fix(csharp): try fallback entrypoint on .NET Framework

csharp/src/Apache.Arrow.Adbc/C/CAdbcDriverImporter.cs

@@ -91,6 +91,12 @@ private static AdbcDriver Load(
                     export = NativeLibrary.GetExport(library, entryPoint);
                 }
                 catch (EntryPointNotFoundException) when (fallbackEntryPoint != null)
+                {
+                    export = IntPtr.Zero;
+                }
+                // The runtime's NativeLibrary.GetExport throws for a missing export,
+                // but the netstandard2.0 NativeLibrary shim returns IntPtr.Zero
+                if (export == IntPtr.Zero && fallbackEntryPoint != null)
                 {
                     export = NativeLibrary.GetExport(library, fallbackEntryPoint);
                 }

csharp/test/Apache.Arrow.Adbc.Tests/CAdbcDriverImporterFallbackTests.cs

@@ -0,0 +1,44 @@
+/*
+ * 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.IO;
+using System.Runtime.InteropServices;
+using Apache.Arrow.Adbc.C;
+using Xunit;
+
+namespace Apache.Arrow.Adbc.Tests
+{
+    public class CAdbcDriverImporterFallbackTests
+    {
+        // When the library doesn't export the requested entrypoint, the fallback
+        // entrypoint should be used. DuckDB only exports duckdb_adbc_init.
+        [Fact]
+        public void LoadWithFallback_MissingEntrypoint_UsesFallback()
+        {
+            string duckDb = Path.Combine(
+                Directory.GetCurrentDirectory(),
+                RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? "duckdb.dll"
+                : RuntimeInformation.IsOSPlatform(OSPlatform.OSX) ? "libduckdb.dylib"
+                : "libduckdb.so");
+
+            using AdbcDriver driver = CAdbcDriverImporter.LoadWithFallback(
+                duckDb, "AdbcTestNoSuchEntrypoint", "duckdb_adbc_init");
+
+            Assert.NotNull(driver);
+        }
+    }
+}