CSHARP-2720: Provide transaction example using new withTransaction API.

Author: DmitryLukyanov

tests/MongoDB.Driver.Examples/TransactionExamplesForDocs/WithTransactionExample1.cs

@@ -0,0 +1,103 @@
+/* Copyright 2019-present MongoDB Inc.
+*
+* Licensed 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.Threading;
+using FluentAssertions;
+using MongoDB.Bson;
+using MongoDB.Driver.Core.Clusters;
+using MongoDB.Driver.Core.Misc;
+using MongoDB.Driver.Core.TestHelpers.XunitExtensions;
+using Xunit;
+
+namespace MongoDB.Driver.Examples.TransactionExamplesForDocs
+{
+    public class WithTransactionExample1
+    {
+        [SkippableFact]
+        public void Example1()
+        {
+            RequireServer.Check().ClusterTypes(ClusterType.ReplicaSet, ClusterType.Sharded).Supports(Feature.Transactions);
+
+            var connectionString = CoreTestConfiguration.ConnectionString.ToString();
+            DropCollections(
+                connectionString, 
+                CollectionNamespace.FromFullName("mydb1.foo"), 
+                CollectionNamespace.FromFullName("mydb2.bar"));
+            string result = null;
+
+            // Start Transactions withTxn API Example 1
+            // For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
+            // string uri = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl";
+            // For a sharded cluster, connect to the mongos instances; e.g.
+            // string uri = "mongodb://mongos0.example.com:27017,mongos1.example.com:27017:27017/";
+            var client = new MongoClient(connectionString);
+
+            // Prereq: Create collections. CRUD operations in transactions must be on existing collections.
+            var database1 = client.GetDatabase("mydb1");
+            var collection1 = database1.GetCollection<BsonDocument>("foo").WithWriteConcern(WriteConcern.WMajority);
+            collection1.InsertOne(new BsonDocument("abc", 0));
+
+            var database2 = client.GetDatabase("mydb2");
+            var collection2 = database2.GetCollection<BsonDocument>("bar").WithWriteConcern(WriteConcern.WMajority);
+            collection2.InsertOne(new BsonDocument("xyz", 0));
+
+            // Step 1: Start a client session.
+            using (var session = client.StartSession())
+            {
+                // Step 2: Optional. Define options to use for the transaction.
+                var transactionOptions = new TransactionOptions(
+                    readPreference: ReadPreference.Primary,
+                    readConcern: ReadConcern.Local,
+                    writeConcern: WriteConcern.WMajority);
+
+                // Step 3: Define the sequence of operations to perform inside the transactions
+                var cancellationToken = CancellationToken.None; // normally a real token would be used
+                result = session.WithTransaction(
+                    (s, ct) =>
+                    {
+                        collection1.InsertOne(s, new BsonDocument("abc", 1), cancellationToken: ct);
+                        collection2.InsertOne(s, new BsonDocument("xyz", 999), cancellationToken: ct);
+                        return "Inserted into collections in different databases";
+                    },
+                    transactionOptions,
+                    cancellationToken);
+            }
+            //End Transactions withTxn API Example 1
+
+            result.Should().Be("Inserted into collections in different databases");
+
+            var collection1Documents = collection1.Find(FilterDefinition<BsonDocument>.Empty).ToList();
+            collection1Documents.Count.Should().Be(2);
+            collection1Documents[0]["abc"].Should().Be(0);
+            collection1Documents[1]["abc"].Should().Be(1);
+
+            var collection2Documents = collection2.Find(FilterDefinition<BsonDocument>.Empty).ToList();
+            collection2Documents.Count.Should().Be(2);
+            collection2Documents[0]["xyz"].Should().Be(0);
+            collection2Documents[1]["xyz"].Should().Be(999);
+        }
+
+        // private methods
+        private void DropCollections(string connectionString, params CollectionNamespace[] collectionNamespaces)
+        {
+            var client = new MongoClient(connectionString);
+            foreach (var collectionNamespace in collectionNamespaces)
+            {
+                var database = client.GetDatabase(collectionNamespace.DatabaseNamespace.DatabaseName);
+                database.DropCollection(collectionNamespace.CollectionName);
+            }
+        }
+    }
+}