Add JsonApiContentTypeFilter to set response content type

JsonApiToolkit/Extensions/ServiceCollectionExtensions.cs

@@ -107,9 +107,11 @@ public static IServiceCollection AddJsonApiToolkit(this IServiceCollection servi
             {
                 jsonInputFormatter.SupportedMediaTypes.Add("application/vnd.api+json");
             }
+            options.Filters.AddService<JsonApiContentTypeFilter>();
         });
 
         services.AddScoped<JsonApiExceptionFilter>();
+        services.AddScoped<JsonApiContentTypeFilter>();
 
         return services;
     }

JsonApiToolkit/Filters/JsonApiContentTypeFilter.cs

@@ -0,0 +1,38 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.Filters;
+
+namespace JsonApiToolkit.Filters;
+
+/// <summary>
+/// A filter that sets the content type of the response to "application/vnd.api+json"
+/// for all JSON API responses.
+/// </summary>
+public class JsonApiContentTypeFilter : IActionFilter
+{
+    private const string JsonApiMediaType = "application/vnd.api+json";
+
+    /// <summary>
+    /// Does nothing before the action executes.
+    /// </summary>
+    public void OnActionExecuting(ActionExecutingContext context)
+    {
+        // No action needed before executing the action
+    }
+
+    /// <summary>
+    /// Sets the content type of the response to "application/vnd.api+json"
+    /// for all JSON API responses.
+    /// </summary>
+    public void OnActionExecuted(ActionExecutedContext context)
+    {
+        if (context.Result is ObjectResult objectResult)
+        {
+            objectResult.ContentTypes.Clear();
+            objectResult.ContentTypes.Add(JsonApiMediaType);
+        }
+        else if (context.Result is StatusCodeResult)
+        {
+            context.HttpContext.Response.ContentType = JsonApiMediaType;
+        }
+    }
+}