Support nullable array element types in parameter schemas

src/OpenApi/src/Extensions/OpenApiSchemaExtensions.cs

@@ -21,6 +21,23 @@ public static IOpenApiSchema CreateOneOfNullableWrapper(this IOpenApiSchema orig
         };
     }
 
+    public static void MakeArrayItemsNullable(this IOpenApiSchema schema)
+    {
+        if (schema is not OpenApiSchema { Items: { } items } arraySchema)
+        {
+            return;
+        }
+
+        if (items is OpenApiSchema { Type: { } itemType } inlineItemSchema)
+        {
+            inlineItemSchema.Type = itemType | JsonSchemaType.Null;
+        }
+        else
+        {
+            arraySchema.Items = items.CreateOneOfNullableWrapper();
+        }
+    }
+
     public static bool IsComponentizedSchema(this OpenApiSchema schema)
         => schema.IsComponentizedSchema(out _);
 

src/OpenApi/src/Extensions/TypeExtensions.cs

@@ -90,6 +90,19 @@ public static bool ShouldApplyNullableRequestSchema(this ApiParameterDescription
         return nullabilityInfo.WriteState == NullabilityState.Nullable;
     }
 
+    public static bool ShouldApplyNullableArrayElementSchema(this ApiParameterDescription apiParameterDescription)
+    {
+        if (apiParameterDescription.Type is not { IsArray: true } ||
+            apiParameterDescription.ParameterDescriptor is not IParameterInfoParameterDescriptor { ParameterInfo: { } parameterInfo })
+        {
+            return false;
+        }
+
+        var nullabilityInfoContext = new NullabilityInfoContext();
+        var nullabilityInfo = nullabilityInfoContext.Create(parameterInfo);
+        return nullabilityInfo.ElementType?.WriteState == NullabilityState.Nullable;
+    }
+
     public static bool ShouldApplyNullablePropertySchema(this JsonPropertyInfo jsonPropertyInfo)
     {
         if (jsonPropertyInfo.AttributeProvider is not PropertyInfo propertyInfo)

src/OpenApi/src/Services/OpenApiDocumentService.cs

@@ -588,6 +588,19 @@ private static bool IsServerSentEventsContentType(string contentType)
                 continue;
             }
 
+            var parameterSchema = await _componentService.GetOrCreateSchemaAsync(
+                document,
+                GetTargetType(description, parameter),
+                scopedServiceProvider,
+                schemaTransformers,
+                parameter,
+                cancellationToken: cancellationToken);
+
+            if (parameter.ShouldApplyNullableArrayElementSchema())
+            {
+                parameterSchema.MakeArrayItemsNullable();
+            }
+
             var openApiParameter = new OpenApiParameter
             {
                 Name = parameter.Name,
@@ -599,7 +612,7 @@ private static bool IsServerSentEventsContentType(string contentType)
                     _ => ParameterLocation.Query
                 },
                 Required = IsRequired(parameter),
-                Schema = await _componentService.GetOrCreateSchemaAsync(document, GetTargetType(description, parameter), scopedServiceProvider, schemaTransformers, parameter, cancellationToken: cancellationToken),
+                Schema = parameterSchema,
                 Description = GetParameterDescriptionFromAttribute(parameter)
             };
 

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.ParameterSchemas.cs

@@ -459,10 +459,7 @@ await VerifyOpenApiDocument(builder, document =>
         [(Guid[] id) => { }, JsonSchemaType.String, false],
         [(Guid?[] id) => { }, JsonSchemaType.String, true],
         [(string[] id) => { }, JsonSchemaType.String, false],
-        // Due to runtime restrictions, we can't resolve nullability
-        // info for reference types as element types so this will still
-        // encode as non-nullable.
-        [(string?[] id) => { }, JsonSchemaType.String, false],
+        [(string?[] id) => { }, JsonSchemaType.String, true],
         [(DateTime[] id) => { }, JsonSchemaType.String, false],
         [(DateTime?[] id) => { }, JsonSchemaType.String, true],
         [(DateTimeOffset[] id) => { }, JsonSchemaType.String, false],