Lesnikowski blog

Email template engine

Pawel Lesnikowski — Fri, 19 Feb 2010 15:47:38 +0000

Newest version of Mail.dll email client includes a simple template engine.

It allows you to easily create html template for your emails:

Loading and rendering such template requires one line of code:

Contact contact = ...;

 string html = Template
     .FromFile("template.txt")
     .DataFrom(contact)
     .Render();

This is how the template looks like:



    Your order


	Hi {FirstName} {LastName},
	

	
	Your account {if Verified}has{else}has not{end} been verified. 

	Your password is: {Password}.
	
	
	Here are your orders:
	
	{foreach Orders}
		
		Order sent to {Street}:
		
		
			{foreach Items}
				
			{end}
		
					{Name} {Price}
				
	{end}
	
		Thank you for your orders.

Here’s the sample of how to load, fill the template and send it using Mail.dll:

Mail.Html(Template
              .FromFile("template.txt")
              .DataFrom(_contact)
              .Render())
    .Text("This is text version of the message.")
    .From(new MailBox("alice@mail.com", "Alice"))
    .To(new MailBox("bob@mail.com", "Bob"))
    .Subject("Your order")
    .UsingNewSmtp()
    .WithCredentials("alice@mail.com", "password")
    .Server("mail.com")
    .WithSSL()
    .Send();

And this is how the data used by template look like:

public class Contact
{
    public List Orders { get; private set; }

    public string FirstName { get; set; }
    public string LastName { get; set; }
    public bool Verified;
    private string Password { get; set; }

    public Contact()
    {
        Orders = new List();
    }
} ;

INotifyPropertyChanged with lambdas

Pawel Lesnikowski — Tue, 16 Feb 2010 21:48:22 +0000

There are several ways of implementing INotifyPropertyChanged in WPF applications.

One that I particularly like is by using PostSharp to implement INotifyPropertyChanged.

If you are not comfortable with PostSharp you can at least get rid of those nasty strings in standard implementation:

public class Model : ViewModeBase
{
    private string _status;
    public string Status
    {
        get { return _status; }
        set
        {
            _status = value;
            OnPropertyChanged("Status");
        }
    }
};

Labda expressions look nicer and are easier to refactor:

public class MyModel : ViewModeBase
{
    private string _status;
    public string Status
    {
        get { return _status; }
        set
        {
            _status = value;
            OnPropertyChanged(() => Status);
        }
    }
};

First thing we need to do is to create base class for all our ViewModels:

public class ViewModelBase
{
    public event PropertyChangedEventHandler PropertyChanged
        = delegate { };

    protected void OnPropertyChanged(
        Expression expression)
    {
        string propertyName = PropertyName.For(expression);
        this.PropertyChanged(
            this,
            new PropertyChangedEventArgs(propertyName));
    }
};

The implementation of PropertyName.For is very straightforward: How to get property name from lambda.

That’s it!
Nice, easy to refactor, compilaile-time checked INotifyPropertyChanged implementation.

How to get property name from lambda

Pawel Lesnikowski — Tue, 09 Feb 2010 16:00:50 +0000

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

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

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

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

    public static string For(
        Expression 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.

Background processing in WinForms

Pawel Lesnikowski — Wed, 03 Feb 2010 16:00:59 +0000

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);
    }
};

Testing DateTime.Now

Pawel Lesnikowski — Mon, 01 Feb 2010 16:07:18 +0000

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;
    }
};

Mark emails as read using IMAP

Pawel Lesnikowski — Wed, 27 Jan 2010 18:31:22 +0000

First you need to download Mail.dll IMAP client.

The following code connects to IMAP server, downloads unique ids of all unseen emails and marks the first one as seen.

// C# version

using(Imap imap = new Imap())
{
	imap.Connect("server");

	imap.User = "user";
	imap.Password = "password";
	imap.Login();

	imap.SelectInbox();
	List uids = client.SearchFlag(Flag.Unseen);
	if (uids.Count > 0)
		client.MarkMessageSeenByUID(uids[0]);
    imap.Close(true);
}

' VB.NET version

Using imap As New Imap()
	imap.Connect("server")

	imap.User = "user"
	imap.Password = "password"
	imap.Login()

	imap.SelectInbox()
	Dim uids As List(Of Long) = client.SearchFlag(Flag.Unseen)
	If uids.Count > 0 Then
		client.MarkMessageSeenByUID(uids(0))
	End If
	imap.Close(True)
End Using

Please note that it is not possible to mark messages as read using POP3 protocol.

Cross-thread operations with PostSharp

Pawel Lesnikowski — Tue, 26 Jan 2010 15:00:43 +0000

When you try to inform User Interface (UI) about the background operation progress or completion, you can not do it from the background thread.

UI doesn’t like to be informed about anything from a different thread: you’ll get “System.InvalidOperationException: Cross-thread operation not valid: Control ‘xxx’ accessed from a thread other than the thread it was created on.” exception from WinForms control, if you try:

public void ShowStatus(ApplicationStatus status)
{
    this._lblServiceAddress.Text = "Connected to: "
        + status.WebServiceAddress;
    this._lblUserId.Text = "Working as: "
        + status.UserId;
}

The easiest sollution is to use BeginInvoke method on the control Control or Form:

public void ShowStatus(ApplicationStatus status)
{
    this.BeginInvoke((MethodInvoker)(() =>
        {
            this._lblServiceAddress.Text = "Connected to: "
                + status.WebServiceAddress;
            this._lblUserId.Text = "Working as: "
                + status.UserId;
        }));
}

Well, it’s fun to write this once, but if you have many operations done in background sooner or later you’d like to have something nicer. Like an attribute for example:

[ThreadAccessibleUI]
public void ShowStatus(ApplicationStatus status)
{
    this._lblServiceAddress.Text = "Connected to: " + status.WebServiceAddress;
    this._lblUserId.Text = "Working as: " + status.UserId;
}

Here’s the attribute implementation of such attribute using PostSharp 1.5:

/// 
/// PostSharp attribute.
/// Use it to mark Control's methods that
/// are invoked from background thread.
/// 
/// 
/// Be careful as BeginInvoke uses the message queue.
/// This means that the interface will be refreshed
/// when application has a chance to process its messages.
/// 
[AttributeUsage(AttributeTargets.Method)]
[Serializable] // required by PostSharp
public class ThreadAccessibleUIAttribute : OnMethodInvocationAspect
{
    public override void OnInvocation(
        MethodInvocationEventArgs eventArgs)
    {
        Control control = eventArgs.Instance as Control;
        if (control == null)
            throw new ApplicationException(
                "ThreadAccessibleUIAttribute" +
                "can be applied only to methods on Control class");

        // The form may be closed before
        // this method is called from another thread.
        if (control.Created == false)
            return;

        control.BeginInvoke((MethodInvoker)eventArgs.Proceed);
    }
};

Download email attachments in .NET

Pawel Lesnikowski — Mon, 25 Jan 2010 16:45:15 +0000

First you’ll need an IMAP client or POP3 client to download emails from the server.

The email attachments are downloaded as a part of the message. Attachments are stored within the email as part of a mime tree. Usually Quoted-Printable or Base64 encoding are used. Mail.dll is going to parse such tree for you and expose all attachments as well known .NET collections.

ISimpleMailMessage uses 2 collections for storing attachments:

Attachments - contains all attached documents
Visuals - contains files that should be ‘displayed’ to the user (like images or music that should be played in background)

Following you’ll find samples of how you can save all attachments to disk using C# and VB.NET via POP3 and IMAP protocols.

When you use IMAP server:

// C# version
using(Imap imap = new Imap())
{
	imap.Connect("server");

	imap.User = "user";
	imap.Password = "password";
	imap.Login();

	imap.SelectInbox();
	List uids = imap.SearchFlag(Flag.All);
	foreach (long uid in uids)
	{
		string eml = imap.GetMessageByUID(uid);
		ISimpleMailMessage email = new SimpleMailMessageBuilder()
			.CreateFromEml(eml);

		Console.WriteLine(email.Subject);

		// save all attachments to disk
		email.Attachments.ForEach(mime => mime.Save(mime.FileName));
		email.Visuals.ForEach(mime => mime.Save(mime.FileName));
	}
	imap.Close(true);
}

You can also save attachment to stream: void MimeData.Save(Stream stream)
Or get direct access to it: MemoryStream MimeData.GetMemoryStream()

' VB.NET version
Using imap As New Imap()
	imap.Connect("server")

	imap.User = "user"
	imap.Password = "password"
	imap.Login()

	imap.SelectInbox()
	Dim uids As List(Of Long) = imap.SearchFlag(Flag.All)
	For Each uid As Long In uids
		Dim eml As String = imap.GetMessageByUID(uid)
		Dim email As ISimpleMailMessage = New SimpleMailMessageBuilder()_
			.CreateFromEml(eml)

		Console.WriteLine(email.Subject)

		' save all attachments to disk
		email.Attachments.ForEach(Function(mime) mime.Save(mime.FileName))
		email.Visuals.ForEach(Function(mime) mime.Save(mime.FileName))
	Next
	imap.Close(True)
End Using

When you use POP3 server:

// C# version
using(Pop3 pop3 = new Pop3())
{
	pop3.Connect("server");

	pop3.User = "user";
	pop3.Password = "password";
	pop3.Login();

	pop3.GetAccountStat();
	for (int i = 1; i <= pop3.MessageCount; i++)
	{
		string eml = pop3.GetMessage(i);
		ISimpleMailMessage email = new SimpleMailMessageBuilder()
			.CreateFromEml(eml);

		Console.WriteLine(email.Subject);

		// save all attachments to disk
		email.Attachments.ForEach(mime => mime.Save(mime.FileName));
		email.Visuals.ForEach(mime => mime.Save(mime.FileName));
	}
	pop3.Close(true);
}

' VB.NET version
Using pop3 As New Pop3()
	pop3.Connect("server")

	pop3.User = "user"
	pop3.Password = "password"
	pop3.Login()

	pop3.GetAccountStat()
	For i As Integer = 1 To pop3.MessageCount
		Dim eml As String = pop3.GetMessage(i)
		Dim email As ISimpleMailMessage = New SimpleMailMessageBuilder()_
			.CreateFromEml(eml)

		Console.WriteLine(email.Subject)

		' save all attachments to disk
		email.Attachments.ForEach(Function(mime) mime.Save(mime.FileName))
		email.Visuals.ForEach(Function(mime) mime.Save(mime.FileName))
	End While
	pop3.Close(True)
End Using

FindAll, ConvertAll are your friends

Pawel Lesnikowski — Wed, 06 Jan 2010 07:40:55 +0000

Let’s take a look at following code:

public List GetDeleteWarnings_ForEach()
{
    List messages = new List();
    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 GetDeleteWarnings_Fluently()
{
    return _itemReferences
        .FindAll(x => !x.CanBeDeleted)
        .ConvertAll(x => x.DeleteMessage);
}

Uploading emails using IMAP

Pawel Lesnikowski — Mon, 04 Jan 2010 16:59:13 +0000

Uploading emails to the IMAP server is fairly easy with Mail.dll IMAP client

First we’ll use Mail.dll to upload an existing email in eml format to the IMAP server:

// C# code

using (Imap imap = new Imap())
{
    imap.Connect("server");

    imap.User = "user";
    imap.Password = "password";
    imap.Login();

    string eml = File.ReadAllText("email.eml");

    // The name of the folder depends on your IMAP server
    imap.UploadMessage("[Gmail]/Sent Mail", eml);

    imap.Close(true);
}

' VB.NET code

Using imap As New Imap()
	imap.Connect("server")

	imap.User = "user"
	imap.Password = "password"
	imap.Login()

	Dim eml As String = File.ReadAllText("email.eml")

	' The name of the folder depends on your IMAP server
	imap.UploadMessage("[Gmail]/Sent Mail", eml)

	imap.Close(True)
End Using

Second sample shows how to create new email message and upload it to IMAP server.

// C# code

using (Imap imap = new Imap())
{
    imap.Connect("server");

    imap.User = "user";
    imap.Password = "password";
    imap.Login();

    // Create new mail message
    SimpleMailMessageBuilder builder = new SimpleMailMessageBuilder();
    builder.Subject = "subject";
    builder.From.Add(new MailBox("alice@email.com", "Alice"));
    builder.To.Add(new MailBox("bob@email.com", "Bob"));
    builder.SetTextData("This is plain text email");

    // Upload
    // The name of the folder depends on your IMAP server
    imap.UploadMessage("[Gmail]/Sent Mail", builder.Create());

    imap.Close(true);
}

' VB.NET code

Using imap As New Imap()
	imap.Connect("server")

	imap.User = "user"
	imap.Password = "password"
	imap.Login()

	' Create new mail message
	Dim builder As New SimpleMailMessageBuilder()
	builder.Subject = "subject"
	builder.From.Add(New MailBox("alice@email.com", "Alice"))
	builder.[To].Add(New MailBox("bob@email.com", "Bob"))
	builder.SetTextData("This is plain text email")

	' Upload
	' The name of the folder depends on your IMAP server
	imap.UploadMessage("[Gmail]/Sent Mail", builder.Create())

	imap.Close(True)
End Using

Please note that only few IMAP servers are going to send the message to the actual recipients. Most servers will only upload the message without sending it.
You should use SMTP protocol for this.
You can download Mail.dll IMAP client here.