Check the test from Jacob Karlén

Author: fredericDelaporte

src/NHibernate.Test/Ado/BatcherFixture.cs

@@ -1,3 +1,4 @@
+using System.Linq;
 using NHibernate.AdoNet;
 using NHibernate.Cfg;
 using NUnit.Framework;
@@ -303,5 +304,56 @@ public void AbstractBatcherLogFormattedSql()
 			Assert.That(Sfi.Statistics.PrepareStatementCount, Is.EqualTo(1));
 			Cleanup();
 		}
+
+		[Test]
+		[Description("Inserting exactly BatchSize entities should not throw on commit. See GH-3725.")]
+		public void InsertExactlyBatchSizeEntitiesShouldNotThrowOnCommit()
+		{
+			// This test verifies that DbBatchBatcher handles empty batches correctly.
+			// The bug (GH-3725): When inserting exactly BatchSize entities, the batch auto-executes
+			// when full (via ExecuteBatchWithTiming), which clears _currentBatch but NOT _batchCommand.
+			// On commit, ExecuteBatch() is called, sees _batchCommand is set, and calls DoExecuteBatch
+			// on an empty _currentBatch, causing InvalidOperationException.
+
+			// BatchSize is configured as 10 in this fixture
+			const int batchSize = 10;
+
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				// Insert exactly BatchSize entities - this fills the batch and triggers auto-execution
+				for (int i = 0; i < batchSize; i++)
+				{
+					session.Save(new VerySimple { Id = 1000 + i, Name = $"Test{i}", Weight = i * 1.1 });
+				}
+
+				// Commit triggers ExecuteBatch() which would fail on empty batch without the fix
+				transaction.Commit();
+			}
+
+			Cleanup();
+		}
+
+		[Test]
+		[Description("Inserting a multiple of BatchSize entities should not throw on commit. See GH-3725.")]
+		public void InsertMultipleOfBatchSizeEntitiesShouldNotThrowOnCommit()
+		{
+			// Same issue as above but with multiple full batches
+			const int batchSize = 10;
+			const int multiplier = 3;
+
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				for (int i = 0; i < batchSize * multiplier; i++)
+				{
+					session.Save(new VerySimple { Id = 2000 + i, Name = $"Test{i}", Weight = i * 1.1 });
+				}
+
+				transaction.Commit();
+			}
+
+			Cleanup();
+		}
 	}
 }

src/NHibernate.Test/Async/Ado/BatcherFixture.cs

@@ -8,6 +8,7 @@
 //------------------------------------------------------------------------------
 
 
+using System.Linq;
 using NHibernate.AdoNet;
 using NHibernate.Cfg;
 using NUnit.Framework;
@@ -275,5 +276,56 @@ public async Task AbstractBatcherLogFormattedSqlAsync()
 			Assert.That(Sfi.Statistics.PrepareStatementCount, Is.EqualTo(1));
 			await (CleanupAsync());
 		}
+
+		[Test]
+		[Description("Inserting exactly BatchSize entities should not throw on commit. See GH-3725.")]
+		public async Task InsertExactlyBatchSizeEntitiesShouldNotThrowOnCommitAsync()
+		{
+			// This test verifies that DbBatchBatcher handles empty batches correctly.
+			// The bug (GH-3725): When inserting exactly BatchSize entities, the batch auto-executes
+			// when full (via ExecuteBatchWithTiming), which clears _currentBatch but NOT _batchCommand.
+			// On commit, ExecuteBatch() is called, sees _batchCommand is set, and calls DoExecuteBatch
+			// on an empty _currentBatch, causing InvalidOperationException.
+
+			// BatchSize is configured as 10 in this fixture
+			const int batchSize = 10;
+
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				// Insert exactly BatchSize entities - this fills the batch and triggers auto-execution
+				for (int i = 0; i < batchSize; i++)
+				{
+					await (session.SaveAsync(new VerySimple { Id = 1000 + i, Name = $"Test{i}", Weight = i * 1.1 }));
+				}
+
+				// Commit triggers ExecuteBatch() which would fail on empty batch without the fix
+				await (transaction.CommitAsync());
+			}
+
+			await (CleanupAsync());
+		}
+
+		[Test]
+		[Description("Inserting a multiple of BatchSize entities should not throw on commit. See GH-3725.")]
+		public async Task InsertMultipleOfBatchSizeEntitiesShouldNotThrowOnCommitAsync()
+		{
+			// Same issue as above but with multiple full batches
+			const int batchSize = 10;
+			const int multiplier = 3;
+
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				for (int i = 0; i < batchSize * multiplier; i++)
+				{
+					await (session.SaveAsync(new VerySimple { Id = 2000 + i, Name = $"Test{i}", Weight = i * 1.1 }));
+				}
+
+				await (transaction.CommitAsync());
+			}
+
+			await (CleanupAsync());
+		}
 	}
 }