Report from the battlefield #9 - async/await + MARS

This post from Report from the battlefield series will be about my own mistake. It is related to async/await and MARS i.e. Multiple Active Result Sets. async/await allows us to use asynchronous programming more easily. MARS is a feature of MSSQL that allows us to have more than one pending request opened per connection at the same time. For example, it may be useful if we have 2 nested loops i.e. internal and external. External loops iterate through one result set and the internal one through another. Ok, but you probably wonder what MARS has in common with async/await.

A few days ago my application started failing due to InvalidOperationException exception with the additional message saying that The connection does not support MultipleActiveResultSets. Well, I used MARS in the past so I simply enabled it in the connection string by setting MultipleActiveResultSets attribute to true.

However, later I realized that my application should not require MARS at all so I started digging into what was wrong. It turned out that the problem was related to my silly mistake in using async/await. Let's look at the following simplified version of the problematic code. We have a trivial Main method:
static void Main()
Start is an async method responsible for opening a connection to DB and executing other async methods:
private static async Task Start()
   using (var c = new SqlConnection(ConnectionString))

      await Func1(c);
      await Func2(c);
      await Func3(c);
Func1, Func2 and Func3 are responsible for reading data and processing them. In our case, for simplification, they all will do the same thing:
private static async Task Func1(SqlConnection c) => await ReadData(c);
private static async Task Func2(SqlConnection c) => ReadData(c);
private static async Task Func3(SqlConnection c) => await ReadData(c);
And here is the ReadData method. It's also simple. It simply reads data from a table:
private static async Task ReadData(SqlConnection c)
   var cmd = c.CreateCommand();

   cmd.CommandText = "SELECT * FROM dbo.Fun";

   using (var reader = await cmd.ExecuteReaderAsync())
      while (await reader.ReadAsync())
         // Process data
If you run this code, the aforementioned InvalidOperationException exception will be thrown in the line with ExecuteReaderAsync. The question is why? Well, in this short code it is rather easy to spot that in Func2 method await is missing before ReadData. But, do you know why it is a problem? If not, don't worry it's a little bit tricky.

Here is an explanation. Without await the simplified flow is as follows:
  • ...
  • Start executes Func2.
  • Func2 executes ReadData.
  • ReadData executes ExecuteReaderAsync.
  • ReadData awaits for the result from ExecuteReaderAsync.
  • The control returns to caller i.e. Func2.
  • However, Func2 does not use await so it returns completed task to Start method.
  • From the point of view of Start processing of Func2 is finished so it executes Func3.
  • Func3 executes ReadData
  • The previous call to ReadData may be still in progress.
  • It also means that ReadData will call ExecuteReaderAsync when another result set is still being processed.
  • The exception is thrown.
Adding missing await fix the problem. Thanks to that the task returned from Func2 will not be completed until call to ReadData is over. And if so Start will not execute Func3 immediately. The final well known conclusion is:

Always async/await all the way down.

*The picture at the beginning of the post comes from own resources and shows Laurel forest on La Gomera.


Roslyn and unit tests suck

Title: Imperial Gardens in Tokyo, Source: own resources, Authors: Agnieszka and Michał Komorowscy

I'm working on the project where I have an opportunity to use Roslyn compiler as a service. It is very good :) However yesterday it took me more than 2 hours to write working unit tests (based on MSTest) for my code! Here are some tips that may save your time.

Let's start with the simle thing. When I run unit tests for the first time the following exception was thrown:

System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime...' or one of its dependencies.

To fix this problem I simply installed the following packages via Nuget:
  • Microsoft.CodeAnalysis.CSharp
  • Microsoft.CodeAnalysis.CSharp.Workspaces
Later it was harder. In my code I use MSBuildWorkspace.OpenSolutionAsync and MSBuildWorkspace.OpenProjectAsync methods to respectively open the entire solution or a single project for further processing.

The next issue was that the first method called from within a unit test was returning an empty solution i.e. without any projects. Whereas the second one was throwing an exception with the message: The language 'C#' is not supported. What was strange these problems occurred only in unit tests! To investigate a problem I opened Exception settings window in Visual Studio and selected a check box next to Common Language Runtime Exceptions. Then, I run the unit tests one more time and Visual Studio quickly reported the exception in the line with MSBuildWorkspace.OpenProjectAsync:

System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.CodeAnalysis.CSharp.Workspaces...' or one of its dependencies.

It was even more strange because my unit test project was actually referencing Microsoft.CodeAnalysis.CSharp.Workspaces.dll! To double check, I went to the unit tests working directory. It is a folder called TestResults which by default is located in the solution directory. To my surprised this dll was missing!

Fortunately, I reminded myself the similar situation from the past. The problem is that MSTest doesn't copy all assemblies to the output directory by default. As far as I know it tries to figure out which assemblies are really needed by the code being tested. Here, I'm not sure but Microsoft.CodeAnalysis.CSharp.Workspaces.dll may be cumbersome because it is not directly referenced by other Roslyn assemblies. Instead, it is probably loaded dynamically when needed.

To fix a problem you can use the simple hack i.e. use directly any code from Microsoft.CodeAnalysis.CSharp.Workspaces.dll in your unit tests in the following way:

public static void ClassInitialize(TestContext ctx)
   var t = typeof(Microsoft.CodeAnalysis.CSharp.Formatting.LabelPositionOptions);

Why did I use LabelPositionOptions? Because majority of types defined in aforementioned assembly is internal and this one was the first public type I found :)


Report from the battlefield #8 - always remember about the context

Title: Sunrise seen from the top of Mount Fuji, , Source: own resources, Authors: Agnieszka and Michał Komorowscy

I decided to change a little bit a character of Report from the battlefield series. Initially, in this series, I was describing my observations from my work as a reviewer for a recruitment company. Now, I'll be also writing about my findings from my day-to-day work. To start, I'll give you a tip how to log useful information.

I worked with an application that is responsible for monitoring folders. If it detects any new files, they are processed and copied somewhere else. The application logs information like the number of files to be processes, the file that is currently being processed etc. This information are logged with severity Information or Debug. It happens that a given file cannot be copied for example because the file with the same name already exists in the destination directory. In that case .NET throws System.IO.IOException. This exception is caught and logged with the severity Error. The simplified version of the log could look in this way:

INFO - 10 files have been found
INFO - Processing file started
INFO - Processing file ended
INFO - Processing file started
ERROR - An error occurred while processing a file: Cannot create a file when that file already exists.
INFO - Processing file ended

It's good that many important information are logged. However, there is a major issue with this log. The lack of the context! For example we know that some files have been processed but we don't know which exactly. This log should look as follows (I used red color to mark changes):

INFO - 10 files have been found in the directory 'C:\Input'
INFO - Processing file 'C:\Input\a.txt' started
INFO - Processing file 'C:\Input\a.txt' ended
INFO - Processing file 'C:\Input\b.txt' started
ERROR - An error occurred while processing a file: Cannot create a file when that file already exists.
INFO - Processing file 'C:\Input\b.txt' ended

It looks much better. Based on the log we can figure out which directory was monitored, which files have been processed successfully and which not. However, it's not everything. There is one more subtle problem here. What if messages with the severity Information or lower won't be logged (for example because of performance issues) and an error will be reported. In this case we'll get the following log:

ERROR - An error occurred while processing a file: Cannot create a file when that file already exists.

It's better than nothing but again we don't know processing of which file has actually failed. The expected result is:

ERROR - An error occurred while processing a file 'C:\Input\b.txt': Cannot create a file when that file already exists.

To sum up, always remember about the context when logging.


Interview Questions by MK #8

Title: Sunset seen from the top of Mount Fuji, Source: own resources, Authors: Agnieszka and Michał Komorowscy

This is the first post from Interview Questions by MK series for a long time. The motivation to write it was a short talk with my colleague. His company really want to hire new .NET developers. The situation on the market is difficult for employers so they are also considering juniors without experience. And still they have a problem to find someone. Why?

The requirements are not extremely high. I'd say that they are standard. They don't demand God knows what. The ideal candidate doesn't have to: know all formatting options available in .NET, enumerate classes in System.DirectoryServices namespace, tell about all new features introduced in any .NET version or any other thing that can be checked in the documentation within seconds. However, they want someone with general knowledge. What I mean by that?
  • It's good to know how to write a class, properties, derived a class... but it's also good to understand and can explain principals of the object oriented programming. For example could you tell why OOP is better than the procedural programming? Or maybe it isn't? Could you justify why encapsulation is actually a good thing?
  • You don't have to know all possible collections available in .NET API but it's worth knowing some of them and their characteristics. Just to mention the list, the dictionary, the stack or the queue.
  • You don't have to be very good in algorithms but knowing how to search a binary tree is not the rocket science.
  • Writing a code that compiles without errors is only a first step. You should also know how to write a readable and a maintainable code. This knowledge comes from experience but at the beginning you should hear about refactoring, knows that the method with 50 parameters is not the best choice...
  • It's not a problem if you have never worked with the continuous integration but you should at least know that there is something like that.
  • As a developer you'll probably not work directly with IT infrastructure but knowing what is the load balancing or the computer cluster does not seem very demanding.
  • ...
I can continue and continue this enumeration. According to me these are basic things, but still many candidates don't know them. Sometimes even developers with a few or more years of experience.

If you are one of them and you want to have better chances on the job market, I recommend one simple thing i.e. reading books, blogs, web sites... whatever you want. Several minutes (better more) every day, regularly, will make a big difference.

You may also say that you don't care because you'll get a job anyway. Well, it's true at least for now. Nonetheless it'll be only some job.


C++ for C# developers - automatic garbage collection

Title: Sushi on the fish market in Tokyo, Source: own resources, Authors: Agnieszka and Michał Komorowscy

In my previous post I wrote that C++ doesn't have the automatic garbage collection. However, it's not entirely true. Let's think about it more carefully. To start, look at the following method in C#. It simply creates an instance of MyObject class.
public void Fun() 
   var x = new MyObject();
When this object will be collected by GC? When it is needed but not sooner than the execution of Fun method is finished. Can we do the same thing in C++? Of course yes, here is an example:
void fun() {
   MyObject x;
If you are not familiar with C++ you may say that I only declared the variable x in this example and that I didn't create any object. Well, in C++ the following 3 commands are equivalent:

MyObject x;
MyObject x();
MyObject x = MyObject();

Personally, I still cannot used to that ;) But let's return to C++ and ask the similar question as for C#. When a destructor will be called for created instance of MyObject? The answer is easy - when the execution of fun method is over. Or in other words when an object goes out of scope. It's worth nothing that this behaviour is called the automatic storage duration. Usually it is implemented by using stack however it's not the rule. Now, let's consider this code in C++:
void fun() {
   MyObject* x = new MyObject();
It looks almost the same like in C#. However, this time we're creating an object dynamically with new keyword. And this kind of objects won't be removed automatically from the memory, even if the execution of fun is over. This is also known as the dynamic storage duration. How to release this kind of objects?

In the past a C++ developer had to use delete keyword to do so. I did the same in the code from my post about virtual destructors. However, since C++ 11 we can use something else i.e. the explicit automatic garbage collection. More precisely I'm talking about smart pointers. Here is the rewritten version of Node class:

class Node {
  static int _count;
  Node(int i) : _value(i) { Node::_count++; }
  ~Node() {
    std::cout << "~Node " << _value;
  int _value;

  std::unique_ptr<Node> _left = nullptr;
  std::unique_ptr<Node> _right = nullptr;

int Node::_count = 0;
In this code I defined ~Node destructor only for its side effects i.e. to decrease a counter. I didn't use delete in this code at all. Instead I wrapped pointers with std::unique_ptr. It works in this way that it releases the pointer when it goes out of scope. Nothing more nothing less. But thanks to that we don't have to remember about delete. Almost like in C#. Here is the testing code:
int main()
    Node* root = new Node(1);
    root->_left = std::unique_ptr<Node>(new Node(2));
    root->_right = std::unique_ptr<Node>(new Node(3));

    std::cout << " Existing nodes: " <<  Node::_count;
    delete root;
    std::cout << " Existing nodes: " <<  Node::_count;
I didn't wrap root in a smart pointer because I wanted to explicitly use delete and verify the final number of nodes. Easy, isn't it?

At the end it's worth mentioning that there are also different types of smart pointers. shared_ptr should be used when the same pointer can have many owners. Whereas std::weak_ptr represents the same concept as C# WeakReference class. Last but not least, except the automatic storage duration and the dynamic storage duration we also have the static storage duration and the thread storage duration. The former is used to store static variables which are release at the end of program (pretty the same as in C#) and the latter to store variables that will survive until the end of a thread (in C# we can use TheradLocalStorage for the similar effect). More reading can be found here.