it doesn't pass all the tests. By that I mean that it provides the basic behaviour but will fail in more complex cases (such as multiple throwing, recapturing, etc...).

As part of Theraot.Core I have made a back port of this class in such way that it will work in modern Mono, old Mono (pre 2.6*) and any Microsoft .NET from 2.0 to 4.0.

*: With a little tip from Miguel de Icaza :)

The code below is currently only on the Feature branch, it will eventually move to the master branch (along with Task for .NET 2.0 :), at which point it will available via the Theraot.Core nuget. I'm leaving it here in case somebody needs it sooner.

Bug reports are welcome on the issue tracker at the project's github (linked above), if you find any.

#if NET20 || NET30 || NET35 || NET40

using System.Reflection;

namespace System.Runtime.ExceptionServices
{
    /// <summary>
    /// The ExceptionDispatchInfo object stores the stack trace information and Watson information that the exception contains at the point where it is captured. The exception can be thrown at another time and possibly on another thread by calling the ExceptionDispatchInfo.Throw method. The exception is thrown as if it had flowed from the point where it was captured to the point where the Throw method is called.
    /// </summary>
    public sealed class ExceptionDispatchInfo
    {
        private static FieldInfo _remoteStackTraceString;

        private Exception _exception;
        private object _stackTraceOriginal;
        private object _stackTrace;

        private ExceptionDispatchInfo(Exception exception)
        {
            _exception = exception;
            _stackTraceOriginal = _exception.StackTrace;
            _stackTrace = _exception.StackTrace;
            if (_stackTrace != null)
            {
                _stackTrace += Environment.NewLine + "---End of stack trace from previous location where exception was thrown ---" + Environment.NewLine;
            }
            else
            {
                _stackTrace = string.Empty;
            }
        }

        /// <summary>
        /// Creates an ExceptionDispatchInfo object that represents the specified exception at the current point in code.
        /// </summary>
        /// <param name="source">The exception whose state is captured, and which is represented by the returned object.</param>
        /// <returns>An object that represents the specified exception at the current point in code. </returns>
        public static ExceptionDispatchInfo Capture(Exception source)
        {
            if (source == null)
            {
                throw new ArgumentNullException("source");
            }
            return new ExceptionDispatchInfo(source);
        }

        /// <summary>
        /// Gets the exception that is represented by the current instance.
        /// </summary>
        public Exception SourceException
        {
            get
            {
                return _exception;
            }
        }

        private static FieldInfo GetFieldInfo()
        {
            if (_remoteStackTraceString == null)
            {
                // ---
                // Code by Miguel de Icaza

                FieldInfo remoteStackTraceString =
                    typeof(Exception).GetField("_remoteStackTraceString",
                    BindingFlags.Instance | BindingFlags.NonPublic); // MS.Net

                if (remoteStackTraceString == null)
                    remoteStackTraceString = typeof(Exception).GetField("remote_stack_trace",
                        BindingFlags.Instance | BindingFlags.NonPublic); // Mono pre-2.6

                // ---
                _remoteStackTraceString = remoteStackTraceString;
            }
            return _remoteStackTraceString;
        }

        private static void SetStackTrace(Exception exception, object value)
        {
            FieldInfo remoteStackTraceString = GetFieldInfo();
            remoteStackTraceString.SetValue(exception, value);
        }

        /// <summary>
        /// Throws the exception that is represented by the current ExceptionDispatchInfo object, after restoring the state that was saved when the exception was captured.
        /// </summary>
        public void Throw()
        {
            try
            {
                throw _exception;
            }
            catch (Exception exception)
            {
                GC.KeepAlive(exception);
                var newStackTrace = _stackTrace + BuildStackTrace(Environment.StackTrace);
                SetStackTrace(_exception, newStackTrace);
                throw;
            }
        }

        private string BuildStackTrace(string trace)
        {
            var items = trace.Split(new[] { Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries);
            var newStackTrace = new Text.StringBuilder();
            var found = false;
            foreach (var item in items)
            {
                // Only include lines that has files in the source code
                if (item.Contains(":"))
                {
                    if (item.Contains("System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()"))
                    {
                        // Stacktrace from here on will be added by the CLR
                        break;
                    }
                    if (found)
                    {
                        newStackTrace.Append(Environment.NewLine);
                    }
                    found = true;
                    newStackTrace.Append(item);
                }
                else if (found)
                {
                    break;
                }
            }
            var result = newStackTrace.ToString();
            return result;
        }
    }
}

#endif