use the same instance when a filter implements both sync and async interfaces

Author: Tom Fiedler

src/Swashbuckle.AspNetCore.SwaggerGen/DependencyInjection/ConfigureSwaggerGeneratorOptions.cs

@@ -17,6 +17,7 @@ internal class ConfigureSwaggerGeneratorOptions : IConfigureOptions<SwaggerGener
         private readonly SwaggerGenOptions _swaggerGenOptions;
         private readonly IServiceProvider _serviceProvider;
         private readonly IWebHostEnvironment _hostingEnv;
+        private readonly Dictionary<Type, object> _filterInstances = [];
 
         public ConfigureSwaggerGeneratorOptions(
             IOptions<SwaggerGenOptions> swaggerGenOptionsAccessor,
@@ -33,7 +34,7 @@ public void Configure(SwaggerGeneratorOptions options)
             DeepCopy(_swaggerGenOptions.SwaggerGeneratorOptions, options);
 
             // Create and add any filters that were specified through the FilterDescriptor lists ...
-
+            // if a filter implements both interfaces, the same instance will be used
             foreach (var filterDescriptor in _swaggerGenOptions.ParameterFilterDescriptors)
             {
                 if (filterDescriptor.Type.IsAssignableTo(typeof(IParameterFilter)))
@@ -120,8 +121,16 @@ public void DeepCopy(SwaggerGeneratorOptions source, SwaggerGeneratorOptions tar
 
         private TFilter GetOrCreateFilter<TFilter>(FilterDescriptor filterDescriptor)
         {
-            return (TFilter)(filterDescriptor.FilterInstance
+            if (_filterInstances.TryGetValue(filterDescriptor.Type, out var value))
+            {
+                return (TFilter)value;
+            }
+
+            var instance =  (TFilter)(filterDescriptor.FilterInstance
                 ?? ActivatorUtilities.CreateInstance(_serviceProvider, filterDescriptor.Type, filterDescriptor.Arguments));
+
+            _filterInstances.Add(filterDescriptor.Type, instance);
+            return instance;
         }
     }
 }