Say you have a table that contains a set of fields in a base model. Then, in another model, you add some new fields as extensions to the table metadata. It seems like a legitimate ask to be able to access the extension fields as easily from C# as you can in X++. As it happens, the story for accessing the extended fields from C# is a little more convoluted; in the future, we may consider adding C# extension methods to fix the problem, but for now you will need to use the setFieldValue and getFieldValue methods on the Common class (from which all tables are derived):
MyTable t; t.setFieldValue("MyExtensionField", expr);
where "MyExtensionField" is the name of the extension field and expr is the value you want to assign to it. There are similar getFieldValue methods on Common, with one overload accepting an integer describing an array index.
You might consider adding your own extension method to hide the gory details. It is unfortunate that you cannot do extension properties in C# yet, which would have been a good solution.