I have this C# base class I want to derive from in F#, which has a
helper method to set property values. This helper method does some
additional work automatically, like change notification, so I would
like to use it. Problem is, it's implemented using a "ref" parameter in
C#.
As far as I'm aware, the only way to call methods with C# ref parameters is by using a ref cell in F#, like so:

C#:

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public void DoSomething(ref string foo) { ... }

F#:

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let var = ref "Hi there"
testclass.DoSomething(var)

Now,
in my real use case I have classes with lots of mutable members and
properties that use said helper method. I'm currently using "mutable"
for all the members. I could make them all ref cells instead and that
would allow me to call the helper method without any fuss. But it
doesn't feel like the right solution, because (a) using ref cells
instead of variables everywhere introduces quite a bit of syntactical
overhead and (b) I'm guessing that ref cells are quite a bit less
performant if you have them everywhere - I'm happy to be corrected
about this.
Assuming I keep my "mutable" fields, I have found this way to implement my property setter code:

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let temp = ref x.var 
x.csharpclass.DoSomething(temp)
x.var <- !temp

Obvously this introduces overhead as well, albeit in a different place. And I don't know how easy it would be to abstract this away, so I wouldn't have to repeat that code in all my property setters - haven't looked into it.
Anyway, (thanks if you're still reading!) here's my question: is there some other way of handling these calls in F#? In C#, passing a parameter by reference or not is a choice I can make at the point of the call. In F#, it seems to influence the whole way I declare my members. Is there something I'm missing?
Thanks
Oliver