[GitHub] [arrow] pitrou commented on a diff in pull request #35496: GH-33856: [C#] Implement C Data Interface for C#

["pitrou (via GitHub)" <gi...@apache.org>]

pitrou commented on code in PR #35496:
URL: https://github.com/apache/arrow/pull/35496#discussion_r1222602105


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csharp/src/Apache.Arrow/C/CArrowArrayExporter.cs:
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@@ -0,0 +1,211 @@
+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License.  You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+
+using System;
+using System.Runtime.InteropServices;
+using Apache.Arrow.Memory;
+
+namespace Apache.Arrow.C
+{
+    public static class CArrowArrayExporter
+    {
+        private unsafe delegate void ReleaseArrowArray(CArrowArray* cArray);
+        private static unsafe readonly NativeDelegate<ReleaseArrowArray> s_releaseArray = new NativeDelegate<ReleaseArrowArray>(ReleaseArray);
+
+        /// <summary>
+        /// Export an <see cref="IArrowArray"/> to a <see cref="CArrowArray"/>. Whether or not the
+        /// export succeeds, the original array becomes invalid. Clone an array to continue using it
+        /// after a copy has been exported.
+        /// </summary>
+        /// <param name="array">The array to export</param>
+        /// <param name="cArray">An allocated but uninitialized CArrowArray pointer.</param>
+        /// <example>
+        /// <code>
+        /// CArrowArray* exportPtr = CArrowArray.Create();
+        /// CArrowArrayExporter.ExportArray(array, exportPtr);
+        /// foreign_import_function(exportPtr);
+        /// </code>
+        /// </example>
+        public static unsafe void ExportArray(IArrowArray array, CArrowArray* cArray)
+        {
+            if (array == null)
+            {
+                throw new ArgumentNullException(nameof(array));
+            }
+            if (cArray == null)
+            {
+                throw new ArgumentNullException(nameof(cArray));
+            }
+            if (cArray->release != null)
+            {
+                throw new ArgumentException("Cannot export array to a struct that is already initialized.", nameof(cArray));
+            }

Review Comment:
   Then `CArrowSchemaExporter` should probably be modified as well.
   
   This is producer code, not consumer code. At this point, the structure is still uninitialized. "Uninitialized" in the C (or C++) sense, that is "may contain any arbitrary bytes", not "zero-initialized".
   
   Producer code therefore shouldn't care about what is already in the structure.
   
   cc @paleolimbot 



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