Fix for strong typed identifier usage w/ CustomProjection

src/EventSourcingTests/Projections/identity_from_event.cs

@@ -0,0 +1,57 @@
+using System;
+using EventSourcingTests.Aggregation;
+using Marten.Events;
+using Shouldly;
+using StronglyTypedIds;
+using Xunit;
+
+namespace EventSourcingTests.Projections;
+
+public class identity_from_event
+{
+    [Fact]
+    public void by_string()
+    {
+        var e = new Event<AEvent>(new AEvent()) { StreamKey = "foo" };
+
+        e.IdentityFromEvent<string>(StreamIdentity.AsString)
+            .ShouldBe("foo");
+    }
+
+    [Fact]
+    public void by_guid()
+    {
+        var streamId = Guid.NewGuid();
+        var e = new Event<AEvent>(new AEvent()) { StreamId = streamId };
+
+        e.IdentityFromEvent<Guid>(StreamIdentity.AsGuid)
+            .ShouldBe(streamId);
+    }
+
+    [Fact]
+    public void by_strong_typed_guid()
+    {
+        var streamId = Guid.NewGuid();
+        var e = new Event<AEvent>(new AEvent()) { StreamId = streamId };
+
+        e.IdentityFromEvent<TripId>(StreamIdentity.AsGuid)
+            .Value.ShouldBe(streamId);
+    }
+
+    [Fact]
+    public void by_strong_typed_string()
+    {
+        var e = new Event<AEvent>(new AEvent()) { StreamKey = "bar" };
+
+        e.IdentityFromEvent<EventSourcingTests.StringId>(StreamIdentity.AsString)
+            .Value.ShouldBe("bar");
+    }
+
+
+}
+
+[StronglyTypedId(Template.Guid)]
+public partial struct TripId;
+
+[StronglyTypedId(Template.String)]
+public partial struct StringId;

src/Marten/Events/Aggregation/CustomProjection.cs

@@ -10,6 +10,7 @@
 using Marten.Events.Daemon.Internals;
 using Marten.Events.Projections;
 using Marten.Exceptions;
+using Marten.Internal;
 using Marten.Internal.Sessions;
 using Marten.Internal.Storage;
 using Marten.Schema;
@@ -419,6 +420,6 @@ public IAggregateCache<TId, TDoc> CacheFor(Tenant tenant)
         }
     }
 
-    public TId IdentityFromEvent(StreamIdentity streamIdentity, IEvent e) =>
-        streamIdentity == StreamIdentity.AsGuid ? e.StreamId.To<TId>() : e.StreamKey.To<TId>();
+    public TId IdentityFromEvent(StreamIdentity streamIdentity, IEvent e)
+        => e.IdentityFromEvent<TId>(streamIdentity);
 }

src/Marten/Events/Event.cs

@@ -1,6 +1,7 @@
 #nullable enable
 using System;
 using System.Collections.Generic;
+using JasperFx.Core.Reflection;
 using Marten.Storage;
 
 namespace Marten.Events;
@@ -250,4 +251,28 @@ public static IEvent<T> WithData<T>(this IEvent @event, T eventData) where T : n
             Timestamp = @event.Timestamp
         };
     }
+
+    public static TId IdentityFromEvent<TId>(this IEvent e, StreamIdentity streamIdentity)
+    {
+        if (streamIdentity == StreamIdentity.AsGuid)
+        {
+            if (typeof(TId) == typeof(Guid))
+            {
+                return e.StreamId.As<TId>();
+            }
+
+            var valueTypeInfo = new StoreOptions().RegisterValueType(typeof(TId));
+            return valueTypeInfo.CreateAggregateIdentitySource<TId>()(e);
+        }
+        else
+        {
+            if (typeof(TId) == typeof(string))
+            {
+                return e.StreamKey.As<TId>();
+            }
+
+            var valueTypeInfo = new StoreOptions().RegisterValueType(typeof(TId));
+            return valueTypeInfo.CreateAggregateIdentitySource<TId>()(e);
+        }
+    }
 }