CSHARP-2221: Throw exception if StartTransaction is called with unacknowledged write concern.

Author: rstam

src/MongoDB.Driver.Core/Core/Bindings/CoreSession.cs

@@ -347,9 +347,12 @@ public void StartTransaction(TransactionOptions transactionOptions = null)
 
             var transactionNumber = AdvanceTransactionNumber();
             var effectiveTransactionOptions = GetEffectiveTransactionOptions(transactionOptions);
-            var transaction = new CoreTransaction(transactionNumber, effectiveTransactionOptions);
+            if (!effectiveTransactionOptions.WriteConcern.IsAcknowledged)
+            {
+                throw new InvalidOperationException("Transactions do not support unacknowledged write concerns.");
+            }
 
-            _currentTransaction = transaction;
+            _currentTransaction = new CoreTransaction(transactionNumber, effectiveTransactionOptions);
         }
 
         /// <inheritdoc />

tests/MongoDB.Driver.Core.Tests/Core/Bindings/CoreSessionTests.cs

@@ -13,6 +13,7 @@
 * limitations under the License.
 */
 
+using System;
 using System.Reflection;
 using FluentAssertions;
 using MongoDB.Bson;
@@ -217,6 +218,19 @@ public void Dispose_should_have_expected_result(
             Mock.Get(subject.ServerSession).Verify(m => m.Dispose(), Times.Once);
         }
 
+        [Fact]
+        public void StartTransaction_should_throw_when_write_concern_is_unacknowledged()
+        {
+            var cluster = CoreTestConfiguration.Cluster;
+            var session = cluster.StartSession();
+            var transactionOptions = new TransactionOptions(writeConcern: WriteConcern.Unacknowledged);
+
+            var exception = Record.Exception(() => session.StartTransaction(transactionOptions));
+
+            var e = exception.Should().BeOfType<InvalidOperationException>().Subject;
+            e.Message.ToLower().Should().Contain("transactions do not support unacknowledged write concerns");
+        }
+
         [Fact]
         public void WasUsed_should_call_serverSession()
         {