From 15525f1c44e0d99ef8fdee73430853e22239181d Mon Sep 17 00:00:00 2001
From: James Newton-King <james@newtonking.com>
Date: Sun, 21 Feb 2021 13:09:54 +1300
Subject: [PATCH] Fix JsonWriter.WriteToken to allow null with string token
 (#2472)

---
 .../JsonTextWriterTest.cs                     |  6 +++--
 .../Linq/JTokenWriterTest.cs                  | 23 +++++++++++++++++++
 Src/Newtonsoft.Json/JsonWriter.cs             |  5 ++--
 3 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/Src/Newtonsoft.Json.Tests/JsonTextWriterTest.cs b/Src/Newtonsoft.Json.Tests/JsonTextWriterTest.cs
index b3bdd4d93..f0c462337 100644
--- a/Src/Newtonsoft.Json.Tests/JsonTextWriterTest.cs
+++ b/Src/Newtonsoft.Json.Tests/JsonTextWriterTest.cs
@@ -945,13 +945,15 @@ public void WriteTokenDirect()
                 jsonWriter.WriteToken(JsonToken.StartArray);
                 jsonWriter.WriteToken(JsonToken.Integer, 1);
                 jsonWriter.WriteToken(JsonToken.StartObject);
-                jsonWriter.WriteToken(JsonToken.PropertyName, "string");
+                jsonWriter.WriteToken(JsonToken.PropertyName, "integer");
                 jsonWriter.WriteToken(JsonToken.Integer, int.MaxValue);
+                jsonWriter.WriteToken(JsonToken.PropertyName, "null-string");
+                jsonWriter.WriteToken(JsonToken.String, null);
                 jsonWriter.WriteToken(JsonToken.EndObject);
                 jsonWriter.WriteToken(JsonToken.EndArray);
             }
 
-            Assert.AreEqual(@"[1,{""string"":2147483647}]", sb.ToString());
+            Assert.AreEqual(@"[1,{""integer"":2147483647,""null-string"":null}]", sb.ToString());
         }
 
         [Test]
diff --git a/Src/Newtonsoft.Json.Tests/Linq/JTokenWriterTest.cs b/Src/Newtonsoft.Json.Tests/Linq/JTokenWriterTest.cs
index d8d7dbe5f..18d195eb5 100644
--- a/Src/Newtonsoft.Json.Tests/Linq/JTokenWriterTest.cs
+++ b/Src/Newtonsoft.Json.Tests/Linq/JTokenWriterTest.cs
@@ -392,5 +392,28 @@ public void DateTimeZoneHandling()
 
             Assert.AreEqual(new DateTime(2000, 1, 1, 1, 1, 1, DateTimeKind.Utc), dt);
         }
+
+        [Test]
+        public void WriteTokenDirect()
+        {
+            JToken token;
+
+            using (JTokenWriter jsonWriter = new JTokenWriter())
+            {
+                jsonWriter.WriteToken(JsonToken.StartArray);
+                jsonWriter.WriteToken(JsonToken.Integer, 1);
+                jsonWriter.WriteToken(JsonToken.StartObject);
+                jsonWriter.WriteToken(JsonToken.PropertyName, "integer");
+                jsonWriter.WriteToken(JsonToken.Integer, int.MaxValue);
+                jsonWriter.WriteToken(JsonToken.PropertyName, "null-string");
+                jsonWriter.WriteToken(JsonToken.String, null);
+                jsonWriter.WriteToken(JsonToken.EndObject);
+                jsonWriter.WriteToken(JsonToken.EndArray);
+
+                token = jsonWriter.Token;
+            }
+
+            Assert.AreEqual(@"[1,{""integer"":2147483647,""null-string"":null}]", token.ToString(Formatting.None));
+        }
     }
 }
\ No newline at end of file
diff --git a/Src/Newtonsoft.Json/JsonWriter.cs b/Src/Newtonsoft.Json/JsonWriter.cs
index d9121f610..a68fc8c43 100644
--- a/Src/Newtonsoft.Json/JsonWriter.cs
+++ b/Src/Newtonsoft.Json/JsonWriter.cs
@@ -579,8 +579,9 @@ public void WriteToken(JsonToken token, object? value)
                     }
                     break;
                 case JsonToken.String:
-                    ValidationUtils.ArgumentNotNull(value, nameof(value));
-                    WriteValue(value.ToString());
+                    // Allow for a null string. This matches JTokenReader behavior which can read
+                    // a JsonToken.String with a null value.
+                    WriteValue(value?.ToString());
                     break;
                 case JsonToken.Boolean:
                     ValidationUtils.ArgumentNotNull(value, nameof(value));