Ensure mechanism data is not fully overwritten

CHANGELOG.md

@@ -7,7 +7,7 @@
 - Reintroduced experimental support for Session Replay on Android ([#4097](https://github.com/getsentry/sentry-dotnet/pull/4097))
 
 ### Fixes
-
+- Ensure user exception data is not removed by AspNetCoreExceptionProcessor ([#4016](https://github.com/getsentry/sentry-dotnet/pull/4106))
 - Prevent users from disabling AndroidEnableAssemblyCompression which leads to untrappable crash ([#4089](https://github.com/getsentry/sentry-dotnet/pull/4089))
 - Fixed MSVCRT build warning on Windows ([#4111](https://github.com/getsentry/sentry-dotnet/pull/4111))
 

src/Sentry.AspNetCore/AspNetCoreExceptionProcessor.cs

@@ -14,11 +14,9 @@ public void Process(Exception exception, SentryEvent @event)
             {
                 foreach (var ex in @event.SentryExceptions)
                 {
-                    ex.Mechanism = new Mechanism
-                    {
-                        Type = "ExceptionHandlerMiddleware",
-                        Handled = false
-                    };
+                    ex.Mechanism ??= new Mechanism();
+                    ex.Mechanism.Type = "ExceptionHandlerMiddleware";
+                    ex.Mechanism.Handled = false;
                 }
             }
         }