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Why is postman sending form data in an HTTP GET?

I received a Postman json collection from an API vendor that works perfectly, but has something mystifying to me: The request is in a GET format, yet there is an x-www-form-urlencoded body.
URL: https://login.microsoftonline.com/d1e<secret>9563/oauth2/token
And when I look at the postman-generated c# code, the mystery continues:
var client = new RestClient("https://login.microsoftonline.com/d1e...d3/oauth2/token");
client.Timeout = -1;
var request = new RestRequest(Method.GET);
request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
request.AddParameter("grant_type", "client_credentials");
request.AddParameter("client_id", "c06bb...79");
request.AddParameter("client_secret", "7~u...D");
request.AddParameter("resource", "https://vault.azure.net");
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);
Note the AddParameter constructions for a GET call. To me, this must be a slight-of-hand for merely adding those values to the querystring. But when I look at the postman console I see:
In the postman console I would have expected to see those params appended to the url as a querystring, and then everything would have made sense. But you can see that it's a bonafide Request Body.
When I make GET calls in my c# code I like to use the simple yet solid WebClient object to call the DownloadString() method. But this method is only for GETs and there's no way to send a form-post style body, understandably.
Is postman truly sending a GET with all those values being appended to the url as a querystring? And should I do the same in my DownloadString() call? Or is there something else going on here? Should I instead, in my c#, be calling the UploadString() method and sending a form post BODY as a GET??

Http protocol supports adding a body to a request, but the WebClient class you use doesn't. Presumably because it isn't considered the norm.
I'm sure there's good reasons for Microsoft using it in the OAuth flow though. Those guys normally do things right!
HTTP GET with request body

API is just an abstraction , you can send what ever you want to the API . It depends on the implementation , how the server handles these data.
Some services considers only what it requires and ignores other information
some services considers the entire requests and validates that it has only the allowed data. what should be allowed depends on the service
Postman is just a client that sends data to server , its upto you to decide what all information it should send . If you dont need any body then keep it as none. if you need some thing then add it.

HTTPResponse not arriving at client

I have a http handler which is registered and working fine. Now i want to process a request, and send a custom html response which is then shown on the client.
So my function is written as follows:
public void ProcessRequest(HttpContext _context)
{
HttpResponse response = _context.Response;
response.Clear();
var requestedUrl = _context.Request.Url;
PhantomModuleController pmc = new PhantomModuleController();
response.BufferOutput = true;
var snapshot = pmc.DoThings(requestedUrl); //this returns a string
response.Write(snapshot); //i put it in the response
response.ContentType = "text/html";
response.End(); //it should send it to the client now
}
But according to my fiddler, the response never arrives on the client. In fact, the httpresponse is never even sent.
Did i forget somethiing

Because the request is not showing up in Fiddler as being sent back to the client (nor an error back to the client), the routing engine may be getting in the way of the request. The scenerio is described by phil hack.
However, there are other cases where you might have requests for files
that don’t exist on disk. For example, if you register an HTTP Handler
directly to a type that implements IHttpHandler. Not to mention
requests for favicon.ico that the browser makes automatically. ASP.NET
Routing attempts to route these requests to a controller. One solution
to this is to add an appropriate ignore route to indicate that routing
should ignore these requests. Unfortunately, we can’t do something
like this: {*path}.aspx/{*pathinfo}
You need to setup the route engine to ignore the route that has the file extension. E.G.
routes.IgnoreRoute("{*allaspx}", new {allaspx=#".*\.aspx(/.*)?"});
routes.IgnoreRoute("{*favicon}", new {favicon=#"(.*/)?favicon.ico(/.*)?"});

Why does my API return {}

I've created a .NET API, but when I try to use it I'm getting strange results.
If I go to the URL of an API call in Chrome I get the result I would expect, in XML format, but if I try it in IE it downloads a file and when I open it it just says {} or sometimes [{},{},{},{}] if I try a call that returns an array.
I've also tried using a webclient.
WebClient web = new WebClient();
var data = web.DownloadString("http://myAPI.example.com/api/MyAPI/APIMethod?parameter1=hiImAParameter");
This also returns empty {}.
I've tried searching online, but I don't see any mentions of this problem anywhere. I'm sure I must be missing something somewhere, but I've tried looking over how I set up my API, and it all looks fine to me.
Edit:
Here's the response I get in Chrome.
<ArrayOfRoute xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/TimePointModel">
<Route>
<RouteId>11235</RouteId>
<RouteName>Fibonacci</RouteName>
<Status i:nil="true"/>
<Width>0</Width>
</Route>
</ArrayOfRoute>
It returns XML on Chrome because of Chrome's accept headers. It's supposed to return JSON on IE, but for some reason the JSON is empty.
This is in my api controller:
[AcceptVerbs("GET")]
public IEnumerable<Route> APIMethod(double parameter)
{
return new Manager(parameter).GetRoutes();
}
This is in my Global.asax.cs:
void Application_Start(object sender, EventArgs e)
{
RouteTable.Routes.MapHttpRoute(
name: "APIMethod",
routeTemplate: "api/{controller}/{action}/{id}",
defaults: new { action = "APIMethod", id = System.Web.Http.RouteParameter.Optional }
);
}
Edit:
This works great when I do an API call which doesn't require parameters.
WebClient web = new WebClient();
var data = web.DownloadString("http://myAPI.example.com/api/MyAPI/SimpleAPIMethod");
I've been doing research, and I tried adding parameters like this:
NameValueCollection myQueryStringCollection = new NameValueCollection();
string myParameter = "hiImAParameter";
myQueryStringCollection.Add("parameter1", myParameter);
web.QueryString = myQueryStringCollection;
var data = web.DownloadString("http://myAPI.example.com/api/MyAPI/APIMethod");
I've noticed that the number of empty {} in my array matches the number of items in the array if I put the full url with the querystring into chrome. It just empties them out for my webclient.
I also tried adding
web.Headers[HttpRequestHeader.ContentType] = "application/json";
before making the call, but there's still no change in the result.
And I tried to follow that tutorial, but it's written for a console application and they're using HttpClient. I can't do that because I can't do Asynchronous calls. This is to be used by a website. That's why I'm using WebClient. I also tried using StreamReader with HttpWebRequest and HttpWebResponse, but it had the same problem as I've been encountered with WebClient.

Without more information, it's a bit hard to diagnose your issue. However, I would say that it is likely your web API is interpreting the expected response type and providing an empty result as it does not support responses of that type, such as happens with ASP .NET Web API websites.
In that sense, the DownloadString is indicating it is expecting a text/html response. You should probably download the Microsoft ASP .NET Web API Client Libraries with NuGet. This library will give you HttpClient which has support for making the queries you want to make with responses such as application/json and application/xml.
You can view a tutorial on how to do the calls right here.
If you want it to work from your web browser, you need to ensure the Accept header field is correct, as you mentioned. Ensure it is being communicated with IE by using something like Fiddler.

I figured out what the problem was. I needed to add [DataMember] attributes to the attributes of the items in the list. I didn't realize it was left out of the return type of that call.

Getting the HTTP Referrer in ASP.NET

I'm looking for a quick, easy and reliable way of getting the browser's HTTP Referrer in ASP.Net (C#). I know the HTTP Referrer itself is unreliable, but I do want a reliable way of getting the referrer if it is present.

You could use the UrlReferrer property of the current request:
Request.UrlReferrer
This will read the Referer HTTP header from the request which may or may not be supplied by the client (user agent).

Request.Headers["Referer"]
Explanation
The Request.UrlReferrer property will throw a System.UriFormatException if the referer HTTP header is malformed (which can happen since it is not usually under your control).
Therefore, the Request.UrlReferrer property is not 100% reliable - it may contain data that cannot be parsed into a Uri class. To ensure the value is always readable, use Request.Headers["Referer"] instead.
As for using Request.ServerVariables as others here have suggested, per MSDN:
Request.ServerVariables Collection
The ServerVariables collection retrieves the values of predetermined environment variables and request header information.
Request.Headers Property
Gets a collection of HTTP headers.
Request.Headers is a better choice than Request.ServerVariables, since Request.ServerVariables contains all of the environment variables as well as the headers, where Request.Headers is a much shorter list that only contains the headers.
So the most reliable solution is to use the Request.Headers collection to read the value directly. Do heed Microsoft's warnings about HTML encoding the value if you are going to display it on a form, though.

Use the Request.UrlReferrer property.
Underneath the scenes it is just checking the ServerVariables("HTTP_REFERER") property.

Like this: HttpRequest.UrlReferrer Property
Uri myReferrer = Request.UrlReferrer;
string actual = myReferrer.ToString();

I'm using .Net Core 2 mvc,
this one work for me ( to get the previews page) :
HttpContext.Request.Headers["Referer"];

Since Google takes you to this post when searching for C# Web API Referrer here's the deal: Web API uses a different type of Request from normal MVC Request called HttpRequestMessage which does not include UrlReferrer. Since a normal Web API request does not include this information, if you really need it, you must have your clients go out of their way to include it. Although you could make this be part of your API Object, a better way is to use Headers.
First, you can extend HttpRequestMessage to provide a UrlReferrer() method:
public static string UrlReferrer(this HttpRequestMessage request)
{
return request.Headers.Referrer == null ? "unknown" : request.Headers.Referrer.AbsoluteUri;
}
Then your clients need to set the Referrer Header to their API Request:
// Microsoft.AspNet.WebApi.Client
client.DefaultRequestHeaders.Referrer = new Uri(url);
And now the Web API Request includes the referrer data which you can access like this from your Web API:
Request.UrlReferrer();

string referrer = HttpContext.Current.Request.UrlReferrer.ToString();

Sometime you must to give all the link like this
System.Web.HttpContext.Current.Request.UrlReferrer.ToString();
(in option when "Current" not founded)

Using .NET Core or .NET 5 I would recommend this:
httpContext.Request.Headers.TryGetValue("Referer", out var refererHeader)

Belonging to other reply, I have added condition clause for getting null.
string ComingUrl = "";
if (Request.UrlReferrer != null)
{
ComingUrl = System.Web.HttpContext.Current.Request.UrlReferrer.ToString();
}
else
{
ComingUrl = "Direct"; // Your code
}

How to capture HTML of redirect page before it redirects?

I am trying to read the HTML of a page that contains a non-delayed redirect. The following snippet (C#) will give me the destination/redirected page, not the initial one I need to see:
using System.Net;
using System.Text;
public class SomeClass {
public static void Main() {
byte[] data = new WebClient().DownloadData("http://SomeUrl.com");
System.Console.WriteLine(Encoding.ASCII.GetString(data));
}
}
Is there a way to get the HTML of a redirecting page? (I prefer .NET but a snippet in Java or Python would be fine too. Thx!)

Unless the redirect is done on the client side you can't. If the redirect is done server side, then no html is actually generated to the client, but the header is redirected at the new server.

It would take more work, but rather than using WebClient, use HttpWebRequest and set the AllowAutoRedirect property to False. A redirect will then throw an exception, but you can get any response text (and some pages do have response text along with the redirect) from the exception's response object. After you get the response from the exception, you can issue another HttpWebRequest for the redirect URL (specified in the Location response header).
You might be able to do something similar with WebRequest if you create a derived object, MyWebRequest, where you overload the GetWebRequest method and set the AllowAutoRedirect property. I don't know what kind of exception, if any, the DownloadData method will return if you do something like that.
As somebody said previously, this will only work for those pages that do client-side redirects (typically 301 or 302). If there is server-side redirection going on, you'd never know it.

Simplest answer would be to add the current page onto the QueryString component of the redirect when redirecting, for instance:
Response.Redirect(newPage + "?FromPage=" + Request.Url);
Then the new page could see where you cane from by simply looking at Request.QueryString("FromPage").

If you want to get the source of an html page you can use this tool:
http://www.selfseo.com/html_source_view.php