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Accessing OneDrive from Desktop App

I'm trying to get to grips with OneDrive, using this tutorial:
https://msdn.microsoft.com/en-us/library/hh826529.aspx
When I run in code, it gets as far as the makeAccessTokenRequest function, sending the following requestURL:
"https: //login.live.com/oauth20_token.srf?client_id=[myclientID] &client_secret=[myclientsecret]&redirect_uri=https:// login.live.com/oauth20_desktop.srf&grant_type=authorization_code&code=[authcode]"
(please ignore the spaces after "https:", I had to add them here to allow the question)
[myclientid], [myclientsecret], and [authcode] all appear to be populated correctly. It seems to get a response, as it runs the function "accessToken_DownloadStringCompleted", but throws a "TargetInvocationException" error, The inner message of the error is ""The remote server returned an error: (400) Bad Request.".
Could anyone throw any light on this? I'm completely new to this, so apologies if my question makes no sense, or is irritatingly vague..

Requests to the oauth20_token.srf end point need to be a POST with the parameters in the body of the post, instead of the query string. Since you didn't mention what code you're using to build the HTTP request it's hard to provide an example, but take a look at RedeemAuthorizationCodeAsync in my sample OAuth 2 project for an idea.
The outgoing request should look like this:
POST https://login.live.com/oauth20_token.srf
Content-Type: application/x-www-form-urlencoded
client_id={client_id}&redirect_uri={redirect_uri}&client_secret={client_secret}&code={code}&grant_type=authorization_code
You may also find this tutorial easier to follow than the one you linked with: https://dev.onedrive.com/auth/msa_oauth.htm.
If you are doing something with OneDrive (you tagged the post OneDrive) then you may want to consider using the OneDrive SDK instead. It includes authentication for several types of .NET projects so you don't need to figure out how to do auth yourself.

Pass data to a URL on a different web server without the QueryString

I've got an .ashx handler which, upon finishing processing will redirect to a success or error page, based on how the processing went. The handler is in my site, but the success or error pages might not be (this is something the user can configure).
Is there any way that I can pass the error details to the error page without putting it in the query string?
I've tried:
Adding a custom header that contains the error details, but since I'm using a Response.Redirect, the headers get cleared
Using Server.Transfer, instead of Response.Redirect, but this will not work for URLs not in my site
I know that I can pass data in the query string, but in some cases the data I need to pass might be too long for the query string. Do I have any other options?

Essentially, no. The only way to pass additional data in a GET request (i.e. a redirect) is to pass it in the query string.
The important thing to realise is that this is not a limitation of WebForms, this is just how HTTP works. If you're redirecting to another page that's outside of your site (and thus don't have the option of cookies/session data), you're going to have to send information directly in the request and that means using a query string.
Things like Server.Transfer and Response.Redirect are just abstractions over a simple HTTP request; no framework feature can defy how HTTP actually works.
You do, of course, have all kinds of options as to what you pass in the query string, but you're going to have to pass something. If you really want to shorten the URL, maybe you can pass an error code and expose an API that will let the receiving page fetch further information:
Store transaction information (or detailed error messages) in a database with an ID.
Pass the ID in the query string.
Expose a web method or similar API to allow the receiving page to request additional information.
There are plenty of hacky ways you could create the illusion of passing data in a redirect outside of a form post (such as returning a page containing a form and Javascript to immediately do a cross-domain form post) but the query string is the proper way of passing data in a GET request, so why try to hack around it?

If you must perform a redirect, you will need to pass some kind of information in the Query String, because that's how browser redirects work. You can be creative about how you pass it, though.
You could pass an error code, and have the consuming system know what various error codes mean.
You could pass a token, and have the consuming system know how to ask your system about the error information for the given token behind-the-scenes.
Also, if you have any flexibility around whether it's actually performing a redirect, you could use an AJAX request in the first place, and send back some kind of JSON object that the browser's javascript could interpret and send via a POST parameter or something like that.

A redirect is executed by most browsers as a GET, which means you'd have to put the data in the query string.
One trick (posted in two other answers) to do a "redirect" as a POST is to turn the response into a form that POSTs itself to the target site:
Response.Clear();
StringBuilder sb = new StringBuilder();
sb.Append("<html>");
sb.AppendFormat(#"<body onload='document.forms[""form""].submit()'>");
sb.AppendFormat("<form name='form' action='{0}' method='post'>",postbackUrl);
<!-- POST values go here -->
sb.AppendFormat("<input type='hidden' name='id' value='{0}'>", id);
sb.Append("</form>");
sb.Append("</body>");
sb.Append("</html>");
Response.Write(sb.ToString());
Response.End();
But I would read the comments on both to understand the limitations.

Basically there are two usual HTTP ways to send some data - GET and POST.
When you redirect to another URL with additional parameters, you make the client browser to send the GET request to the target server. Technically, your server responds to the browser with specific HTTP error code 307 + the URL to go (including the GET parameters).
Alternatively, you may want/need to make a POST request to the target URL. In that case you should respond with a simple HTML form, which consists of several hidden fields pre-filled with certain values. The form's action should point the target URL, method should be "POST", and of course your HTML should include javascript, which automatically submits the form once the document is loaded. This way the client browser would send the POST request instead of the GET one.

How to use Yahoo REST Api in C#

I'm just trying to make an yahoo boot that send to registered user of my application an instant message. I've spent some hours searching the web on how to do it but yahoo developer documentation sucks.First of all I don't know what servers I should use for authorization, log in, and messaging. I have a consumer key and I've tried to follow this steps but nothing works.
Any advice/suggestion is welcome.

The documentation looks to be very good, I think the issue here is that your knowledge of how REST API's work in general is a bit lacking.
Let's talk about diagram #2: Get a request token using: get_request_token.
get_request_token is part of an HTTP endpoint, and in their diagram they want you to pass in a handful of parameters to validate your request.
oauth_consumer_key
oauth_nonce
oauth_signature_method
etc
(If you need more clarification of any step you can find it in the tree view on the left hand side of the page)
The request URL:
https://api.login.yahoo.com/oauth/v2/get_request_token.
Now at this point you can either use the HTTP GET or POST verb. If you decide to use GET you will need to include those above parameters as a query string.
?oath_consumer_key=myConsumerKey&oauth_nonce=oathNonce etc
I will leave it to you to write the associated C# code. You'll want to start off with the HttpWebRequest.Create() method

PHP-C# two way communication

I am creating a PHP website connected to a MySQL database. Next I will need to write a C# desktop app that will use the same DB. Unfortunately I cannot connect to the DB directly from a remote location and my hosting company won't allow SSH neither.
So what options do I have? If the hosting company supported .NET, it wouldn't be a problem, but I'm not that experienced with PHP. Will I have to write a PHP service (SOAP?) and then consume it in my desktop app? Also, how do I communicate with server from the desktop app?
Any help appreciated!

Depending on security requirement, could you write a generic SQL executing page in PHP, that took the SQL as a String parameter, and returned the results as an array of Strings (Might need some meta data too or something)?
Other than that the only thing I can think of is a web service of some kind.
Also SOAP can work both ways, you can read and write from the C# app, no need to write a WebService on both ends, unless you need to notify your c# app about something from the server (In which case you could always try frequent polling from the c# app)

Best option would be creating a set of RESTful services in your PHP site.
One of most important things to take in account is REST is more configuration by convention, and there's no need of things like SOAP which may be an absolute overkill for your solution.
You just send JSON from PHP and .NET Windows application will parse it as a CLR object.
A sample scenario would be:
Service operation: http://yourdomainhere.com/API/Message/34894
** This returns something like { "text": "hello world" }
.NET client receives this JSON and using a JSON parser like Newton JSON parser, you'd be doing this:
MessageDto dto = JsonConvert.DeserializeObject([JSON received from the service call]);
MessageBox.Show(dto.Text); // This will show "hello world"
It's just a very simple example, but it'd give you an idea of what's next.
You can query your REST API using WebRequest/WebResponse .NET BCL classes.
PHP only needs to send a web response including your JSON in the output stream, that's all. No SOAP, no XML, no complication. Keep it simple.

I think the following link will be of helpful to you!
Developing SOAP Web Services with PHP/C#

What you can do is providing some PHP-wrappers which you can access from your C# code. As an example you can use this discussion, regarding C# / PHP communication.
Basically you can send a HTTP request to PHP and retrieve it's return value with C#. PHP would then perform the DB requests. If you're using AJAX on the Website it should be easy using the same communication interfaces.

this is the first paragraph of Matt Fellows answer.
But in what form do you send the data back to the application in?
Maybe JSON?
PHP webpage
<?php
$host = "host.host.com";
$user = "XXXXX";
$password = "XXXX";//plaintext :)
$connection = mysql_connect($host, $user, $password);
$database = "XXXXX";
$syntax = $_GET['syntax']; //www.example.com/help.php?syntax=DROP%20TABLE%20XXX
$result = mysql_query($syntax);
//somehow output the $result in C# readable form
?>

Getting a raw socket from an IIS request

I'm trying to get the raw data sent to IIS using a HttpHandler. However, because the request is an "GET"-request without the "Content-Length" header set it reports that there is no data to read (TotalBytes), and the inputstream is empty. Is there any way I can plug into the IIS-pipeline (maybe even before the request is parsed) and just kind of take control over the request and read it's raw data? I don't care if I need to parse headers and stuff like that myself, I just want to get my hands on the actual request and tell IIS to ignore this one. Is that at all possible? Cause right now it looks like I need to do the alternative, which is developing a custom standalone server, and I really don't want to do that.

Most web servers will ignore (and rarely give you access to) the body of a GET request, because the HTTP semantics imply that it is to be ignored anyway. You should consider another method (for example POST or PUT).
See this question and the link in this answer:
HTTP GET with request body