Handle setting RichMultiString keys to null (#1791)

* Handle setting RichMultiString keys to null

* create a helper method to get a WsId from an object

* Create WsId from key, not value

---------

Co-authored-by: Kevin Hahn <kevin_hahn@sil.org>

Author: myieye

backend/FwLite/MiniLcm.Tests/RichMultiStringTests.cs

@@ -223,6 +223,16 @@ public void JsonPatchCanUpdateRichMultiStringWhenValueIsString()
         ms["en"].Should().BeEquivalentTo(new RichString("updated", "en"));
     }
 
+    [Fact]
+    public void JsonPatchCanUpdateRichMultiStringWhenValueIsEmptyString()
+    {
+        var ms = new RichMultiString() { { "en", new RichString("test") } };
+        var patch = new JsonPatchDocument<RichMultiString>();
+        patch.Operations.Add(new Operation<RichMultiString>("replace", "/en", null, ""));
+        patch.ApplyTo(ms);
+        ms.Should().BeEmpty();
+    }
+
     [Fact]
     public void JsonPatchCanRemoveRichMultiString()
     {

backend/FwLite/MiniLcm/Models/MultiString.cs

@@ -75,18 +75,7 @@ void IDictionary.Add(object key, object? value)
         {
             var valStr = value as string ??
                          throw new ArgumentException($"unable to convert value {value?.GetType().Name ?? "null"} to string", nameof(value));
-            if (key is WritingSystemId keyWs)
-            {
-                Add(keyWs, valStr);
-            }
-            else if (key is string keyStr)
-            {
-                Add(keyStr, valStr);
-            }
-            else
-            {
-                throw new ArgumentException("unable to convert key to writing system id", nameof(key));
-            }
+            Add(WritingSystemId.FromUnknown(key), valStr);
         }
     }
 

backend/FwLite/MiniLcm/Models/RichMultiString.cs

@@ -41,18 +41,7 @@ void IDictionary.Add(object key, object? value)
         var valStr = value as RichString ??
                      throw new ArgumentException($"unable to convert value {value?.GetType().Name ?? "null"} to RichString",
                          nameof(value));
-        if (key is WritingSystemId keyWs)
-        {
-            Add(keyWs, valStr);
-        }
-        else if (key is string keyStr)
-        {
-            Add(keyStr, valStr);
-        }
-        else
-        {
-            throw new ArgumentException("unable to convert key to writing system id", nameof(key));
-        }
+        Add(WritingSystemId.FromUnknown(key), valStr);
     }
 
     public void Add(WritingSystemId key, RichString value)
@@ -152,28 +141,22 @@ void IDictionary.Remove(object key)
 
     object? IDictionary.this[object key]
     {
-        get =>
-            key switch
-            {
-                WritingSystemId keyWs => this[keyWs],
-                string keyStr => this[keyStr],
-                _ => throw new ArgumentException("unable to convert key to writing system id", nameof(key))
-            };
+        get => this[WritingSystemId.FromUnknown(key)];
         set
         {
-            var valStr = value as RichString ??
-                         throw new ArgumentException("unable to convert value to string", nameof(value));
-            if (key is WritingSystemId keyWs)
-            {
-                this[keyWs] = valStr;
-            }
-            else if (key is string keyStr)
+            // value will be null if an empty string was deserialized as a RichString (e.g. from a JsonPatch operation).
+            // Usually that's what we want, because when deserializing a whole RichMultiString it will result in the key being dropped.
+            // So we mimic that behaviour.
+            var wsId = WritingSystemId.FromUnknown(key);
+            if (value is null)
             {
-                this[keyStr] = valStr;
+                Remove(wsId);
             }
             else
             {
-                throw new ArgumentException("unable to convert key to writing system id", nameof(key));
+                var valStr = value as RichString ??
+                             throw new ArgumentException("unable to convert value to string", nameof(value));
+                this[wsId] = valStr;
             }
         }
     }

backend/FwLite/MiniLcm/Models/WritingSystemId.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using SIL.WritingSystems;
@@ -37,6 +38,13 @@ public override void WriteAsPropertyName(Utf8JsonWriter writer, WritingSystemId
 
     public static readonly WritingSystemId Default = "default";
 
+    public static WritingSystemId FromUnknown(object? obj)
+    {
+        if (obj is WritingSystemId wsId) return wsId;
+        if (obj is string str) return new WritingSystemId(str);
+        throw new ArgumentException($"Unable to convert object: '{obj}' to writing system id", nameof(obj));
+    }
+
     public WritingSystemId(string code)
     {
         //__key is used by the LfClassicMiniLcmApi to smuggle non guid ids with possibilitie lists