Add tests.

Author: peombwa

src/Authentication/Authentication.Test/Helpers/GraphSessionTests.cs

@@ -0,0 +1,41 @@
+namespace Microsoft.Graph.Authentication.Test.Helpers
+{
+    using Microsoft.Graph.PowerShell.Authentication;
+    using System;
+    using Xunit;
+    public class GraphSessionTests
+    {
+        [Fact]
+        public void GraphSessionShouldBeInitilizedAfterInitializerIsCalled()
+        {
+            GraphSessionInitializer.InitializeSession();
+
+            Assert.NotNull(GraphSession.Instance);
+            Assert.Null(GraphSession.Instance.AuthContext);
+        }
+        
+        [Fact]
+        public void ShoudlOverwriteExistingGraphSession()
+        {
+            GraphSessionInitializer.InitializeSession();
+            Guid originalSessionId = GraphSession.Instance._graphSessionId;
+
+            GraphSession.Initialize(() => new GraphSession(), true);
+
+            Assert.NotNull(GraphSession.Instance);
+            Assert.NotEqual(GraphSession.Instance._graphSessionId, originalSessionId);
+        } 
+        
+        [Fact]
+        public void ShoudlNotOverwriteExistingGraphSession()
+        {
+            GraphSessionInitializer.InitializeSession();
+            Guid originalSessionId = GraphSession.Instance._graphSessionId;
+
+            GraphSession.Initialize(() => new GraphSession());
+
+            Assert.NotNull(GraphSession.Instance);
+            Assert.Equal(GraphSession.Instance._graphSessionId, originalSessionId);
+        }
+    }
+}

src/Authentication/Authentication/Common/GraphSession.cs

@@ -6,11 +6,19 @@ namespace Microsoft.Graph.PowerShell.Authentication
 {
     using System;
     using System.Threading;
+    /// <summary>
+    /// The current <see cref="GraphSession"/>.
+    /// </summary>
     public class GraphSession: IGraphSession
     {
         static GraphSession _instance;
         static bool _initialized = false;
         static ReaderWriterLockSlim sessionLock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);
+        internal Guid _graphSessionId;
+
+        /// <summary>
+        /// Gets or Sets <see cref="IAuthContext"/>.
+        /// </summary>
         public IAuthContext AuthContext { get; set; }
         public static GraphSession Instance
         {
@@ -42,7 +50,16 @@ public static GraphSession Instance
                 }
             }
         }
+        public GraphSession()
+        {
+            _graphSessionId = Guid.NewGuid();
+        }
 
+        /// <summary>
+        /// Initialize <see cref="GraphSession"/>.
+        /// </summary>
+        /// <param name="instanceCreator">A func to create an instance.</param>
+        /// <param name="overwrite">If true, overwrite the current instance. Otherwise do not initialize.</param>
         public static void Initialize(Func<GraphSession> instanceCreator, bool overwrite)
         {
             try
@@ -71,11 +88,19 @@ public static void Initialize(Func<GraphSession> instanceCreator, bool overwrite
             }
         }
 
+        /// <summary>
+        /// Initialize the current instance if none exists.
+        /// </summary>
+        /// <param name="instanceCreator">A func to create an instance.</param>
         public static void Initialize(Func<GraphSession> instanceCreator)
         {
             Initialize(instanceCreator, false);
         }
 
+        /// <summary>
+        /// Modify the current instance of <see cref="GraphSession"/>.
+        /// </summary>
+        /// <param name="modifier">A func to modify the <see cref="GraphSession"/> instance.</param>
         public static void Modify(Action<GraphSession> modifier)
         {
             try

src/Authentication/Authentication/Common/GraphSessionInitializer.cs

@@ -15,7 +15,7 @@ public static void InitializeSession()
         }
 
         /// <summary>
-        /// Creates or gets a instance of a <see cref="GraphSession"/>.
+        /// Creates a new instance of a <see cref="GraphSession"/>.
         /// </summary>
         /// <returns><see cref="GraphSession"/></returns>
         internal static GraphSession CreateInstance()

src/Authentication/Authentication/Properties/AssemblyInfo.cs

@@ -0,0 +1,5 @@
+using System.Runtime.CompilerServices;
+
+#if DEBUG
+[assembly: InternalsVisibleTo("Microsoft.Graph.Authentication.Test")]
+#endif