From https://github.com/dotnet/roslyn/issues/347 (Details > Mutability):

  > Should tuples be mutable or immutable? The nice thing about them being 
  > structs is that the user can choose. If a reference to the tuple is 
  > readonly then the tuple is readonly. 
  
  > Now a local variable cannot be readonly, unless we adopt #115 (which is 
  > likely), but that isn't too big of a deal, because locals are only used 
  > locally, and so it is easier to stick to an immutable discipline if you 
  > so choose. 
  
  > If tuples are used as fields, then those fields can be readonly if 
  > desired.