AVRO 4091: [C#] Allow previously parsed schemas to be referenced when parsing a schema (#3242)

* Allow previously parsed schemas to be referenced when parsing a schema

* Incorporate review feedback

* Fix comment

Author: rayokota

lang/csharp/src/apache/main/Schema/Schema.cs

@@ -228,7 +228,7 @@ internal static Schema ParseJson(JToken jtok, SchemaNames names, string encspace
         public static Schema Parse(string json)
         {
             if (string.IsNullOrEmpty(json)) throw new ArgumentNullException(nameof(json), "json cannot be null.");
-            return Parse(json.Trim(), new SchemaNames(), null); // standalone schema, so no enclosing namespace
+            return ParseInternal(json.Trim(), new SchemaNames(), null); // standalone schema, so no enclosing namespace
         }
 
         /// <summary>
@@ -238,7 +238,20 @@ public static Schema Parse(string json)
         /// <param name="names">list of named schemas already read</param>
         /// <param name="encspace">enclosing namespace of the schema</param>
         /// <returns>new Schema object</returns>
-        internal static Schema Parse(string json, SchemaNames names, string encspace)
+        public static Schema Parse(string json, SchemaNames names, string encspace = null)
+        {
+            if (string.IsNullOrEmpty(json)) throw new ArgumentNullException(nameof(json), "json cannot be null.");
+            return ParseInternal(json.Trim(), names, encspace); // standalone schema, so no enclosing namespace
+        }
+
+        /// <summary>
+        /// Parses a JSON string to create a new schema object
+        /// </summary>
+        /// <param name="json">JSON string</param>
+        /// <param name="names">list of named schemas already read</param>
+        /// <param name="encspace">enclosing namespace of the schema</param>
+        /// <returns>new Schema object</returns>
+        internal static Schema ParseInternal(string json, SchemaNames names, string encspace)
         {
             Schema sc = PrimitiveSchema.NewInstance(json);
             if (null != sc) return sc;

lang/csharp/src/apache/test/Schema/SchemaTests.cs

@@ -408,6 +408,25 @@ public void TestRecordCreationWithRecursiveRecord()
             Assert.AreEqual(schema, recordSchema.ToString());
         }
 
+        [TestCase]
+        public void TestRecordWithNamedReference()
+        {
+            string nestedSchema = "{\"name\":\"NestedRecord\",\"type\":\"record\",\"fields\":[{\"name\":\"stringField\",\"type\":\"string\"}]}";
+            // The root schema references the nested schema above by name only.
+            // This mimics tools that allow schemas to have references to other schemas.
+            string rootSchema = "{\"name\":\"RootRecord\",\"type\":\"record\",\"fields\":[{\"name\": \"nestedField\",\"type\":\"NestedRecord\"}]}";
+
+            NamedSchema nestedRecord = (NamedSchema) Schema.Parse(nestedSchema);
+
+            SchemaNames names = new SchemaNames();
+            names.Add(nestedRecord.SchemaName, nestedRecord);
+
+            // Pass the schema names when parsing the root schema and its reference.
+            RecordSchema rootRecord = (RecordSchema) Schema.Parse(rootSchema, names);
+            Assert.AreEqual("RootRecord", rootRecord.Name);
+            Assert.AreEqual("NestedRecord", rootRecord.Fields[0].Schema.Name);
+        }
+
         [TestCase("{\"type\":\"enum\",\"name\":\"Test\",\"symbols\":[\"A\",\"B\"]}",
                 new string[] { "A", "B" })]