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How to call one service from another with parameter and get response

Currently, I have two services running at different endpoints. For example (this is not my real scenario):
StudentService
CheckHomeWorkService
CheckHomeWorkService can be used from many services (For example TeacherService, WorkerService). It has one controller with CheckHomeWork action. It has some parameters:
HomeWorkNumber (int)
ProvidedSolution (string).
It will return success or failed.
Now, In my StudentService, I have a controller with SubmitHomeWork Action. It needs to check homework using CHeckHomeWorkService and save results to a database. How can I implement this?
I was using Ocelot Api GateWay but it can 'redirect' to another service and I need to save the result of the response to the database

As suggested in the comments, you can use HttpClient to call the other APIs. However, if you want a more safe and performant solution, you can use also the HttpClientFactory.
see this why to use it
and see the official docu

After your comment you say that you have separate WEB API services
in this case you can do this service to retrieve the result :
public async Task<List<HomeWorkModel>> CheckHomeWorkService(int
homeWorkNumber, string providedSolution)
{
using (var httpClient = new HttpClient())
{
using (var response = await
httpClient.GetAsync(Constants.ApiBaseUrl + "/CheckHomeWork?n=" +
homeWorkNumber + "&sol=" + providedSolution))
{
if (response.StatusCode ==
System.Net.HttpStatusCode.OK)
{
string apiResponse = await
response.Content.ReadAsStringAsync();
return
JsonConvert.DeserializeObject<List<HomeWorkModel>>(apiResponse);
}
else
{
return null;
}
}
}
}
public class HomeworkModel
{
Public bool Result {get; set;}
}
whereas :
"Constants.ApiBaseUrl" is http address for base Url of another API
But in case of the same API you will use old solution :
you can pass "CHeckHomeWorkService" in your controller constructor ("Dependency Injection")
and call service methods as you like

Why is my header data missing from my Azure Function Http Trigger in .Net 5 when calling from HttpClient.GetAsync

I have a client using HttpClient.GetAsync to call into a Azure Function Http Trigger in .Net 5.
When I call the function using PostMan, I get my custom header data.
However, when I try to access my response object (HttpResponseMessage) that is returned from HttpClient.GetAsync, my header data empty.
I have my Content data and my Status Code. But my custom header data are missing.
Any insight would be appreciated since I have looking at this for hours.
Thanks for you help.
Edit: Here is the code where I am making the http call:
public async Task<HttpResponseMessage> GetQuotesAsync(int? pageNo, int? pageSize, string searchText)
{
var requestUri = $"{RequestUri.Quotes}?pageNo={pageNo}&pageSize={pageSize}&searchText={searchText}";
return await _httpClient.GetAsync(requestUri);
}
Edit 8/8/2021: See my comment below. The issue has something to do with using Blazor Wasm Client.

For anyone having problems after following the tips on this page, go back and check the configuration in the host.json file. you need the Access-Control-Expose-Headers set to * or they won't be send even if you add them. Note: I added the "extensions" node below and removed my logging settings for clarity.
host.json (sample file):
{
"version": "2.0",
"extensions": {
"http": {
"customHeaders": {
"Access-Control-Expose-Headers": "*"
}
}
}
}

This is because HttpResponseMessage's Headers property data type is HttpResponseHeaders but HttpResponseData's Headers property data type is HttpHeadersCollection. Since, they are different, HttpResponseHeaders could not bind to HttpHeadersCollection while calling HttpClient.GetAsync(as it returns HttpResponseMessage).
I could not find a way to read HttpHeadersCollection through HttpClient.

As long as your Azure function code is emitting the header value, you should be able to read that in your client code from the Headers collection of HttpResponseMessage. Nothing in your azure function (which is your remote endpoint you are calling) makes it any different. Remember, your client code has no idea how your remote endpoint is implemented. Today it is azure functions, tomorrow it may be a full blown aspnet core web api or a REST endpoint written in Node.js. Your client code does not care. All it cares is whether the Http response it received has your expected header.
Asumming you have an azure function like this where you are adding a header called total-count to the response.
[Function("quotes")]
public static async Task<HttpResponseData> RunAsync(
[HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequestData req,
FunctionContext executionContext)
{
var logger = executionContext.GetLogger("Quotes");
logger.LogInformation("C# HTTP trigger function processed a request for Quotes.");
var quotes = new List<Quote>
{
new Quote { Text = "Hello", ViewCount = 100},
new Quote { Text = "Azure Functions", ViewCount = 200}
};
var response = req.CreateResponse(HttpStatusCode.OK);
response.Headers.Add("total-count", quotes.Count.ToString());
await response.WriteAsJsonAsync(quotes);
return response;
}
Your existing client code should work as long as you read the Headers property.
public static async Task<HttpResponseMessage> GetQuotesAsync()
{
var requestUri = "https://shkr-playground.azurewebsites.net/api/quotes";
return await _httpClient.GetAsync(requestUri);
}
Now your GetQuotesAsync method can be called somewhere else where you will use the return value of it (HttpResponseMessage instance) and read the headers. In the below example, I am reading that value and adding to a string variable. HttpResponseMessage implements IDisposable. So I am using a using construct to implicitly call the Dispose method.
var msg = "Total count from response headers:";
using (var httpResponseMsg = await GetQuotesAsync())
{
if (httpResponseMsg.Headers.TryGetValues("total-count", out var values))
{
msg += values.FirstOrDefault();
}
}
// TODO: use "msg" variable as needed.
The types which Azure function uses for dealing with response headers is more of an implementation concern of azure functions. It has no impact on your client code where you are using HttpClient and HttpResponseMessage. Your client code is simply dealing with standard http call response (response headers and body)

The issue is not with Blazor WASM, rather if that header has been exposed on your API Side. In your azure function, add the following -
Note: Postman will still show the headers even if you don't expose the headers like below. That's because, Postman doesn't care about CORS headers. CORS is just a browser concept and not a strong security mechanism. It allows you to restrict which other web apps may use your backend resources and that's all.
First create a Startup File to inject the HttpContextAccessor
Package Required: Microsoft.Azure.Functions.Extensions
[assembly: FunctionsStartup(typeof(FuncAppName.Startup))]
namespace FuncAppName
{
public class Startup : FunctionsStartup
{
public override void Configure(IFunctionsHostBuilder builder)
{
builder.Services.AddScoped<HttpContextAccessor>();
}
}
}
Next, inject it into your main Function -
using Microsoft.AspNetCore.Http;
namespace FuncAppName
{
public class SomeFunction
{
private readonly HttpContext _httpContext;
public SomeFunction(HttpContextAccessor contextAccessor)
{
_httpContext = contextAccessor.HttpContext;
}
[FunctionName("SomeFunc")]
public override Task<IActionResult> Run([HttpTrigger(AuthorizationLevel.Anonymous, new[] { "post" }, Route = "run")] HttpRequest req)
{
var response = "Some Response"
_httpContext.Response.Headers.Add("my-custom-header", "some-custom-value");
_httpContext.Response.Headers.Add("my-other-header", "some-other-value");
_httpContext.Response.Headers.Add("Access-Control-Expose-Headers", "my-custom-header, my-other-header");
return new OkObjectResult(response)
}
If you want to allow all headers you can use wildcard (I think, not tested) -
_httpContext.Response.Headers.Add("Access-Control-Expose-Headers", "*");
You still need to add your web-app url to the azure platform CORS. You can add * wildcard, more info here - https://iotespresso.com/allowing-all-cross-origin-requests-azure-functions/
to enable CORS for Local Apps during development - https://stackoverflow.com/a/60109518/9276081
Now to access those headers in your Blazor WASM, as an e.g. you can -
protected override async Task OnInitializedAsync()
{
var content = JsonContent.Create(new { query = "" });
using (var client = new HttpClient())
{
var result = await client.PostAsync("https://func-app-name.azurewebsites.net/api/run", content);
var headers = result.Headers.ToList();
}
}

ASP.NET Web-api core: Handling client connections problems and find conflicts

Background
I have a web api server (asp.net core v2.1) that serve some basic operation, like managing entities on the server. This is the interface:
[HttpPost]
[Route("create")]
public async Task<ActionResult<NewEntityResponse>> Create(CreateEntityModel model)
{
// 1) Validate the request.
// 2) Create a new row on the database
// 3) Return the new entity in response.
}
The user running this REST method in this way:
POST https://example.com/create
Content-Type: application/json
{
"firstName": "Michael",
"lastName": "Jorden"
}
And getting response like this:
Status 200
{
"id": "123456" // The newly created entity id
}
The Problem
When sending thousands of requests like this, at some point it will fail because of network connections. When connection fails, it can leads us into two different situations:
The network call was ended on the way to the server - In this case, the server don't know about this request. Therefore, the entity wasn't created. The user just have to send the same message again.
The network call was sent from the server to back to the client but never rich the destination - In this case the request was fulfill completely, but the client don't aware for this. The expected solution is to send the same request again. In this case, it will create the same entity twice - and this is the problem.
The Requested Solution
I want to create an generic solution for web-api that "remmeber" which commands it already done. if he got same request twice, it's return HTTP status code Conflict.
Where I got so far
I thought to add the client an option to add a unique id to the request, in this way:
POST https://example.com/create?call-id=XXX
Add to my server a new filter that check if the key XXX is already fulfill. If yes, return Conflict. Otherwise - continue.
Add another server filter that checks the response of the method and marking it as "completed" for further checks.
The problem with this solution on concurrency calls. If my method takes 5 seconds to be returned and the client sent the same message again after 1 second - it will create two entities with same data.
The Questions:
Do you think that this is good approach to solve this problem?
Do you familiar with ready to use solutions that doing this?
How to solve my "concurrency" problem?
Any other tips will be great!
thanks.

Isnt the easiest solution to make the REST action idempotent?
I mean by that: the call should check if the resource already exists and either create a new resource if it doesnt or return the existing if it does?

OK, I just figure it up how to make it right. So, I implemented it by myself and share it with you.
In order to sync all requests between different servers, I used Redis as cache service. If you have only one server, you can use Dictionary<string, string> instead.
This filter do:
Before processing the request - add a new empty value key to Redis.
After the server processed the request - store the server response in Redis. This data will be used when the user will ask again for same request.
public class ConflictsFilter : ActionFilterAttribute
{
const string CONFLICT_KEY_NAME = "conflict-checker";
static readonly TimeSpan EXPIRE_AFTER = TimeSpan.FromMinutes(30);
private static bool ShouldCheck(ActionDescriptor actionDescription, IQueryCollection queries)
{
return queries.ContainsKey(CONFLICT_KEY_NAME);
}
private string BuildKey(string uid, string requestId)
{
return $"{uid}_{requestId}";
}
public override void OnActionExecuting(ActionExecutingContext context)
{
if (ShouldCheck(context.ActionDescriptor, context.HttpContext.Request.Query))
{
using (var client = RedisConnectionPool.ConnectionPool.GetClient())
{
string key = BuildKey(context.HttpContext.User.GetId(), context.HttpContext.Request.Query[CONFLICT_KEY_NAME]);
string existing = client.Get<string>(key);
if (existing != null)
{
var conflict = new ContentResult();
conflict.Content = existing;
conflict.ContentType = "application/json";
conflict.StatusCode = 409;
context.Result = conflict;
return;
}
else
{
client.Set(key, string.Empty, EXPIRE_AFTER);
}
}
}
base.OnActionExecuting(context);
}
public override void OnResultExecuted(ResultExecutedContext context)
{
base.OnResultExecuted(context);
if (ShouldCheck(context.ActionDescriptor, context.HttpContext.Request.Query) && context.HttpContext.Response.StatusCode == 200)
{
string key = BuildKey(context.HttpContext.User.GetId(), context.HttpContext.Request.Query[CONFLICT_KEY_NAME]);
using (var client = RedisConnectionPool.ConnectionPool.GetClient())
{
var responseBody = string.Empty;
if (context.Result is ObjectResult)
{
ObjectResult result = context.Result as ObjectResult;
responseBody = JsonConvert.SerializeObject(result.Value);
}
if (responseBody != string.Empty)
client.Set(key, responseBody, EXPIRE_AFTER);
}
}
}
}
The code is executed only if the query ?conflict-checker=XXX is exists.
This code is provide you under MIT license.
Enjoy the ride :)

Returning an HttpResponseMessage contents from a pass through API in ASP.NET Core [duplicate]

I'm developing an ASP.Net Core web application where I need to create a kind of "authentication proxy" to another (external) web service.
What I mean by authentication proxy is that I will receive requests through a specific path of my web app and will have to check the headers of those requests for an authentication token that I'll have issued earlier, and then redirect all the requests with the same request string / content to an external web API which my app will authenticate with through HTTP Basic auth.
Here's the whole process in pseudo-code
Client requests a token by making a POST to a unique URL that I sent him earlier
My app sends him a unique token in response to this POST
Client makes a GET request to a specific URL of my app, say /extapi and adds the auth-token in the HTTP header
My app gets the request, checks that the auth-token is present and valid
My app does the same request to the external web API and authenticates the request using BASIC authentication
My app receives the result from the request and sends it back to the client
Here's what I have for now. It seems to be working fine, but I'm wondering if it's really the way this should be done or if there isn't a more elegant or better solution to this? Could that solution create issues in the long run for scaling the application?
[HttpGet]
public async Task GetStatement()
{
//TODO check for token presence and reject if issue
var queryString = Request.QueryString;
var response = await _httpClient.GetAsync(queryString.Value);
var content = await response.Content.ReadAsStringAsync();
Response.StatusCode = (int)response.StatusCode;
Response.ContentType = response.Content.Headers.ContentType.ToString();
Response.ContentLength = response.Content.Headers.ContentLength;
await Response.WriteAsync(content);
}
[HttpPost]
public async Task PostStatement()
{
using (var streamContent = new StreamContent(Request.Body))
{
//TODO check for token presence and reject if issue
var response = await _httpClient.PostAsync(string.Empty, streamContent);
var content = await response.Content.ReadAsStringAsync();
Response.StatusCode = (int)response.StatusCode;
Response.ContentType = response.Content.Headers.ContentType?.ToString();
Response.ContentLength = response.Content.Headers.ContentLength;
await Response.WriteAsync(content);
}
}
_httpClient being a HttpClient class instantiated somewhere else and being a singleton and with a BaseAddressof http://someexternalapp.com/api/
Also, is there a simpler approach for the token creation / token check than doing it manually?

If anyone is interested, I took the Microsoft.AspNetCore.Proxy code and made it a little better with middleware.
Check it out here: https://github.com/twitchax/AspNetCore.Proxy. NuGet here: https://www.nuget.org/packages/AspNetCore.Proxy/. Microsoft archived the other one mentioned in this post, and I plan on responding to any issues on this project.
Basically, it makes reverse proxying another web server a lot easier by allowing you to use attributes on methods that take a route with args and compute the proxied address.
[ProxyRoute("api/searchgoogle/{query}")]
public static Task<string> SearchGoogleProxy(string query)
{
// Get the proxied address.
return Task.FromResult($"https://www.google.com/search?q={query}");
}

I ended up implementing a proxy middleware inspired by a project in Asp.Net's GitHub.
It basically implements a middleware that reads the request received, creates a copy from it and sends it back to a configured service, reads the response from the service and sends it back to the caller.

This post talks about writing a simple HTTP proxy logic in C# or ASP.NET Core. And allowing your project to proxy the request to any other URL. It is not about deploying a proxy server for your ASP.NET Core project.
Add the following code anywhere of your project.
public static HttpRequestMessage CreateProxyHttpRequest(this HttpContext context, Uri uri)
{
var request = context.Request;
var requestMessage = new HttpRequestMessage();
var requestMethod = request.Method;
if (!HttpMethods.IsGet(requestMethod) &&
!HttpMethods.IsHead(requestMethod) &&
!HttpMethods.IsDelete(requestMethod) &&
!HttpMethods.IsTrace(requestMethod))
{
var streamContent = new StreamContent(request.Body);
requestMessage.Content = streamContent;
}
// Copy the request headers
foreach (var header in request.Headers)
{
if (!requestMessage.Headers.TryAddWithoutValidation(header.Key, header.Value.ToArray()) && requestMessage.Content != null)
{
requestMessage.Content?.Headers.TryAddWithoutValidation(header.Key, header.Value.ToArray());
}
}
requestMessage.Headers.Host = uri.Authority;
requestMessage.RequestUri = uri;
requestMessage.Method = new HttpMethod(request.Method);
return requestMessage;
}
This method covert user sends HttpContext.Request to a reusable HttpRequestMessage. So you can send this message to the target server.
After your target server response, you need to copy the responded HttpResponseMessage to the HttpContext.Response so the user's browser just gets it.
public static async Task CopyProxyHttpResponse(this HttpContext context, HttpResponseMessage responseMessage)
{
if (responseMessage == null)
{
throw new ArgumentNullException(nameof(responseMessage));
}
var response = context.Response;
response.StatusCode = (int)responseMessage.StatusCode;
foreach (var header in responseMessage.Headers)
{
response.Headers[header.Key] = header.Value.ToArray();
}
foreach (var header in responseMessage.Content.Headers)
{
response.Headers[header.Key] = header.Value.ToArray();
}
// SendAsync removes chunking from the response. This removes the header so it doesn't expect a chunked response.
response.Headers.Remove("transfer-encoding");
using (var responseStream = await responseMessage.Content.ReadAsStreamAsync())
{
await responseStream.CopyToAsync(response.Body, _streamCopyBufferSize, context.RequestAborted);
}
}
And now the preparation is complete. Back to our controller:
private readonly HttpClient _client;
public YourController()
{
_client = new HttpClient(new HttpClientHandler()
{
AllowAutoRedirect = false
});
}
public async Task<IActionResult> Rewrite()
{
var request = HttpContext.CreateProxyHttpRequest(new Uri("https://www.google.com"));
var response = await _client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, HttpContext.RequestAborted);
await HttpContext.CopyProxyHttpResponse(response);
return new EmptyResult();
}
And try to access it. It will be proxied to google.com

A nice reverse proxy middleware implementation can also be found here: https://auth0.com/blog/building-a-reverse-proxy-in-dot-net-core/
Note that I replaced this line here
requestMessage.Content?.Headers.TryAddWithoutValidation(header.Key, header.Value.ToArray());
with
requestMessage.Headers.TryAddWithoutValidation(header.Key, header.Value.ToString());
Original headers (e.g. like an authorization header with a bearer token) would not be added without my modification in my case.

I had luck using twitchax's AspNetCore.Proxy NuGet package, but could not get it to work using the ProxyRoute method shown in twitchax's answer. (Could have easily been a mistake on my end.)
Instead I defined the mapping in Statup.cs Configure() method similar to the code below.
app.UseProxy("api/someexternalapp-proxy/{arg1}", async (args) =>
{
string url = "https://someexternalapp.com/" + args["arg1"];
return await Task.FromResult<string>(url);
});

Piggy-backing on James Lawruk's answer https://stackoverflow.com/a/54149906/6596451 to get the twitchax Proxy attribute to work, I was also getting a 404 error until I specified the full route in the ProxyRoute attribute. I had my static route in a separate controller and the relative path from Controller's route was not working.
This worked:
public class ProxyController : Controller
{
[ProxyRoute("api/Proxy/{name}")]
public static Task<string> Get(string name)
{
return Task.FromResult($"http://www.google.com/");
}
}
This does not:
[Route("api/[controller]")]
public class ProxyController : Controller
{
[ProxyRoute("{name}")]
public static Task<string> Get(string name)
{
return Task.FromResult($"http://www.google.com/");
}
}
Hope this helps someone!

Twitchax's answer seems to be the best solution at the moment. In researching this, I found that Microsoft is developing a more robust solution that fits the exact problem the OP was trying to solve.
Repo: https://github.com/microsoft/reverse-proxy
Article for Preview 1 (they actually just released prev 2): https://devblogs.microsoft.com/dotnet/introducing-yarp-preview-1/
From the Article...
YARP is a project to create a reverse proxy server. It started when we noticed a pattern of questions from internal teams at Microsoft who were either building a reverse proxy for their service or had been asking about APIs and technology for building one, so we decided to get them all together to work on a common solution, which has become YARP.
YARP is a reverse proxy toolkit for building fast proxy servers in .NET using the infrastructure from ASP.NET and .NET. The key differentiator for YARP is that it is being designed to be easily customized and tweaked to match the specific needs of each deployment scenario. YARP plugs into the ASP.NET pipeline for handling incoming requests, and then has its own sub-pipeline for performing the steps to proxy the requests to backend servers. Customers can add additional modules, or replace stock modules as needed.
...
YARP works with either .NET Core 3.1 or .NET 5 preview 4 (or later). Download the preview 4 (or greater) of .NET 5 SDK from https://dotnet.microsoft.com/download/dotnet/5.0
More specifically, one of their sample apps implements authentication (as for the OP's original intent)
https://github.com/microsoft/reverse-proxy/blob/master/samples/ReverseProxy.Auth.Sample/Startup.cs

Here is a basic implementation of Proxy library for ASP.NET Core:
This does not implement the authorization but could be useful to someone looking for a simple reverse proxy with ASP.NET Core. We only use this for development stages.
using System;
using System.Globalization;
using System.Linq;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Primitives;
namespace Sample.Proxy
{
public class Startup
{
public void ConfigureServices(IServiceCollection services)
{
services.AddLogging(options =>
{
options.AddDebug();
options.AddConsole(console =>
{
console.IncludeScopes = true;
});
});
services.AddProxy(options =>
{
options.MessageHandler = new HttpClientHandler
{
AllowAutoRedirect = false,
UseCookies = true
};
options.PrepareRequest = (originalRequest, message) =>
{
var host = GetHeaderValue(originalRequest, "X-Forwarded-Host") ?? originalRequest.Host.Host;
var port = GetHeaderValue(originalRequest, "X-Forwarded-Port") ?? originalRequest.Host.Port.Value.ToString(CultureInfo.InvariantCulture);
var prefix = GetHeaderValue(originalRequest, "X-Forwarded-Prefix") ?? originalRequest.PathBase;
message.Headers.Add("X-Forwarded-Host", host);
if (!string.IsNullOrWhiteSpace(port)) message.Headers.Add("X-Forwarded-Port", port);
if (!string.IsNullOrWhiteSpace(prefix)) message.Headers.Add("X-Forwarded-Prefix", prefix);
return Task.FromResult(0);
};
});
}
private static string GetHeaderValue(HttpRequest request, string headerName)
{
return request.Headers.TryGetValue(headerName, out StringValues list) ? list.FirstOrDefault() : null;
}
public void Configure(IApplicationBuilder app)
{
app.UseWebSockets()
.Map("/api", api => api.RunProxy(new Uri("http://localhost:8833")))
.Map("/image", api => api.RunProxy(new Uri("http://localhost:8844")))
.Map("/admin", api => api.RunProxy(new Uri("http://localhost:8822")))
.RunProxy(new Uri("http://localhost:8811"));
}
public static void Main(string[] args)
{
var host = new WebHostBuilder()
.UseKestrel()
.UseIISIntegration()
.UseStartup<Startup>()
.Build();
host.Run();
}
}
}

SimpleGeo DotnetOpenAuth for C#

I have moved simpleGeo sample from VB(langsamu) to C#. This is an amazing feature for my needs but i not abled to fix the OAuth class that i can't initialize correctly to test-it. I haved posted my complete project in this bottom link including sample. The simpleGeo.dll implement base classes to connect and query simpleGeo. Client.cs is the main point to go & implementing ConsumerBase. need and Gmaps key and simpleGeo key easier to get. Can someone help me to fix.
Thanks for your help, Romi.
here VB : http github.com / simplegeo / simplegeo-dotnet
here C# : C# simpleGeo that need your help
namespace SimpleGeo
{
public class Client : DotNetOpenAuth.OAuth.ConsumerBase
{
private SimpleGeo.Description Services;
private SimpleGeo.TokenManager Tokens;
public Client(Version Version, string Key, string Secret): base(new Description(Version), new TokenManager(Key, Secret))
{
//added because not firing Public proprieties of base.
Services = new Description(Version);//remplace Services
Tokens = new TokenManager(Key, Secret);//remplace Token
}
...
public ServiceProviderDescription Service
{
get { return base.ServiceProvider; }
}
private IConsumerTokenManager Token
{
get { return base.TokenManager; }
}
....

Well, I love simplegeo but the guys dont see .NET as a viable API consumer :) - VB sample is pretty much useless and SimpleGeo.NET seems abandoned (?). I decided to roll out my own client for a project I have in mind and since there is an excellent REST library supporting OAuth (RestSharp) I gave it a try with simplegeo. A very rude example follows:
//create client and pass OAuth credentials
RestClient client = new RestClient("http://api.simplegeo.com");
client.Authenticator = OAuth1Authenticator.ForRequestToken(yourapikey, yoursecret);
//sample of creating a request for a specific simplegeo endpoint (places near an IP)
RestRequest request = new RestRequest(Method.GET) {Resource = "{version}/places/{ip}.json", RequestFormat.Json};
request.AddParameter("version", "1.0", ParameterType.UrlSegment);
request.AddParameter("ip", anyiphere, ParameterType.UrlSegment);
//call the api function
RestResponse response = client.Execute(request);
//get the simplegeo response in json
string json = response.Content;
Use Json.NET for deserializing into any custom classes of yours.
You may have a helper class constructing the request for you according to parameters passed - keep version somewhere not hardcoded just in case :)