The whole idea is simple, we want this test to pass:

[Test]
void CanGetPropertyName_UsingLambda()
{
  Assert.AreEqual("Name", PropertyName.For<Person>(x => x.Name));
}

It seams nice and convenient way of getting property name.
Whole test fixture looks as follows:

[TestFixture]
public class PropertyNameTests
{
    public string NameForTest { get; set; }

    [Test]
    public void CanGetPropertyName_SameType_UsingLambda()
    {
        Assert.AreEqual("NameForTest",
            PropertyName.For(() => NameForTest));
    }

    [Test]
    public void CanGetPropertyName_UsingLambda()
    {
        Assert.AreEqual("Name",
            PropertyName.For<Person>(x => x.Name));
        Assert.AreEqual("Age",
            PropertyName.For<Person>(x => x.Age));
    }

    [Test]
    public void CanGetPropertyName_Composite_UsingLambda()
    {
        Assert.AreEqual("Home.City",
            PropertyName.For<Person>(x => x.Home.City));
        Assert.AreEqual("Home.FlatNumber",
            PropertyName.For<Person>(x => x.Home.FlatNumber));
    }
}

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
    public Home Home { get; set; }
}

public class Home
{
    public string City { get; set; }
    public string FlatNumber { get; set; }
}

Implementation uses .NET 3.5 feature called expression trees. Expression trees represent language-level code in the form of data. The data is stored in a tree-shaped structure.

/// <summary>
/// Gets property name using lambda expressions.
/// </summary>
internal class PropertyName
{
    public static string For<T>(
        Expression<Func<T, object>> expression)
    {
        Expression body = expression.Body;
        return GetMemberName(body);
    }

    public static string For(
        Expression<Func<object>> expression)
    {
        Expression body = expression.Body;
        return GetMemberName(body);
    }

    public static string GetMemberName(
        Expression expression)
    {
        if (expression is MemberExpression)
        {
            var memberExpression = (MemberExpression)expression;

            if (memberExpression.Expression.NodeType ==
                ExpressionType.MemberAccess)
            {
                return GetMemberName(memberExpression.Expression)
                    + "."
                    + memberExpression.Member.Name;
            }
            return memberExpression.Member.Name;
        }

        if (expression is UnaryExpression)
        {
            var unaryExpression = (UnaryExpression)expression;

            if (unaryExpression.NodeType != ExpressionType.Convert)
                throw new Exception(string.Format(
                    "Cannot interpret member from {0}",
                    expression));

            return GetMemberName(unaryExpression.Operand);
        }

        throw new Exception(string.Format(
            "Could not determine member from {0}",
            expression));
    }
}

UnaryExpression part is needed for value types to work.

Many times developing windows applications, you’ll need to perform some operations in background.

The problem you’ll face sooner or later is that those operations need to inform User Interface (UI) about their progress and completion.

UI doesn’t like to be informed about anything from a different thread: you’ll get nasty “Cross-thread operation not valid” exception from WinForms controls, if you try.

Let’s take a look at the sample Presenter code:

public void Start()
{
        TaskStatus taskStatus = this._backupService.CreateTask();
        taskStatus.Completed += BackupFinished;
        this._backupService.Start(taskStatus);
}

TaskStatus contains single event Completed.
What’ll do is that we’ll subscribe to this event to display some information on the View:

public void BackupFinished(object sender, EventArgs e)
{
        // If the operation is done on different thread,
        // you'll get "Cross-thread operation not valid"
        // exception from WinForms controls here.
        this.View.ShowMessage("Finished!");
}

So, what are the options:

• Use Control.BeginInvoke in View – makes your code unreadable
• Use Control.BeginInvoke with PostSharp – you need PostSharp
• SynchronizationContext

Lets examine the last concept as SynchronizationContext is not a well-know-class in the .NET world.
Generally speaking this class is useful for synchronizing calls from worker thread to UI thread.

It has a static Current property that gets the synchronization context for the current thread or null if there is no UI thread (e.g. in Console application)

This is the TaskStatus class that utilizes SynchronizationContext.Current if it is not null:

public class TaskStatus
{
    private readonly SynchronizationContext _context;

    public event EventHandler Completed = delegate { };

    public TaskStatus()
    {
        _context = SynchronizationContext.Current;
    }

    internal void OnCompleted()
    {
        Synchronize(x => this.Completed(this, EventArgs.Empty));
    }

    private void Synchronize(SendOrPostCallback callback)
    {
        if (_context != null)
            _context.Post(callback, null);
        else
            callback(null);
    }
};

Now lets see some tests.

First we’ll check if the event is executed:

[Test]
public void Completed_RaisesCompleted()
{
    using(SyncContextHelper.No())
    {
        bool wasFired = false;
        TaskStatus status = new TaskStatus();
        status.Completed += (sender, args) => { wasFired = true; };
        status.OnCompleted();
        Assert.IsTrue(wasFired);
    }
}

The following test shows that in WindowsForms application, although operation is executed on different thread, Completed event is routed back (using windows message queue) to the UI thread:

[Test]
public void Completed_WithSyncContext_IsExecutedOnSameThread()
{
    using (SyncContextHelper.WinForms())
    {
         int completedOnThread = -1;
         int thisThread = Thread.CurrentThread.GetHashCode();

         TaskStatus status = new TaskStatus();
         status.Completed += (sender, args) =>
             {
                 completedOnThread =
                    Thread.CurrentThread.GetHashCode();
             };

         Scenario.ExecuteOnSeparateThread(status.OnCompleted);

         // process messages send from background thread
         // (like Completed event)
         Application.DoEvents();

         Assert.AreEqual(thisThread, completedOnThread);
    }
}

When there is no SynchronizationContext (SynchronizationContext.Current == null) Completed event is executed on the different thread:

[Test]
public void Completed_InMultiThreadedScenario_IsExecuedOnDifferentThread()
{
    using(SyncContextHelper.No())
    {
        int completedOnThread = -1;
        int thisThread = Thread.CurrentThread.GetHashCode();

        TaskStatus status = new TaskStatus();
        status.Completed += (sender, args) =>
            {
                completedOnThread =
                    Thread.CurrentThread.GetHashCode();
            };

        Scenario.ExecuteOnSeparateThread(status.OnCompleted);

        Assert.AreNotEqual(thisThread, completedOnThread);
    }
}

Finally unit test helper classes:

public class SyncContextHelper : IDisposable
{
    private readonly SynchronizationContext _previous;

    private SyncContextHelper(SynchronizationContext context)
    {
        _previous = SynchronizationContext.Current;
        SynchronizationContext.SetSynchronizationContext(context);
    }

    public static SyncContextHelper WinForms()
    {
        return new SyncContextHelper(
            new WindowsFormsSynchronizationContext());
    }

    public static SyncContextHelper No()
    {
        return new SyncContextHelper(null);
    }

    public void Dispose()
    {
        SynchronizationContext.SetSynchronizationContext(_previous);
    }
};

When you are doing Test Driven Development (TDD) many time during the testing phase you’ll find yourself with a code that depends on current time.

It’s not possible write tests for this code as DateTime.Now changes constantly.

Let’s take a look at following method:

public string CreateName(...)
{
   return "name " + DateTime.Now;
}

The solution to this problem is simple. We’ll introduce Clock class with the same interface as DateTime:

public string CreateName(...)
{
   return "name " + Clock.Now;
}

We’d like the test to look something like this:

[Test]
public void CreateName_AddsCurrentTimeAtEnd()
{
    using (Clock.NowIs(new DateTime(2010, 12, 31, 23, 59, 00)))
    {
        string name = new ReportNameService().CreateName(...);
        Assert.AreEqual("name 2010-12-31 23:59:00", name);
    }
}

Tests should not leave any side effects. This is why we are using IDisposable pattern. After text execution Clock.Now is reverted and again returns current time.

Finally this is how Clock class looks like:

public class Clock : IDisposable
{
    private static DateTime? _nowForTest;

    public static DateTime Now
    {
        get { return _nowForTest ?? DateTime.Now; }
    }

    public static IDisposable NowIs(DateTime dateTime)
    {
        _nowForTest = dateTime;
        return new Clock();
    }

    public void Dispose()
    {
        _nowForTest = null;
    }
};

Let’s take a look at following code:

public List<string> GetDeleteWarnings_ForEach()
{
    List<string> messages = new List<string>();
    foreach (ItemReference reference in _itemReferences)
    {
        if (!reference.CanBeDeleted)
        {
            messages.Add(reference.DeleteMessage);
        }
    }
    return messages;
}

Using FindAll and ConvertAll you can do it in one, very obvious, line:

public List<string> GetDeleteWarnings_Fluently()
{
    return _itemReferences
        .FindAll(x => !x.CanBeDeleted)
        .ConvertAll(x => x.DeleteMessage);
}

When you to inherit your custom dictionary from Dictionary class you’ll get nasty exception during deserialization:

System.Runtime.Serialization.SerializationException : The constructor to deserialize an object of type ‘SafeDictionary`2[System.String,System.String]‘ was not found.

[Serializable]
internal class SafeDictionary<K, V> : Dictionary<K, V>
{
    public new V this[K key]
    {
            get
            {
                V value;
                if (this.TryGetValue(key, out value) == true)
                    return value;
                return default(V);
            }
            set
            {
                base[key] = value;
            }
    }

     public SafeDictionary()
    {
    }
};

Here’s the test:

[Test]
public void IsSerializable()
{
    SafeDictionary<string,string> dictionary =
        new SafeDictionary<string, string>();
    dictionary["key"] = "value";
    dictionary =
        SerializationHelper.SerializeAndDeserialize(dictionary);
    Assert.AreEqual("value", dictionary["key"]);
}

And serialization utility class:

class SerializationHelper
{
    public static T SerializeAndDeserialize<T>(T sm)
    {
        BinaryFormatter formatter = new BinaryFormatter();
        using (MemoryStream stream = new MemoryStream())
        {
            formatter.Serialize(stream, sm);
            stream.Position = 0;
            return (T)formatter.Deserialize(stream);
        }
    }
}

Dictionary class implements ISerializable interface.

The ISerializable interface implies a constructor with the signature constructor (SerializationInfo information, StreamingContext context).

At deserialization time, the current constructor is called only after the data in the SerializationInfo has been deserialized by the formatter. In general, this constructor should be protected if the class is not sealed.

So what you need to do is to add the following constructor:

    //Needed for deserialization.
    protected SafeDictionary(SerializationInfo information, StreamingContext context)
            : base(information,context)
    {
    }

Now the test will pass.