fix: Remove DateTime in favour of TimeProvider and DateTimeOffset

.editorconfig

@@ -110,7 +110,7 @@ csharp_style_prefer_null_check_over_type_check = true : suggestion
 csharp_style_implicit_object_creation_when_type_is_apparent = true : suggestion
 
 # primary constructors (C# 12+)
-csharp_style_prefer_primary_constructors = true : suggestion
+csharp_style_prefer_primary_constructors = true : warning
 
 # collection expressions (C# 12+)
 dotnet_style_prefer_collection_expression = true : suggestion

src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryServiceTests.cs

@@ -115,7 +115,7 @@ public void RecordDeliveryAttempt_UpdatesDeliveryStatus()
         var attempt = new DeliveryAttempt
         {
             AttemptNumber = 1,
-            AttemptTime = DateTime.UtcNow,
+            AttemptTime = DateTimeOffset.UtcNow,
             Outcome = DeliveryOutcome.Success,
             HttpStatusCode = 200,
         };
@@ -136,7 +136,7 @@ public void RecordDeliveryAttempt_NonExistingEvent_DoesNotThrow()
         var attempt = new DeliveryAttempt
         {
             AttemptNumber = 1,
-            AttemptTime = DateTime.UtcNow,
+            AttemptTime = DateTimeOffset.UtcNow,
             Outcome = DeliveryOutcome.Success,
         };
 
@@ -283,7 +283,7 @@ private static SimulatorEvent CreateTestEvent(string id)
                 Id = id,
                 Subject = "/test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = DateTimeOffset.UtcNow.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { Property = "Value" },
             }

src/AzureEventGridSimulator.Tests/Domain/Services/Dashboard/EventHistoryStoreTests.cs

@@ -258,7 +258,7 @@ private static EventHistoryRecord CreateTestRecord(string id, string topicName =
             EventType = "Test.EventType",
             Subject = "/test/subject",
             Source = "/test/source",
-            EventTime = DateTime.UtcNow.ToString("o"),
+            EventTime = DateTimeOffset.UtcNow.ToString("o"),
             InputSchema = EventSchema.EventGridSchema,
             PayloadJson = "{}",
         };

src/AzureEventGridSimulator.Tests/Helpers/FakeTimeProvider.cs

@@ -0,0 +1,31 @@
+namespace AzureEventGridSimulator.Tests.Helpers;
+
+/// <summary>
+/// A fake TimeProvider for testing time-dependent code.
+/// Allows tests to control the current time for deterministic behavior.
+/// </summary>
+public class FakeTimeProvider(DateTimeOffset startTime) : TimeProvider
+{
+    private DateTimeOffset _utcNow = startTime;
+
+    public override DateTimeOffset GetUtcNow()
+    {
+        return _utcNow;
+    }
+
+    /// <summary>
+    /// Sets the current time to a specific value.
+    /// </summary>
+    public void SetUtcNow(DateTimeOffset value)
+    {
+        _utcNow = value;
+    }
+
+    /// <summary>
+    /// Advances the current time by the specified duration.
+    /// </summary>
+    public void Advance(TimeSpan duration)
+    {
+        _utcNow = _utcNow.Add(duration);
+    }
+}

src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/CloudEventSchemaFormatterTests.cs

@@ -72,8 +72,8 @@ public void GivenEventGridEvent_WhenSerializedAsCloudEvent_ThenConvertedCorrectl
             EventType = "Test.Event.Type",
             EventTime = "2025-01-15T10:30:00Z",
             Data = new { Property = "Value" },
-            Topic = "/test/topic",
         };
+        eventGridEvent.SetTopic("/test/topic");
 
         var simulatorEvent = SimulatorEvent.FromEventGridEvent(eventGridEvent);
         var json = _formatter.Serialize(simulatorEvent);

src/AzureEventGridSimulator.Tests/UnitTests/CloudEvents/SimulatorEventTests.cs

@@ -18,8 +18,8 @@ public void GivenEventGridEvent_WhenWrappedInSimulatorEvent_ThenPropertiesMapped
             EventTime = "2025-01-15T10:30:00Z",
             Data = new { Property = "Value" },
             DataVersion = "1.0",
-            Topic = "/test/topic",
         };
+        eventGridEvent.SetTopic("/test/topic");
 
         var simulatorEvent = SimulatorEvent.FromEventGridEvent(eventGridEvent);
 

src/AzureEventGridSimulator.Tests/UnitTests/Commands/SendNotificationEventsToSubscriberCommandHandlerTests.cs

@@ -407,7 +407,7 @@ private static SimulatorEvent CreateTestEventGridEvent(string id)
                 Id = id,
                 Subject = "/test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = DateTimeOffset.UtcNow.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { Property = "Value" },
             }

src/AzureEventGridSimulator.Tests/UnitTests/Commands/ValidateSubscriptionCommandHandlerTests.cs

@@ -16,7 +16,7 @@ public class ValidateSubscriptionCommandHandlerTests
     public ValidateSubscriptionCommandHandlerTests()
     {
         _logger = Substitute.For<ILogger<ValidateSubscriptionCommandHandler>>();
-        _handler = new ValidateSubscriptionCommandHandler(_logger);
+        _handler = new ValidateSubscriptionCommandHandler(TimeProvider.System, _logger);
     }
 
     [Fact]

src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridSchemaFormatterTests.cs

@@ -9,7 +9,7 @@ namespace AzureEventGridSimulator.Tests.UnitTests.EventGrid;
 [Trait("Category", "unit")]
 public class EventGridSchemaFormatterTests
 {
-    private readonly EventGridSchemaFormatter _formatter = new();
+    private readonly EventGridSchemaFormatter _formatter = new(TimeProvider.System);
 
     [Fact]
     public void GivenEventGridEvent_WhenSerialized_ThenJsonContainsAllFields()
@@ -22,9 +22,9 @@ public void GivenEventGridEvent_WhenSerialized_ThenJsonContainsAllFields()
             EventTime = "2025-01-15T10:30:00Z",
             Data = new { Property = "Value" },
             DataVersion = "1.0",
-            Topic = "/test/topic",
             MetadataVersion = "1",
         };
+        eventGridEvent.SetTopic("/test/topic");
 
         var simulatorEvent = SimulatorEvent.FromEventGridEvent(eventGridEvent);
         var json = _formatter.Serialize(simulatorEvent);

src/AzureEventGridSimulator.Tests/UnitTests/EventGrid/EventGridValidationTests.cs

@@ -1,3 +1,4 @@
+using System.Text.Json;
 using AzureEventGridSimulator.Domain.Entities;
 using Shouldly;
 using Xunit;
@@ -258,36 +259,55 @@ string metadataVersion
     public void GivenEventWithNonNullTopic_WhenValidated_ThenExceptionThrown()
     {
         // Topic should be null/empty when publishing - it's set by the service
-        var eventGridEvent = new EventGridEvent
-        {
-            Id = "test-id-123",
-            Subject = "/test/subject",
-            EventType = "Test.EventType",
-            EventTime = "2025-01-15T10:30:00Z",
-            Topic = "/subscriptions/some/topic",
-        };
-
-        var exception = Should.Throw<InvalidOperationException>(() => eventGridEvent.Validate());
+        // Use JSON deserialization to simulate a publisher sending a Topic value
+        var json = """
+            {
+                "id": "test-id-123",
+                "subject": "/test/subject",
+                "eventType": "Test.EventType",
+                "eventTime": "2025-01-15T10:30:00Z",
+                "topic": "/subscriptions/some/topic"
+            }
+            """;
+        var eventGridEvent = JsonSerializer.Deserialize<EventGridEvent>(json);
+
+        var exception = Should.Throw<InvalidOperationException>(() => eventGridEvent!.Validate());
         exception.Message.ShouldContain("Topic");
     }
 
-    [Theory]
-    [InlineData(null)]
-    [InlineData("")]
-    public void GivenEventWithNullOrEmptyTopic_WhenValidated_ThenNoExceptionThrown(string topic)
+    [Fact]
+    public void GivenEventWithNullTopic_WhenValidated_ThenNoExceptionThrown()
     {
+        // Topic defaults to null when not set
         var eventGridEvent = new EventGridEvent
         {
             Id = "test-id-123",
             Subject = "/test/subject",
             EventType = "Test.EventType",
             EventTime = "2025-01-15T10:30:00Z",
-            Topic = topic,
         };
 
         Should.NotThrow(() => eventGridEvent.Validate());
     }
 
+    [Fact]
+    public void GivenEventWithEmptyTopic_WhenValidated_ThenNoExceptionThrown()
+    {
+        // Use JSON deserialization to set empty Topic
+        var json = """
+            {
+                "id": "test-id-123",
+                "subject": "/test/subject",
+                "eventType": "Test.EventType",
+                "eventTime": "2025-01-15T10:30:00Z",
+                "topic": ""
+            }
+            """;
+        var eventGridEvent = JsonSerializer.Deserialize<EventGridEvent>(json);
+
+        Should.NotThrow(() => eventGridEvent!.Validate());
+    }
+
     [Fact]
     public void GivenEventWithNullData_WhenValidated_ThenNoExceptionThrown()
     {

src/AzureEventGridSimulator.Tests/UnitTests/Filtering/AdvancedFilterEventAcceptanceTests.cs

@@ -9,25 +9,31 @@ namespace AzureEventGridSimulator.Tests.UnitTests.Filtering;
 [Trait("Category", "unit")]
 public class AdvancedFilterEventAcceptanceTests
 {
-    private static readonly EventGridEvent _gridEvent = new()
+    private static readonly EventGridEvent _gridEvent = CreateTestEvent();
+
+    private static EventGridEvent CreateTestEvent()
     {
-        Id = "EventId",
-        Data = new
+        var evt = new EventGridEvent
         {
-            NumberValue = 1,
-            IsTrue = true,
-            Name = "StringValue",
-            DoubleValue = 0.12345d,
-            NumberMaxValue = ulong.MaxValue,
-            SubObject = new { Id = 1, Name = "Test" },
-        },
-        DataVersion = "5.0",
-        EventTime = DateTime.UtcNow.ToString("O"),
-        EventType = "this.is.a.test.event.type",
-        MetadataVersion = "2.3.4",
-        Subject = "TheEventSubject",
-        Topic = "THE_EVENT_TOPIC",
-    };
+            Id = "EventId",
+            Data = new
+            {
+                NumberValue = 1,
+                IsTrue = true,
+                Name = "StringValue",
+                DoubleValue = 0.12345d,
+                NumberMaxValue = ulong.MaxValue,
+                SubObject = new { Id = 1, Name = "Test" },
+            },
+            DataVersion = "5.0",
+            EventTime = DateTimeOffset.UtcNow.ToString("O"),
+            EventType = "this.is.a.test.event.type",
+            MetadataVersion = "2.3.4",
+            Subject = "TheEventSubject",
+        };
+        evt.SetTopic("THE_EVENT_TOPIC");
+        return evt;
+    }
 
     [Theory]
     [ClassData(typeof(PositiveFilterTestCaseContainer))]

src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAegSasTokenTests.cs

@@ -14,7 +14,7 @@ public void GivenValidAegSasToken_WhenValidated_ThenReturnsTrue()
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
@@ -29,7 +29,7 @@ public void GivenExpiredAegSasToken_WhenValidated_ThenReturnsFalse()
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(-5)
+            DateTimeOffset.UtcNow.AddMinutes(-5)
         );
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
@@ -44,7 +44,7 @@ public void GivenAegSasTokenWithWrongSignature_WhenValidated_ThenReturnsFalse()
         var token = GenerateValidSasToken(
             "WrongKey123456789012345=",
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
@@ -59,7 +59,7 @@ public void GivenInvalidAegSasToken_WhenValidated_ThenLogsError()
         var token = GenerateValidSasToken(
             "WrongKey123456789012345=",
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
@@ -82,7 +82,7 @@ public void GivenAegSasTokenWithDifferentCase_WhenValidated_ThenHeaderIsFoundCas
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary { { "AEG-SAS-TOKEN", token } };
 
@@ -94,7 +94,7 @@ public void GivenAegSasTokenWithDifferentCase_WhenValidated_ThenHeaderIsFoundCas
     [Fact]
     public void GivenTokenExpiringExactlyNow_WhenValidated_ThenReturnsFalse()
     {
-        var token = GenerateValidSasToken(ValidTopicKey, "http://localhost", DateTime.UtcNow);
+        var token = GenerateValidSasToken(ValidTopicKey, "http://localhost", DateTimeOffset.UtcNow);
         var headers = new HeaderDictionary { { Constants.AegSasTokenHeader, token } };
 
         var result = Validator.IsValid(headers, ValidTopicKey);

src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorAuthorizationHeaderTests.cs

@@ -15,7 +15,7 @@ public void GivenValidAuthorizationHeader_WhenValidated_ThenReturnsTrue()
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary
         {
@@ -33,7 +33,7 @@ public void GivenExpiredAuthorizationHeader_WhenValidated_ThenReturnsFalse()
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(-5)
+            DateTimeOffset.UtcNow.AddMinutes(-5)
         );
         var headers = new HeaderDictionary
         {
@@ -51,7 +51,7 @@ public void GivenAuthorizationHeaderWithWrongSignature_WhenValidated_ThenReturns
         var token = GenerateValidSasToken(
             "WrongKey123456789012345=",
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary
         {
@@ -69,7 +69,7 @@ public void GivenInvalidAuthorizationHeader_WhenValidated_ThenLogsError()
         var token = GenerateValidSasToken(
             "WrongKey123456789012345=",
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary
         {
@@ -108,7 +108,7 @@ public void GivenAuthorizationHeaderWithDifferentCase_WhenValidated_ThenHeaderIs
         var token = GenerateValidSasToken(
             ValidTopicKey,
             "http://localhost",
-            DateTime.UtcNow.AddMinutes(5)
+            DateTimeOffset.UtcNow.AddMinutes(5)
         );
         var headers = new HeaderDictionary
         {

src/AzureEventGridSimulator.Tests/UnitTests/Middleware/SasKeyValidatorTestBase.cs

@@ -15,12 +15,16 @@ public abstract class SasKeyValidatorTestBase
     protected SasKeyValidatorTestBase()
     {
         Logger = Substitute.For<ILogger<SasKeyValidator>>();
-        Validator = new SasKeyValidator(Logger);
+        Validator = new SasKeyValidator(TimeProvider.System, Logger);
     }
 
-    protected static string GenerateValidSasToken(string key, string resource, DateTime expiry)
+    protected static string GenerateValidSasToken(
+        string key,
+        string resource,
+        DateTimeOffset expiry
+    )
     {
-        var decodedExpiration = expiry.ToString("o");
+        var decodedExpiration = expiry.UtcDateTime.ToString("o");
 
         var encodedResource = HttpUtility.UrlEncode(resource);
         var encodedExpiration = HttpUtility.UrlEncode(decodedExpiration);

src/AzureEventGridSimulator.Tests/UnitTests/Retry/DeadLetterServiceTests.cs

@@ -68,7 +68,7 @@ private static PendingDelivery CreatePendingDelivery(
                 Id = eventId ?? Guid.NewGuid().ToString(),
                 Subject = "test/subject",
                 EventType = "Test.EventType",
-                EventTime = DateTime.UtcNow.ToString("o"),
+                EventTime = DateTimeOffset.UtcNow.ToString("o"),
                 DataVersion = "1.0",
                 Data = new { test = "data" },
             }
@@ -130,7 +130,7 @@ public async Task GivenDelivery_WhenWriting_ThenFileContainsCorrectJson()
             new DeliveryAttempt
             {
                 AttemptNumber = 5,
-                AttemptTime = DateTime.UtcNow,
+                AttemptTime = DateTimeOffset.UtcNow,
                 Outcome = DeliveryOutcome.HttpError,
                 HttpStatusCode = 503,
                 ErrorMessage = "Service Unavailable",