If you are using solution from http://blogs.msdn.com/infopath/archive/2005/03/04/385577.aspx. You need to make sure that your textbox is formatted to be able to display paragraph breaks. To do this double-click on the text box control, go to the Display tab, and check the checkbox labeled “Allow paragraph breaks”.

After this, your code should work, but if it still doesn’t try using “\n” instead of the System.Environment.Newline.

There is another way to do that.
http://www.infopathdev.com/forums/t/2013.aspx

Quite often you will need to get selected values from a Multiple-Selection list box to a field. I haven’t found a way to do in rules but the following code will solve the problem.

Suppose you have a Multiple-Selection List Box named myListBox on an InfoPath form and an after change event to fill a Text Box named myTextBox with the values or display names of the items that were selected in the Multiple-Selection List Box.

Retrieving the values of selected items
You could use the following C# code to retrieve the values that were selected:

        public void field13_Changed(object sender, XmlEventArgs e)
        {
            XPathNavigator root = MainDataSource.CreateNavigator();
            XPathNodeIterator iter = root.Select("//my:myListBox ",
              NamespaceManager);

            System.Text.StringBuilder sb = new System.Text.StringBuilder();

            while (iter.MoveNext())
            {
                string value = iter.Current.Value;
                sb.Append(value);
                if (sb.Length > 0)
                {
                    sb.Append(";");
                }
            }

            root.SelectSingleNode("//my:myTextBox",
              NamespaceManager).SetValue(sb.ToString());
        }

Retrieving the display names of selected items

If you want to retrieve the Display Names of the items that were selected and you’ve bound the Multiple-Selection List Box to a repeating node either in the Main data source or a secondary data source, you’ll have to do a programmatic lookup to retrieve the Display Names.

You could use the following C# code to retrieve the display names of the items that were selected:

XPathNavigator root = MainDataSource.CreateNavigator();
XPathNodeIterator iter = root.Select("//my:myListBox",
  NamespaceManager);

System.Text.StringBuilder sb = new System.Text.StringBuilder();

while (iter.MoveNext())
{
  string value = iter.Current.Value;

  XPathNavigator secDS = DataSources["Fruits"].CreateNavigator();
  string displayName = secDS.SelectSingleNode(
    "/dfs:myFields/dfs:dataFields/d:Fruits[@ID = '"
    + value + "']/@Name", NamespaceManager).Value;

  sb.Append(displayName);
  sb.Append(";");
}

root.SelectSingleNode("//my:myTextBox",
 NamespaceManager).SetValue(sb.ToString());

where Fruits is a secondary data source, @ID the value of an item, and @Name the display name of an item.

Note: You cannot do a lookup for items if you’ve manually entered them into the list box.

More useful code you can find from http://www.bizsupportonline.net/blog/2009/03/programmatically-retrieve-selected-items-multiple-selection-list-box-infopath/

I always be asked to get a Numberic Id for new infopath form. Instead of writing code in Form, I created a Web Service which will go to the SharePoint List, grab the number and increment by 1, then update the list. By doing that, you can set rule in your Form Submit Button without code required.
Below is the code

[WebMethod]
        public int GetAndIncrementCount(string ListUrl, string CountName)
        {
            int count = 0;

            SPWeb oSPWeb = null;
            try
            {
                SPSecurity.RunWithElevatedPrivileges(delegate()
                {
                    using (SPSite oSPSite = new SPSite(ListUrl))
                    {
                        oSPWeb = oSPSite.OpenWeb(ListUrl.Substring(ListUrl.IndexOf("/") + 1));
                    }
                });
                SPList oSPList = oSPWeb.GetList(ListUrl);

                if (oSPList != null)
                {
                    oSPWeb.AllowUnsafeUpdates = true;
                    SPListItemCollection items = oSPList.Items;
                    if (items.Count > 0)
                    {
                        SPListItem item = items[0];
                        count = int.Parse(item[CountName].ToString()) + 1;
                        item[CountName] = count;
                        item.Update();
                    }
                }
            }
            catch (Exception exception)
            {
                Log.WriteLogEvent(string.Format("An Error Occured In GetAndIncrementCount Method | Exception Message:{0} StackTrace: {1}", exception.Message, exception.StackTrace));
            }
            finally
            {
                oSPWeb.Dispose();
            }
            return count;
        }

I was working on a way to close an InfoPath form using C# or VB.NET from the form code-behind after the user submits the form. The steps below help in running some custom code to save the data and then close the form.

Here are the steps to close the Form

* Set the Submit Action[Tools->Submit Options] within InfoPath for the form to “Perform custom action using Code” . Click the Edit Code option to submit the form using code
* Set the After Submit option under Advanced to “Close the Form”

Click the Edit Code button next to the perform custom action to submit the data. Editing the code adds an handler for the Form Submit event and a FormEvents_Submit method is created in the code behind. Once the code completes the submission, set the CancelableArgs.Cancel property of the SubmitEventArgs to false.

Here is a sample FormEvents Submit method

In the SubmitConnection method, I retrieve the required data connection and call the execute method for submission

Dim fileSubmit as FileSubmitConnection = CType(Me.Connections(”SharePoint Save”),Microsoft.Office.InfoPath.FileSubmitConnection)

fileSubmit.Execute()

In the above sample, the data connection that I want to use for Submit is named “SharePoint Save”. I typecast it as a FileSubmitConnection.

I found this solution on one of the forum posts

http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=2605818&SiteID=17

Issue

User get’s a listed below error when opening a new or existing InfoPath form because of an error in the form’s code

Error
“The required version of the Microsoft .NET Framework is not installed on your computer or the InfoPath Primary Interop Assembly (PIA) is not registered. Use Add or Remove Programs in Control Panel to make sure that the required version of the Microsoft .NET Framework is installed. Or install it using Windows Update and run the Setup program again to confirm that the corresponding version of the .NET Programmability Support is installed, or contact your system administrator.”
Cause\Resolution

As the error message suggests either the Microsoft .NET Framework or the InfoPath Primary Interop Assembly (PIA) is not installed. If Microsoft .NET framework is not present on the system then this will need to be installed. If the user does not have the .NET Framework already installed (when installing Office 2007) then PIAs will not be installed. Additionally, the option to install the PIAs doesn’t show up in the Custom setup for Office. If the user does a complete install of Office 2007, then PIAs will get installed into the GAC automatically. It’s strongly recommended that a complete install of Office 2007 be performed.

If .Net Framework is installed after installing Office 2007, then PIA does not get installed automatically and this can to be installed by following the steps listed below

1. Go to ‘Add or Remove Programs…’
2. Select the Microsoft Office 2007 installation and click on change.
3. Select “Add or remove features” and click Continue.
4. Expand the “Microsoft Office InfoPath” and “.NET Programmability Support” node. Add the ‘.NET programmability support’. To add, right click on the node and select “Run from my computer”. Press continue and follow the on screen instructions to complete the setup.

When loading an InfoPath form from an intranet/Sharepoint etc, the form template is downloaded and cached locally. Sometimes after the template has been upgraded on the server the local client doesn’t always get the upgraded form template but uses the local cached copy instead.

You can rectify this by clearing the local cache from Start -> Run;

Infopath /cache clearall
or VB Script

Dim wshShell
Dim sRun, sParam, sMsg

sParam = “C:\Program Files\Microsoft Office\OFFICE11\INFOPATH.EXE /cache clearall”
Set wshShell = Createobject(“WScript.Shell”)

wshShell.Exec sParam

Set wshShell = Nothing

Verify that the cache has been cleared by navigating to the following folder for XP+

C:\Documents and Settings\[User]\Local Settings\Application Data\Microsoft\InfoPath\FormCache2

Or for Vista;

C:\Users\[User]\AppData\Local\Microsoft\InfoPath\FormCache2

Underneath the FormCache2 folder there will be a folder with a GUID name, check inside here, it should be empty, apart from a file called _SCVer.dat.

I have an interesting problem when trying to update an InfoPath Form Data. If I use XMLDOCUMNET and node to update the xml file. I got problem to load the data. So I have to use

StreamReader sr = new StreamReader(filename);
String file = sr.ReadToEnd();
sr.Close();
file = file.Replace(oldtext, newtext);
StreamWriter sw = new StreamWriter(filename);
sw.Write(file);
// Close StreamWriter
sw.Close();