Change Json.NET to System.Text.Json (#3758)

Author: tdykstra

xml/System.Web.Script.Serialization/JavaScriptSerializer.xml

@@ -16,14 +16,19 @@
   <Interfaces />
   <Docs>
     <summary>
-      <see href="https://www.newtonsoft.com/json">Json.NET</see> should be used for serialization and deserialization. Provides serialization and deserialization functionality for AJAX-enabled applications.</summary>
+      For .NET Framework 4.7.2 and later versions, use the APIs in the <see cref="N:System.Text.Json" /> namespace for serialization and deserialization. For earlier versions of .NET Framework, use <see href="https://www.newtonsoft.com/json">Newtonsoft.Json</see>. This type was intended to provide serialization and deserialization functionality for AJAX-enabled applications.
+    </summary>
     <remarks>
       <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The <xref:System.Web.Script.Serialization.JavaScriptSerializer> class is used internally by the asynchronous communication layer to serialize and deserialize the data that is passed between the browser and the Web server. You cannot access that instance of the serializer. However, this class exposes a public API. Therefore, you can use the class when you want to work with JavaScript Object Notation (JSON) in managed code.  
+
+> [!IMPORTANT]
+> For .NET Framework 4.7.2 and later versions, the APIs in the <xref:System.Text.Json> namespace should be used for serialization and deserialization. For earlier versions of .NET Framework, use [Newtonsoft.Json](https://www.newtonsoft.com/json).
+
+The <xref:System.Web.Script.Serialization.JavaScriptSerializer> class is used internally by the asynchronous communication layer to serialize and deserialize the data that is passed between the browser and the Web server. You cannot access that instance of the serializer. However, this class exposes a public API. Therefore, you can use the class when you want to work with JavaScript Object Notation (JSON) in managed code.  
   
- To serialize an object, use the <xref:System.Web.Script.Serialization.JavaScriptSerializer.Serialize%2A> method. To deserialize a JSON string, use the <xref:System.Web.Script.Serialization.JavaScriptSerializer.Deserialize%2A> or <xref:System.Web.Script.Serialization.JavaScriptSerializer.DeserializeObject%2A> methods. To serialize and deserialize types that are not natively supported by <xref:System.Web.Script.Serialization.JavaScriptSerializer>, implement custom converters by using the <xref:System.Web.Script.Serialization.JavaScriptConverter> class. Then register the converters by using the <xref:System.Web.Script.Serialization.JavaScriptSerializer.RegisterConverters%2A> method.  
+To serialize an object, use the <xref:System.Web.Script.Serialization.JavaScriptSerializer.Serialize%2A> method. To deserialize a JSON string, use the <xref:System.Web.Script.Serialization.JavaScriptSerializer.Deserialize%2A> or <xref:System.Web.Script.Serialization.JavaScriptSerializer.DeserializeObject%2A> methods. To serialize and deserialize types that are not natively supported by <xref:System.Web.Script.Serialization.JavaScriptSerializer>, implement custom converters by using the <xref:System.Web.Script.Serialization.JavaScriptConverter> class. Then register the converters by using the <xref:System.Web.Script.Serialization.JavaScriptSerializer.RegisterConverters%2A> method.  
   
 ## Mapping Between Managed Types and JSON  
  The following table shows the mapping between managed types and JSON for the serialization process. These managed types are natively supported by <xref:System.Web.Script.Serialization.JavaScriptSerializer>. When you are deserializing from a JSON string to a managed type, the same mapping applies. However, deserialization can be asymmetric; not all serializable managed types can be deserialized from JSON.