title: C# Driver MongoDb: How to use "UpdateOneAsync"
body: <p>I would like to update only a single field of a document. I mean to understand that I have to use <code>UpdateOneAsync</code>. When I try to do this, I always get <code>MongoDB.Bson.BsonSerializationException : Element name 'Test' is not valid'.</code>.</p>
  
  <p>The following code reproduces my problem (xUnit, .NET Core, MongoDb in Docker).</p>
  
  <pre class="lang-cs prettyprint-override"><code>public class Fixture
  {
      public class DummyDocument : IDocument
      {
          public string Test { get; set; }
  
          public Guid Id { get; set; }
  
          public int Version { get; set; }
      }
  
      [Fact]
      public async Task Repro()
      {
          var db = new MongoDbContext("mongodb://localhost:27017", "myDb");
          var document = new DummyDocument { Test = "abc", Id = Guid.Parse("69695d2c-90e7-4a4c-b478-6c8fb2a1dc5c") };
          await db.GetCollection&lt;DummyDocument&gt;().InsertOneAsync(document);
          FilterDefinition&lt;DummyDocument&gt; u = new ExpressionFilterDefinition&lt;DummyDocument&gt;(d =&gt; d.Id == document.Id);
          await db.GetCollection&lt;DummyDocument&gt;().UpdateOneAsync(u, new ObjectUpdateDefinition&lt;DummyDocument&gt;(new DummyDocument { Test = "bla" }));
      }
  }
  </code></pre>