Solve the "writing comments" issue in CSharp generator

Now we are writing expressions and statements, therefore we do not have a good way to write comment at all the places we want, for example in between two expressions like this:
```
Foo(1, /* something */ 2)
```
or in between one big structure like this:
```
switch (foo)
{
// something
case 1:
break;
}
```