use is operator instead of reflection

Author: SimonCropp

src/Microsoft.OpenApi/Extensions/OpenApiReferencableExtensions.cs

@@ -31,17 +31,17 @@ public static IOpenApiReferenceable ResolveReference(this IOpenApiReferenceable
             var mapKey = pointer.Tokens.ElementAtOrDefault(1);
             try
             {
-                if (element.GetType() == typeof(OpenApiHeader))
+                if (element is OpenApiHeader header)
                 {
-                    return ResolveReferenceOnHeaderElement((OpenApiHeader)element, propertyName, mapKey, pointer);
+                    return ResolveReferenceOnHeaderElement(header, propertyName, mapKey, pointer);
                 }
-                if (element.GetType() == typeof(OpenApiParameter))
+                if (element is OpenApiParameter parameter)
                 {
-                    return ResolveReferenceOnParameterElement((OpenApiParameter)element, propertyName, mapKey, pointer);
+                    return ResolveReferenceOnParameterElement(parameter, propertyName, mapKey, pointer);
                 }
-                if (element.GetType() == typeof(OpenApiResponse))
+                if (element is OpenApiResponse response)
                 {
-                    return ResolveReferenceOnResponseElement((OpenApiResponse)element, propertyName, mapKey, pointer);
+                    return ResolveReferenceOnResponseElement(response, propertyName, mapKey, pointer);
                 }
             }
             catch (KeyNotFoundException)

test/Microsoft.OpenApi.Tests/Writers/OpenApiJsonWriterTests.cs

@@ -208,10 +208,10 @@ private void WriteValueRecursive(OpenApiJsonWriter writer, object value)
 
                 writer.WriteEndObject();
             }
-            else if (typeof(IEnumerable).IsAssignableFrom(value.GetType()))
+            else if (value is IEnumerable enumerable)
             {
                 writer.WriteStartArray();
-                foreach (var elementValue in (IEnumerable)value)
+                foreach (var elementValue in enumerable)
                 {
                     WriteValueRecursive(writer, elementValue);
                 }

test/Microsoft.OpenApi.Tests/Writers/OpenApiYamlWriterTests.cs

@@ -276,10 +276,10 @@ private void WriteValueRecursive(OpenApiYamlWriter writer, object value)
 
                 writer.WriteEndObject();
             }
-            else if (typeof(IEnumerable).IsAssignableFrom(value.GetType()))
+            else if (value is IEnumerable enumerable)
             {
                 writer.WriteStartArray();
-                foreach (var elementValue in (IEnumerable)value)
+                foreach (var elementValue in enumerable)
                 {
                     WriteValueRecursive(writer, elementValue);
                 }