Question

It has been a long question, so here is the summary first,

• I have a Client class for my messenger project.
• My Client class has a Socket.
• I use its BeginReceive method to get messages from server.
• In async callback of BeginReceive, I get the message using EndReceive.
• When I get the message, I fire the MessageReceived event with message itself.
• I use this Client class in my message form.
• In message form, I can get the received message by attaching a method to Client.MessageReceived.
• I use OnMessageReceived method for this purpose.
• When I get the message, I can show it on a TextBox (using Control.Invoke) or MessageBox.
• I can also add a new tab to my TabControl in OnMessageReceived.
• When I try to initialize a WebBrowser control, I get ThreadStateException.
• The control I use to display messages derives from WebBrowser control, so I need it.
• Threading.Thread.CurrentThread.ThreadState is “Background”.

I don’t think you’ll need the details but you can find the detailed question I first intented to post below.

Many thanks.

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I’m working on a messenger project (Server is a Windows Service and Client is a Windows Forms Application) using Net.Socket’s async methods.
I fire Client’s MessageReceived event in callback of Socket.BeginReceive;
Everything is how I want them to be until here.

I use MessageReceived event in my form (the one that two people writes to each other) I can do anything I want to the UI using Control.Invoke method (if required) with one annoying exception.

I have tabbed conversations in the form so when a message arrives, I check if there is an open conversation (tab) with the sender. If yes, I select that tab and display the message. If no, I create a new ConversationTab.

Now, I’m sorry if it’s being a long question than it should be or if I can’t explain myself sufficently. English is not my first language and this is my first question in Stack Overflow.
So, here is the ConversationTab:

public class ConversationTab : TabPage
    {
        public User Friend { get; private set; }
        public MessageBrowser MessageBrowser { get; private set; }

        public ConversationTab(User friend) : base(friend.DisplayName)
        {
            Friend = friend;
            MessageBrowser = new MessageBrowser();
            Controls.Add(MessageBrowser);
            MessageBrowser.Dock = DockStyle.Fill;   
        }
    }

MessageBrowser derives from WebBrowser and the reason I use this is because I could not apply custom styles (color, font, size) ‘per message’ using RichTextBox. RichTextBox.SelectedColor doesn’t always work or I couldn’t make it work as intended. MessageBrowser let’s me use CSS instead. Wandering off of the subject? Sorry.

Here is the NewConversation method I call when MessageReceived event fires:

public void NewConversation(User friend)
    {
        ConversationTab tab = Conversations.FirstOrDefault(c => c.Friend.Id == friend.Id);

        if (tab != null)
            ActiveConversation = tab;
        else
        {
            tab = new ConversationTab(friend);
            // add tab to TabControl
        }

        // bla
    }

“Conversations” gets the tab pages of the TabControl and “ActiveConversation” gets or sets the SelectedTab property of the TabControl.
My point in creating these properties are mostly thread-safety logic inside.

So the question: It’s throwing ThreadStateException in “tab = new ConversationTab(friend)” part of the above code. It is the “MessageBrowser = new MessageBrowser()” part of the first code and the constructor of MessageBrowser. The reason of why I didn’t provide MessageBrowser’s constructor code is because the exception is thrown before any line of inner code gets executed (It is about WebBrowser’s constructor, I get this exception when I try to initalize a WebBrowser, too.)
Actually I don’t even get an exception, the application just closes there without notifying me about anything. I saw the exception when I try to call “MessageBrowser = new MessageBrowser()” on ConversationTab’s constructor in Watch window.

I’m kind of new to using threads and asynchronous methods.
MSDN says:

ThreadStateException is thrown by methods that cannot perform the requested operation due to the current state of a thread.

In my case, the thread’s state is “Background”.
I have no clue about what am I doing wrong.

Thank you very much if you read the whole thing and thank you much more if you can help.

Answer 1

This seems to be related to using COM (web browser control uses COM) in .NET where thread apartment needs to be set to STA.

Try adding [STAThread] to your entry point.

Have a look at this.