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Implement MS Word functionality in C#

I am trying to reference MS Word in my C# program using the Microsoft.Office.Interop namespace. In Word, we can insert images in a document and use the "Remove Background" feature to specify a portion of the image and remove other parts. Is there a way to do this with the Interop namespace?? Or am I supposed to resort to manual methods??

Word allows to record a VBA macro in the background. Thus, you will find what properties and methods can be used to get the job done. Note, the Word object model is common for all kind of applications. See Create or run a macro for more information.
Open XML SDK is another alternative way to get the job done. But in some cases it is easier to automate Word than deal with XML. See Welcome to the Open XML SDK 2.5 for Office for more information.
Where do you run the code? A web server? ASP.NET?

editable word document attachment

Its a general scenario when we provide an option of attaching a file (MS .doc) to end user. This file is stored in DB as binary. When user try to access this attachment next time, we allow them to download it. Now, here I want to give a feature to user where he should be able to open this doc file on click, edit it and save it without downloading.

.doc is a binary format and not easy to work with - a library such as Aspose, as mentioned by Christian, is definitely the way to go.
However, if .DOCX is acceptable (and that's Office 2007 and higher), then you can achieve what you want in three steps:
Convert .docx to HTML
Convert Word to HTML then render HTML on webpage
Display the HTML using any rich text control of your choice
What is the best rich textarea editor for jQuery?
Finally, convert HTML back to .docx:
Convert Html to Docx in c#

You would have to "reinvent" Microsoft Office Online (look into your skydrive account). I am unsure if there are any "out of the box" libraries for that, but you could build a simple editing app by leveraging Aspose word (or some other library). But that would be far from simple.
Link to aspose: http://www.aspose.com/.net/word-component.aspx

Word will only open files that are locally stored. What you are looking for is something similar to editing items that SharePoint provides using the WebDAV interface.
You may be able to use this approach to support your requirement. You should be cautious about the security aspects of the solution unless you have fully authenticated access to the shared folder on the server.

I am not sure if a standalone MS Word Document editor exists. However, this can be done with using a combination of rich text formatting / converting tool (for example, the DevExpress ASPxHtmlEditor + Document Server):
Load binary data from a DB;
Import loaded data (MS Word content) as HTML content into the ASPxHtmlEditor;
Edit imported data via the WYSIWYG ASPxHtmlEditor;
Convert the edited HTML back to MS Word content;
Save the converted / edited MS Word content back to the DB.
I believe, it is possible to do something like this if you have such products (free or commercial analogs) in your project.

How to create *.docx files from a template in C#

I have a working ASP.NET MVC web application to manage projects and customers. Now I want to generate a word file for some customers. In this file should be displayed some data about the customer. Every generated file should have the same data and the same design. So I want to craete a new Word Template with the fields and want to fill the placeholders programmatically.
My problem is that I couldn't find a clear way to do that. Does anybody know a good learning resources?

Try this page:
Building Office Open XML Files
Open XML files (docx) are ZIP packages containing XML files. In your case, I would create a copy of your original template, then use the System.IO.Packaging API to open the file and modify it. By opening, the correct XML file and replacing certain placeholders in XML, you should be able to achieve the result you want.

While trying to do the same I have found some libraries to create/edit DOC or DOCX in .Net
GemBox.Document
TemplateEngine.Docx
DocXTemplateEngine
Templater - Nuget
Spire.Doc - Nuget

for the new document generation from a template file (.dot) should be very easy, I think it's a parameter you specify in the word application file open or so, when you pass the path of the .dot file, telling word to create a new .doc based on that file and not edit the actual template document.
for form fields and bookmarks filling, lots of examples online on how to do it from C#, see here:
MS Word Office Automation - Filling Text Form Fields And Check Box Form Fields And Mail Merge

What's the best way to generate a file and print it in .net?

I am working in a desktop project in C# with .net. This project has a function that generates some information and i would like to print this generated info as a document (may be .doc, .pdf, etc). Summarizing, i need:
Get the data generated by a function;
Generate a document containing these information structured with title, texts and tables (things that every document have);
Print it;
I thought generating an .html file (because it's simple to generate this kind of file), but i couldn't find a way to print it directly from my program.
Which extension of file would you recommend to insert this kind of information and print it directly from my program??
Thanks in advance.

Here's an easy way that uses a RichTextBox
http://www.codeproject.com/KB/printing/simpleprintingcs.aspx
It's not trivial to print a PDF, HTML, or a doc unless you are going to use external programs or third-party libraries. ImageMagick/GhostScript could help you print PDF.
Disclaimer: I work at Atalasoft -- If you are willing to use commercial software, my company makes PDF rendering components for .NET. There are companies that do the same for HTML.

Directly? Open printer port...
Or you can do it with framework classes:
How to: Print with a WebBrowser Control
http://msdn.microsoft.com/en-us/library/b0wes9a3.aspx

Using C# to create a Word document from an existing template

I have several Word templates and I wish to use these to dynamically create Word documents in my app. I wish to avoid using automation at all costs as this is no good. I know that I can use both HTML and XML to create word documents but I just don't know where to start with regards to using a template that may well have images in the footer or the header of a document.

I use the OpenXML SDK with Word 2007. After you get the hang of it, it's not so bad. I have several template docx files that I scan through to search and replace for placeholder strings with what I want, and then can stitch together multiple templates into one document if I want to. It's nice because I can start with docx files as the template and modify them while the whole time staying within the realm of the docx format. If an image is in the docx when you start modifying it, it'll be there after you re-save it after modification (provided you didn't programmatically remove it of course).
If you have more details with what you'll be doing, let us know.

You could use DocX. It's free, very easy to use, with nice tutorials and is feature reach. It works with only DOCX documents thou. Also development is currently on hold until the author will finish his semester. Here's detailed blog about it.
It has good example of using template in his Invoice Example.

MigraDoc http://www.pdfsharp.net/MigraDocOverview.ashx is a free utility for exporting PDF/Word/HTML files. I've not worked with it using templates as yet however, you could use the DDL files to persists a layout for your files to be re-used.