handle add column request (#1272)

* handle add column request

* comments

* rename data to ViewModel

src/Microsoft.SqlTools.ServiceLayer/TableDesigner/Contracts/TableChangeInfo.cs

@@ -3,6 +3,7 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 //
 
+using System;
 using System.Runtime.Serialization;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Converters;
@@ -27,8 +28,41 @@ namespace Microsoft.SqlTools.ServiceLayer.TableDesigner.Contracts
     {
         public DesignerEditType Type { get; set; }
 
+        [JsonConverter(typeof(TableDesignerPropertyConverter))]
         public object Property { get; set; }
 
         public object Value { get; set; }
     }
+
+    /// <summary>
+    /// The property "Property" of <c>TableDesignerChangeInfo</c> could be string or <c>TableDesignerPropertyIdentifier</c>, use this custom converter to set the property value.
+    /// </summary>
+    public class TableDesignerPropertyConverter : JsonConverter
+    {
+        public override bool CanConvert(Type objectType)
+        {
+            return true;
+        }
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        {
+            object property;
+            if (reader.TokenType == JsonToken.StartObject)
+            {
+                TableDesignerPropertyIdentifier obj = serializer.Deserialize(reader, typeof(TableDesignerPropertyIdentifier)) as TableDesignerPropertyIdentifier;
+                property = obj;
+            }
+            else
+            {
+                property = reader.Value;
+            }
+            return property;
+        }
+
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            // We don't need to serialize this class.
+            throw new NotImplementedException();
+        }
+    }
 }