序列化对象列表时出现错误

在调用按钮单击事件处理程序的方法时收到此错误:

System.Runtime.Serialization。InvalidDataContractException'发生在Mscorlib.dll，但未在用户代码中处理

这是我的处理器:

private async Task WriteAsync() 
        {
            var objectGraph = getObjectGraph();
           var serializer = new  DataContractSerializer(typeof(List<Car>));
            using (var stream = await ApplicationData.Current.LocalFolder.OpenStreamForWriteAsync(myFile, CreationCollisionOption.ReplaceExisting)) 
            {
                serializer.WriteObject(stream, objectGraph);
            }
            txt.Text = "Write Succeed !!!";
        }