MediatR exceptions handling

1.ExceptionExtensions.cs

public static class ExceptionExtensions
{
	public static List<Exception> FlattenInnerExceptions(this Exception exception)
	{
		return exception
				 .DepthFirstSearchInnerExceptions()
				 .Distinct()
				 .Reverse()
				 .ToList();
	}

	private static IEnumerable<Exception> DepthFirstSearchInnerExceptions(this Exception exception)
	{
		if (exception is not AggregateException) yield return exception;
		var bypassInnerException = exception;

		do
		{
			switch (bypassInnerException)
			{
				case AggregateException { InnerExceptions: { } innerExceptions } aggregateException when innerExceptions.Any():
					foreach (var innerException in aggregateException.FlattenAggregateExceptions())
					{
						yield return innerException;
					}
					break;

				case { InnerException: AggregateException { InnerExceptions: { } innerExceptions } aggregateException } when innerExceptions.Any():
					foreach (var innerException in aggregateException.FlattenAggregateExceptions())
					{
						yield return innerException;
					}
					break;

				case { InnerException: { } innerException }:
					yield return innerException;
					break;

				default:
					yield return bypassInnerException;
					break;
			}

			bypassInnerException = bypassInnerException.InnerException;
		} while (bypassInnerException is not null);
	}

	private static IEnumerable<Exception> FlattenAggregateExceptions(this AggregateException aggregateException)
	{
		return aggregateException
				 .Flatten().InnerExceptions
				 .SelectMany(DepthFirstSearchInnerExceptions);
	}
}

2.ExceptionsHandlerBehavior.cs

public class ExceptionsHandlerBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
where TRequest : notnull
{
	public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
	{
		try
		{
			return await next().ConfigureAwait(false);
		}
		catch (InvalidOperationException exception)
		{
			var exceptions = exception.FlattenInnerExceptions();

			foreach (var ex in exceptions)
			{
				Log.Write(LogEventLevel.Warning, string.Concat("{Exception}: ", ex.Message), ex.GetType().FullName);
			}

			throw new AggregateException("One or more errors occurred", exceptions);
		}
	}
}

3.SetupExtensions.cs

public static void AddMediatR(this IServiceCollection services)
{
	//...
	services.AddTransient(typeof(IPipelineBehavior<,>), typeof(ExceptionsHandlerBehavior<,>));
	//...
}

4.ExceptionExtensionsTests.cs

public class ExceptionExtensionsTests
{
	private readonly ITestOutputHelper _output;

	public ExceptionExtensionsTests(ITestOutputHelper output)
	{
		_output = output;
	}

	public static readonly object[][] AggregateException_InnerNineExceptions =
	{
		new object[]
		{
			ExceptionsExtensionsFixture.CreateAggregateException_InnerNineExceptions(),
		}
	};

	[Theory, MemberData(nameof(AggregateException_InnerNineExceptions))]
	public void FlattenInnerExceptions_ShouldReturnDistinctOrderedListExceptions((Exception exception, List<Exception> expected) valueTuple)
	{
		var (exception, expected) = valueTuple;
		var actual = exception.FlattenInnerExceptions();

		Assert.Equal(expected.Count, actual.Count);

		for (var i = 0; i < expected.Count; i++)
		{
			_output.WriteLine($"{actual[i].GetType().FullName}: {actual[i].Message}");
			Assert.Equal(expected[i], actual[i]);
		}
	}
}

5.ExceptionsExtensionsFixture.cs

public static class ExceptionsExtensionsFixture
{
	/// <summary>
	///		Created nine exceptions in "Exception.InnerException" each other
	/// </summary>
	/// <returns>
	///		System.NullReferenceException: NullReferenceException <br/>
	///		System.ArgumentException: ArgumentException <br/>
	///		System.Exceptions.RecordNotFoundException: RecordNotFoundException <br/>
	///		MongoDB.Bson.BsonSerializationException: BsonSerializationException <br/>
	///		MongoDB.Bson.BsonException: BsonException <br/>
	///		Microsoft.EntityFrameworkCore.Storage.RetryLimitExceededException: RetryLimitExceededException <br/>
	///		Castle.DynamicProxy.Generators.GeneratorException: GeneratorException <br/>
	///		System.TypeLoadException: TypeLoadException <br/>
	///		System.InvalidOperationException: InvalidOperationException <br/>
	/// </returns>
	public static (Exception, List<Exception>) CreateAggregateException_InnerNineExceptions()
	{
		var nullReference = new NullReferenceException(nameof(NullReferenceException));
		var argument = new ArgumentException(nameof(ArgumentException), nullReference);
		var recordNotFound = new RecordNotFoundException(nameof(RecordNotFoundException), argument);

		var bsonSerialization = new BsonSerializationException(nameof(BsonSerializationException));
		var bson = new BsonException(nameof(BsonException), bsonSerialization);

		var aggregate = new AggregateException(bson, recordNotFound);

		var retryLimitExceeded = new RetryLimitExceededException(nameof(RetryLimitExceededException), aggregate);
		var generator = new GeneratorException(nameof(GeneratorException), retryLimitExceeded);
		var typeLoad = new TypeLoadException(nameof(TypeLoadException), generator);
		var invalidOperation = new InvalidOperationException(nameof(InvalidOperationException), typeLoad);

		var exception = new AggregateException(nameof(AggregateException), aggregate, invalidOperation);

		List<Exception> exceptions = new()
		{
			nullReference,
			argument,
			recordNotFound,
			bsonSerialization,
			bson,
			retryLimitExceeded,
			generator,
			typeLoad,
			invalidOperation
		};

		return (exception, exceptions);
	}
}