[release/3.1] JsonSerializer.Serialize should check whether the passed-in Utf8JsonWriter is null.

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Write.Helpers.cs

@@ -97,6 +97,11 @@ private static void WriteValueCore(Utf8JsonWriter writer, object value, Type typ
                 options = JsonSerializerOptions.s_defaultOptions;
             }
 
+            if (writer == null)
+            {
+                throw new ArgumentNullException(nameof(writer));
+            }
+
             WriteCore(writer, value, type, options);
         }
 
@@ -111,6 +116,7 @@ private static void WriteCore(PooledByteBufferWriter output, object value, Type
         private static void WriteCore(Utf8JsonWriter writer, object value, Type type, JsonSerializerOptions options)
         {
             Debug.Assert(type != null || value == null);
+            Debug.Assert(writer != null);
 
             if (value == null)
             {

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Write.Utf8JsonWriter.cs

@@ -12,6 +12,9 @@ public static partial class JsonSerializer
         /// <param name="writer">The writer to write.</param>
         /// <param name="value">The value to convert and write.</param>
         /// <param name="options">Options to control the behavior.</param>
+        /// <exception cref="ArgumentNullException">
+        ///   <paramref name="writer"/> is null.
+        /// </exception>
         public static void Serialize<TValue>(Utf8JsonWriter writer, TValue value, JsonSerializerOptions options = null)
         {
             WriteValueCore(writer, value, typeof(TValue), options);
@@ -24,6 +27,9 @@ public static void Serialize<TValue>(Utf8JsonWriter writer, TValue value, JsonSe
         /// <param name="value">The value to convert and write.</param>
         /// <param name="inputType">The type of the <paramref name="value"/> to convert.</param>
         /// <param name="options">Options to control the behavior.</param>
+        /// <exception cref="ArgumentNullException">
+        ///   <paramref name="writer"/> is null.
+        /// </exception>
         public static void Serialize(Utf8JsonWriter writer, object value, Type inputType, JsonSerializerOptions options = null)
         {
             VerifyValueAndType(value, inputType);

src/System.Text.Json/tests/Serialization/WriteValueTests.cs

@@ -9,6 +9,13 @@ namespace System.Text.Json.Serialization.Tests
 {
     public static partial class WriteValueTests
     {
+        [Fact]
+        public static void NullWriterThrows()
+        {
+            Assert.Throws<ArgumentNullException>(() => JsonSerializer.Serialize(null, 1));
+            Assert.Throws<ArgumentNullException>(() => JsonSerializer.Serialize(null, 1, typeof(int)));
+        }
+
         [Fact]
         public static void CanWriteValueToJsonArray()
         {