Proposing a fix for #100 (#108)

This does not seem to make sense to have an absolute date to invaliidate the tokens, as when it's passed, no token can be added to the cache,

Proposed to have a sliding duration.

Author: jmprieur

Microsoft.Identity.Web/Client/TokenCacheProviders/InMemory/MSALAppMemoryTokenCacheProvider.cs

@@ -128,7 +128,7 @@ private void AppTokenCacheAfterAccessNotification(TokenCacheNotificationArgs arg
             if (args.HasStateChanged)
             {
                 // Reflect changes in the persistence store
-                this.memoryCache.Set(this.AppCacheId, args.TokenCache.SerializeMsalV3(), CacheOptions.AbsoluteExpiration);
+                this.memoryCache.Set(this.AppCacheId, args.TokenCache.SerializeMsalV3(), CacheOptions.SlidingExpiration);
             }
         }
     }

Microsoft.Identity.Web/Client/TokenCacheProviders/InMemory/MSALAppMemoryTokenCacheProviderExtension.cs

@@ -39,7 +39,7 @@ public static class MSALAppMemoryTokenCacheProviderExtension
         /// <returns></returns>
         public static IServiceCollection AddInMemoryTokenCaches(this IServiceCollection services, MSALMemoryTokenCacheOptions cacheOptions = null)
         {
-            var memoryCacheoptions = (cacheOptions == null) ? new MSALMemoryTokenCacheOptions { AbsoluteExpiration = DateTimeOffset.Now.AddDays(14) }
+            var memoryCacheoptions = (cacheOptions == null) ? new MSALMemoryTokenCacheOptions { SlidingExpiration = TimeSpan.FromDays(14) }
             : cacheOptions;
 
             AddInMemoryAppTokenCache(services, memoryCacheoptions);

Microsoft.Identity.Web/Client/TokenCacheProviders/InMemory/MSALMemoryTokenCacheOptions.cs

@@ -38,15 +38,15 @@ public class MSALMemoryTokenCacheOptions
         /// <value>
         /// The AbsoluteExpiration value.
         /// </value>
-        public DateTimeOffset AbsoluteExpiration
+        public TimeSpan SlidingExpiration
         {
             get;
             set;
         }
 
         public MSALMemoryTokenCacheOptions()
         {
-            this.AbsoluteExpiration = DateTimeOffset.Now.AddHours(12);
+            this.SlidingExpiration = TimeSpan.FromHours(12);
         }
     }
 }

Microsoft.Identity.Web/Client/TokenCacheProviders/InMemory/MSALPerUserMemoryTokenCacheProvider.cs

@@ -104,7 +104,7 @@ private void UserTokenCacheAfterAccessNotification(TokenCacheNotificationArgs ar
                     return;
 
                 // Ideally, methods that load and persist should be thread safe.MemoryCache.Get() is thread safe.
-                this.memoryCache.Set(cacheKey, args.TokenCache.SerializeMsalV3(), this.CacheOptions.AbsoluteExpiration);
+                this.memoryCache.Set(cacheKey, args.TokenCache.SerializeMsalV3(), this.CacheOptions.SlidingExpiration);
 
             }
         }