Add support for nullable request/response

src/Immediate.Cache.Shared/ApplicationCache.cs

@@ -23,8 +23,8 @@ public abstract class ApplicationCache<TRequest, TResponse>(
 	IMemoryCache memoryCache,
 	Owned<IHandler<TRequest, TResponse>> handler
 )
-	where TRequest : class
-	where TResponse : class
+	where TRequest : class?
+	where TResponse : class?
 {
 	private readonly Lock _lock = new();
 
@@ -57,8 +57,6 @@ protected virtual MemoryCacheEntryOptions GetCacheEntryOptions() =>
 
 	private CacheValue GetCacheValue(TRequest request)
 	{
-		ArgumentNullException.ThrowIfNull(request);
-
 		var key = TransformKey(request);
 
 		if (!memoryCache.TryGetValue(key, out var result))
@@ -76,7 +74,10 @@ private CacheValue GetCacheValue(TRequest request)
 			}
 		}
 
-		return (CacheValue)result!;
+		if (result is not CacheValue cacheValue)
+			throw new InvalidOperationException($"An unknown type has been stored as the cache value for key `{key}`; Immediate.Cache is unable to operate.");
+
+		return cacheValue;
 	}
 
 	/// <summary>

tests/Immediate.Cache.FunctionalTests/NullableTypesCache.cs

@@ -0,0 +1,23 @@
+using Immediate.Cache.Shared;
+using Immediate.Handlers.Shared;
+
+namespace Immediate.Cache.FunctionalTests;
+
+[CacheFor<NullableTypesHandler>]
+public sealed partial class NullableTypesCache
+{
+	protected override string TransformKey(NullableTypesHandler.Query? request) =>
+		$"NullableTypesHandler(query: {request})";
+}
+
+[Handler]
+public sealed partial class NullableTypesHandler
+{
+	public sealed record Query;
+	public sealed record Response;
+
+	private async ValueTask<Response?> HandleAsync(Query? _, CancellationToken __)
+	{
+		return null;
+	}
+}