// You need "using" a lot System.* namespaces
   namespace Whatever
   {
       class RequestHelper
       {
           public void makeRequest()
           {
               String method = "GET";
               String url = "https://api.cardmarket.com/ws/v1.1/account";
               
               HttpWebRequest request = WebRequest.CreateHttp(url) as HttpWebRequest;
               OAuthHeader header = new OAuthHeader();
               request.Headers.Add(HttpRequestHeader.Authorization, header.getAuthorizationHeader(method, url));
               request.Method = method;
               
               HttpWebResponse response = request.GetResponse() as HttpWebResponse;
               XmlDocument doc = new XmlDocument();
               doc.Load(response.GetResponseStream());
               // proceed further
           }
       }
       
       
       /// <summary>
       /// Class encapsulates tokens and secret to create OAuth signatures and return Authorization headers for web requests.
       /// </summary>
       class OAuthHeader
       {
           /// <summary>App Token</summary>
           protected String appToken = "bfaD9xOU0SXBhtBP";
           /// <summary>App Secret</summary>
           protected String appSecret = "pChvrpp6AEOEwxBIIUBOvWcRG3X9xL4Y";
           /// <summary>Access Token (Class should also implement an AccessToken property to set the value)</summary>
           protected String accessToken = "lBY1xptUJ7ZJSK01x4fNwzw8kAe5b10Q";
           /// <summary>Access Token Secret (Class should also implement an AccessToken property to set the value)</summary>
           protected String accessSecret = "hc1wJAOX02pGGJK2uAv1ZOiwS7I9Tpoe";
           /// <summary>OAuth Signature Method</summary>
           protected String signatureMethod = "HMAC-SHA1";
           /// <summary>OAuth Version</summary>
           protected String version = "1.0";
           /// <summary>All Header params compiled into a Dictionary</summary>
           protected IDictionary<String, String> headerParams;
           
           /// <summary>
           /// Constructor
           /// </summary>
           public OAuthHeader ()
           {
               // String nonce = Guid.NewGuid().ToString("n");
               String nonce = "53eb1f44909d6";
               // String timestamp = (DateTime.UtcNow.Subtract(new DateTime(1970, 1, 1))).TotalSeconds.ToString();
               String timestamp = "1407917892";
               /// Initialize all class members
               this.headerParams = new Dictionary<String, String>();
               this.headerParams.Add("oauth_consumer_key", this.appToken);
               this.headerParams.Add("oauth_token", this.accessToken);
               this.headerParams.Add("oauth_nonce", nonce);
               this.headerParams.Add("oauth_timestamp", timestamp);
               this.headerParams.Add("oauth_signature_method", this.signatureMethod);
               this.headerParams.Add("oauth_version", this.version);
           }
           
           /// <summary>
           /// Pass request method and URI parameters to get the Authorization header value
           /// </summary>
           /// <param name="method">Request Method</param>
           /// <param name="url">Request URI</param>
           /// <returns>Authorization header value</returns>
           public String getAuthorizationHeader (String method, String url)
           {
               /// Add the realm parameter to the header params
               this.headerParams.Add("realm", url);
               
               /// Start composing the base string from the method and request URI
               String baseString = method.ToUpper() 
                                 + "&"
                                 + Uri.EscapeDataString(url)
                                 + "&";
               
               /// Gather, encode, and sort the base string parameters
               SortedDictionary<String, String> encodedParams = new SortedDictionary<String, String>();
               foreach (KeyValuePair<String, String> parameter in this.headerParams)
               {
                   if (false == parameter.Key.Equals("realm"))
                   {
                       encodedParams.Add(Uri.EscapeDataString(parameter.Key), Uri.EscapeDataString(parameter.Value));
                   }
               }
               
               /// Expand the base string by the encoded parameter=value pairs
               List<String> paramStrings = new List<String>();
               foreach (KeyValuePair<String, String> parameter in encodedParams)
               {
                   paramStrings.Add(parameter.Key + "=" + parameter.Value);
               }
               String paramString = Uri.EscapeDataString(String.Join<String>("&", paramStrings));
               baseString += paramString;
               
               /// Create the OAuth signature
               String signatureKey = Uri.EscapeDataString(this.appSecret) + "&" + Uri.EscapeDataString(this.accessSecret);
               HMAC hasher = HMACSHA1.Create();
               hasher.Key = Encoding.UTF8.GetBytes(signatureKey);
               Byte[] rawSignature = hasher.ComputeHash(Encoding.UTF8.GetBytes(baseString));
               String oAuthSignature = Convert.ToBase64String(rawSignature);
               
               /// Include the OAuth signature parameter in the header parameters array
               this.headerParams.Add("oauth_signature", oAuthSignature);
               
               /// Construct the header string
               List<String> headerParamStrings = new List<String>();
               foreach (KeyValuePair<String, String> parameter in this.headerParams)
               {
                   headerParamStrings.Add(parameter.Key + "=\"" + parameter.Value + "\"");
               }
               String authHeader = "OAuth " + String.Join<String>(", ", headerParamStrings);
               
               return authHeader;
           }
       }
   }