First a warning, this is a difficult article which goes really deep inside the .Net machinery so if you don’t get it the first time (or even the second or third time…) don’t worry and come back later. 🙂
For a training session I’ve taught at the end of last year I wanted to demonstrate some subtleties of multi-threading, and more specifically some memory visibility issues that should cause a program to hang.
So I developed a small sample that I expected would be showing the issue, but instead of hanging as expected the program completed!
After manipulating the program further I obtained the behavior I wanted, the program was hanging, but it still didn’t explained why it managed to complete with my original version.
I suspected some JITter optimizations, and indeed it was the case, but I needed more information to completely explain this strange behavior.
As often, the StackOverflow platform was of great help; if you’re curious you can have a look at the original SO thread.
In this article I’ll “build” and explain the issue step by step, trying to make it more understandable than the SO thread which is indeed quite dry.
This is a small article about an issue I had recently trying to save some big documents represented as .Net objects in MongoDB using the MongoDB .Net driver.
While saving a “relatively” big document I’ve received the following exception:
System.IO.FileFormatException: Size 32325140 is larger than MaxDocumentSize 16777216.
at MongoDB.Bson.IO.BsonBinaryWriter.BackpatchSize() in c:\projects\mongo-csharp-driver\MongoDB.Bson\IO\BsonBinaryWriter.cs:line 697
at MongoDB.Bson.IO.BsonBinaryWriter.WriteEndArray() in c:\projects\mongo-csharp-driver\MongoDB.Bson\IO\BsonBinaryWriter.cs:line 294
at MongoDB.Bson.Serialization.Serializers.EnumerableSerializerBase`1.Serialize(BsonWriter bsonWriter, Type nominalType, Object value, IBsonSerializationOptions options) in c:\projects\mongo-csharp-driver\MongoDB.Bson\Serialization\Serializers\EnumerableSerializerBase.cs:line 408
at MongoDB.Bson.Serialization.BsonClassMapSerializer.SerializeMember(BsonWriter bsonWriter, Object obj, BsonMemberMap memberMap) in c:\projects\mongo-csharp-driver\MongoDB.Bson\Serialization\Serializers\BsonClassMapSerializer.cs:line 684
at MongoDB.Bson.Serialization.BsonClassMapSerializer.Serialize(BsonWriter bsonWriter, Type nominalType, Object value, IBsonSerializationOptions options) in c:\projects\mongo-csharp-driver\MongoDB.Bson\Serialization\Serializers\BsonClassMapSerializer.cs:line 432
at MongoDB.Driver.Internal.MongoInsertMessage.AddDocument(BsonBuffer buffer, Type nominalType, Object document) in c:\projects\mongo-csharp-driver\MongoDB.Driver\Communication\Messages\MongoInsertMessage.cs:line 53
at MongoDB.Driver.Operations.InsertOperation.Execute(MongoConnection connection) in c:\projects\mongo-csharp-driver\MongoDB.Driver\Operations\InsertOperation.cs:line 97
at MongoDB.Driver.MongoCollection.InsertBatch(Type nominalType, IEnumerable documents, MongoInsertOptions options) in c:\projects\mongo-csharp-driver\MongoDB.Driver\MongoCollection.cs:line 1149
at MongoDB.Driver.MongoCollection.Insert(Type nominalType, Object document, MongoInsertOptions options) in c:\projects\mongo-csharp-driver\MongoDB.Driver\MongoCollection.cs:line 1004
at MongoDB.Driver.MongoCollection.Save(Type nominalType, Object document, MongoInsertOptions options) in c:\projects\mongo-csharp-driver\MongoDB.Driver\MongoCollection.cs:line 1426
Well the message is clear: seems like I’ve exceeded the MongoDB max document size threshold which is 16MB, fair enough this is quite a sane design decision.
First I’ll explain why I had this issue, then how I’ve solved it.
As you may know event handlers are a common source of memory leaks caused by the persistence of objects that are not used anymore, and you may think should have been collected, but are not, and for good reason.
In this (hopefully) short article, I’ll present the issue with event handlers in the context of the .Net framework, then I’ll show you how you can implement the standard solution to this issue, the weak event pattern, in two ways, either using:
the “legacy” (well, before .Net 4.5, so not that old) approach which is quite cumbersome to implement
the new approach provided by the .Net 4.5 framework which is as simple as it can be
Recently I’ve worked with a web API, the Cometdocs API, in order to use their converter of documents, particularly for automating some conversions from PDF documents to Excel spreadsheets for data extraction.
I wanted to use this API from my two favorite development platforms: Java and .Net/C#, so I needed to build what is called a language binding, i.e. a small library that acts as a proxy between the application code and the web API.
The development of these two bindings was really interesting from a technical point of view, and I’ve learned a bunch of things during the process.
I’d like to share the interesting stuff with you, it should be of interest even if you don’t have any plan for interacting with a web API because all the technologies and techniques I’ve used (the HTTP protocol, JSON data binding, SSL/TLS…) are applicable to other types of developments.
It seemed technically feasible because Python has a remarkable tool to interact with native code: the ctypes module.
The only issue is that ctypes only supports C interfaces not C++ classes so in this case it can’t directly use the YahooAPIWrapper class.
In fact it’s a minor issue because this kind of situation is well known and a well documented pattern exists to circumvent it: building a C wrapper around the C++ API.
This looks a little crazy because you now have 2 layers between the Python client code and the C# Yahoo API:
Python -> C wrapper -> C++/CLI wrapper -> C# API
So, while I don’t think this layering could have any usefulness in real-life, this was a challenging and interesting question.
Looks simple no? Well, as you know when you start to pile up heterogeneous layersunexpected issues can appear and this is exactly what happened there, and it has revealed one that is worth talking about.
So keep reading!
Sometimes we need to modify some global state, but when we do that it’s critical to ensure we leave things as we’ve found them; you know like when you lift the toilet seat: forgetting to put it down afterwards could get you in trouble!
It’s similar in programming and you need to use the right tools and practices.
In this article I’ll show you how they can be used to scope global state changes in a fluent manner, a pattern I’ve used for years to enhance reusability and readability of this kind of code.
In the first part I’ll illustrate this pattern with a small and hopefully fun example, and then in the second part I’ll describe how it can be applied to other situations like culture settings and Excel development. Continue reading →