[Fixes #5243] Handle null values in TempData properly

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/SessionStateTempDataProvider.cs

@@ -21,7 +21,9 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
     /// </summary>
     public class SessionStateTempDataProvider : ITempDataProvider
     {
-        private const string TempDataSessionStateKey = "__ControllerTempData";
+        // Internal for testing
+        internal const string TempDataSessionStateKey = "__ControllerTempData";
+
         private readonly JsonSerializer _jsonSerializer = JsonSerializer.Create(
             new JsonSerializerSettings()
             {
@@ -61,15 +63,22 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             // Accessing Session property will throw if the session middleware is not enabled.
             var session = context.Session;
 
-            var tempDataDictionary = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
+            Dictionary<string, object> tempDataDictionary = null;
             byte[] value;
 
             if (session.TryGetValue(TempDataSessionStateKey, out value))
             {
+                // If we got it from Session, remove it so that no other request gets it
+                session.Remove(TempDataSessionStateKey);
+
                 using (var memoryStream = new MemoryStream(value))
                 using (var writer = new BsonReader(memoryStream))
                 {
                     tempDataDictionary = _jsonSerializer.Deserialize<Dictionary<string, object>>(writer);
+                    if (tempDataDictionary == null)
+                    {
+                        return new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
+                    }
                 }
 
                 var convertedDictionary = new Dictionary<string, object>(
@@ -142,9 +151,6 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 }
 
                 tempDataDictionary = convertedDictionary;
-
-                // If we got it from Session, remove it so that no other request gets it
-                session.Remove(TempDataSessionStateKey);
             }
             else
             {
@@ -153,7 +159,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 session.Set(TempDataSessionStateKey, new byte[] { });
             }
 
-            return tempDataDictionary;
+            return tempDataDictionary ?? new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
         }
 
         /// <inheritdoc />
@@ -172,8 +178,11 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             {
                 foreach (var item in values.Values)
                 {
-                    // We want to allow only simple types to be serialized in session.
+                    if (item != null)
-                    EnsureObjectCanBeSerialized(item);
+                    {
+                        // We want to allow only simple types to be serialized in session.
+                        EnsureObjectCanBeSerialized(item);
+                    }
                 }
 
                 using (var memoryStream = new MemoryStream())

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/ViewFeatures/SessionStateTempDataProviderTest.cs

@@ -54,6 +54,21 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             Assert.Empty(tempDataDictionary);
         }
 
+        [Fact]
+        public void Load_ReturnsEmptyDictionary_WhenSessionDataIsEmpty()
+        {
+            // Arrange
+            var testProvider = new SessionStateTempDataProvider();
+            var httpContext = GetHttpContext();
+            httpContext.Session.Set(SessionStateTempDataProvider.TempDataSessionStateKey, new byte[] { });
+
+            // Act
+            var tempDataDictionary = testProvider.LoadTempData(httpContext);
+
+            // Assert
+            Assert.Empty(tempDataDictionary);
+        }
+
         public static TheoryData<object, Type> InvalidTypes
         {
             get
@@ -369,6 +384,26 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             Assert.Null(emptyDictionary);
         }
 
+        [Fact]
+        public void SaveAndLoad_NullValue_RoundTripsSuccessfully()
+        {
+            // Arrange
+            var testProvider = new SessionStateTempDataProvider();
+            var input = new Dictionary<string, object>
+            {
+                { "NullKey", null }
+            };
+            var context = GetHttpContext();
+
+            // Act
+            testProvider.SaveTempData(context, input);
+            var TempData = testProvider.LoadTempData(context);
+
+            // Assert
+            Assert.True(TempData.ContainsKey("NullKey"));
+            Assert.Null(TempData["NullKey"]);
+        }
+
         private class TestItem
         {
             public int DummyInt { get; set; }