Index: Src/GoogleApis.Auth/OAuth2/AuthorizationCodeInstalledApp.cs
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+/*
+Copyright 2013 Google Inc
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+using System.Threading;
+using System.Threading.Tasks;
+
+using Google.Apis.Auth.OAuth2.Responses;
+using Google.Apis.Auth.OAuth2.Requests;
+using Google.Apis.Logging;
+
+namespace Google.Apis.Auth.OAuth2
+{
+    /// <summary>
+    /// Thread-safe OAuth 2.0 authorization code flow for an installed application that persists end-user credentials.
+    /// </summary>
+    public class AuthorizationCodeInstalledApp : IAuthorizationCodeInstalledApp
+    {
+        private static readonly ILogger Logger = ApplicationContext.Logger.ForType<AuthorizationCodeInstalledApp>();
+
+        private readonly IAuthorizationCodeFlow flow;
+        private readonly ICodeReceiver codeReceiver;
+
+        /// <summary>
+        /// Constructs a new authorization code installed application with the given flow and code receiver.
+        /// </summary>
+        public AuthorizationCodeInstalledApp(IAuthorizationCodeFlow flow, ICodeReceiver codeReceiver)
+        {
+            this.flow = flow;
+            this.codeReceiver = codeReceiver;
+        }
+
+        #region IAuthorizationCodeInstalledApp Members
+
+        /// <summary>Gets the authorization code flow.</summary>
+        public IAuthorizationCodeFlow Flow
+        {
+            get { return flow; }
+        }
+
+        /// <summary>Gets the code receiver which is responsible for receiving the authorization code.</summary>
+        public ICodeReceiver CodeReceiver
+        {
+            get { return codeReceiver; }
+        }
+
+        public async Task<UserCredential> Authorize(string userId, CancellationToken taskCancellationToken)
+        {
+            // Try to load a token from the data store.
+            var token = await Flow.LoadTokenAsync(userId, taskCancellationToken).ConfigureAwait(false);
+
+            // If the stored token is null or it doesn't have a refresh token and the access token is expired we need 
+            // to retrieve a new authorization code.
+            if (token == null || (token.RefreshToken == null && token.IsExpired(flow.Clock)))
+            {
+                // Create a authorization code request.
+                var redirectUri = CodeReceiver.RedirectUri;
+                AuthorizationCodeRequestUrl codeRequest = Flow.CreateAuthorizationCodeRequest(redirectUri);
+
+                // Receive the code.
+                var response = await CodeReceiver.ReceiveCodeAsync(codeRequest, taskCancellationToken)
+                    .ConfigureAwait(false);
+
+                if (string.IsNullOrEmpty(response.Code))
+                {
+                    var errorResponse = new TokenErrorResponse(response);
+                    Logger.Info("Received an error. The response is: {0}", errorResponse);
+                    throw new TokenResponseException(errorResponse);
+                }
+
+                Logger.Debug("Received \"{0}\" code", response.Code);
+
+                // Get the token based on the code.
+                token = await Flow.ExchangeCodeForTokenAsync(userId, response.Code, CodeReceiver.RedirectUri,
+                    taskCancellationToken).ConfigureAwait(false);
+            }
+
+            return new UserCredential(flow, userId, token);
+        }
+
+        #endregion
+    }
+}
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