Updated Example C# code showing MongoDB Transactions using the MongoDB .NET Driver (2.7+)

CSharpTransactions.cs

using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
using System;
using System.Threading.Tasks;

namespace MongoDBTransaction
{
    public static class Program
    {
        public class Product
        {
            [BsonId]
            public ObjectId Id { get; set; }
            [BsonElement("SKU")]
            public int SKU { get; set; }
            [BsonElement("Description")]
            public string Description { get; set; }
            [BsonElement("Price")]
            public Double Price { get; set; }
        }

        // replace with your connection string if it is different
        const string MongoDBConnectionString = "mongodb://localhost"; 

        public static async Task Main(string[] args)
        {
            if (!await UpdateProductsAsync()) { Environment.Exit(1); }
            Console.WriteLine("Finished updating the product collection");
            Console.ReadKey();
        }

        private static async Task<bool> UpdateProductsAsync()
        {
            // Create client connection to our MongoDB database
            var client = new MongoClient(MongoDBConnectionString);

            // Create the collection object that represents the "products" collection
            var database = client.GetDatabase("MongoDBStore");
            var products = database.GetCollection<Product>("products");

            // Clean up the collection if there is data in there
            await database.DropCollectionAsync("products");

            // collections can't be created inside a transaction so create it first
            await database.CreateCollectionAsync("products"); 

            // Create a session object that is used when leveraging transactions
            using (var session = await client.StartSessionAsync())
            {
                // Begin transaction
                session.StartTransaction();

                try
                {
                    // Create some sample data
                    var tv = new Product { Description = "Television", 
                                  SKU = 4001, 
                                  Price = 2000 };
                    var book = new Product { Description = "A funny book", 
                                  SKU = 43221, 
                                  Price = 19.99 };
                    var dogBowl = new Product { Description = "Bowl for Fido", 
                                  SKU = 123, 
                                  Price = 40.00 };

                    // Insert the sample data 
                    await products.InsertOneAsync(session, tv);
                    await products.InsertOneAsync(session, book);
                    await products.InsertOneAsync(session, dogBowl);

                    var resultsBeforeUpdates = await products
                                  .Find<Product>(session, Builders<Product>.Filter.Empty)
                                  .ToListAsync();
                    Console.WriteLine("Original Prices:\n");
                    foreach (Product d in resultsBeforeUpdates)
                    {
                        Console.WriteLine(
                                  String.Format("Product Name: {0}\tPrice: {1:0.00}", 
                                        d.Description, d.Price)
                        );
                    }

                    // Increase all the prices by 10% for all products
                    var update = new UpdateDefinitionBuilder<Product>()
                           .Mul<Double>(r => r.Price, 1.1);
                    await products.UpdateManyAsync(session, 
                           Builders<Product>.Filter.Empty, 
                           update); //,options);

                    // Made it here without error? Let's commit the transaction
                    await session.CommitTransactionAsync();
                }
                catch (Exception e)
                {
                    Console.WriteLine("Error writing to MongoDB: " + e.Message);
                    await session.AbortTransactionAsync();
                    return false;
                }

                // Let's print the new results to the console
                Console.WriteLine("\n\nNew Prices (10% increase):\n");
                var resultsAfterCommit = await products
                       .Find<Product>(session, Builders<Product>.Filter.Empty)
                       .ToListAsync();
                foreach (Product d in resultsAfterCommit)
                {
                    Console.WriteLine(
                        String.Format("Product Name: {0}\tPrice: {1:0.00}", 
                                                 d.Description, d.Price)
                    );
                }

                return true;
            }
        }
    }
}