hubFS: THE place for F#

Hi,

I'm trying to figure a type inference problem but struggling somewhat. I'm interacting with a C# generic class

    abstract class BaseClass<StoreType, FunctionType>
    {
        public void AddFunction(int i, FunctionType f) { ... }
    }
    
    public delegate double SimpleFunction(double S);
    public delegate double ComplexFunction(double S, double T);
    
    class SimpleType : BaseClass<double, SimpleFunction>
    { ... }
    class ComplexType : BaseClass<long, ComplexFunction>
    { ... }
    
I'm trying to call AddFunction on the types

What I have at the minute is

let rec apply_between start_date end_date (var : #BaseClass<'a, 'b>) fn =
    if start_date < end_date then     
        var.AddFunction(start_date, fn)
        apply_between (start_date + 1) end_date var fn

type FSExample =
    inherit CSBaseClass as base
    
    let simple = new SimpleType()
    let complex = new ComplexType()
    
    let simple_fn x = x + 1.0;
    let complex_fn x y = x + y + 1.0;
        
    override self.Initialise() =
        // 10 20 are actually DateTimes but I don't think changes anything
        apply_between 10 20 simple (new SimpleFunction(simple_fn))
        apply_between 10 20 complex (new ComplexFunction(complex_fn))
    

I appear to need the (var : #BaseClass<'a, 'b>) to stop errors about ambiguous
types (correct?). It seems to me that I should be able to get rid of the explicit need
to create the Simple/ComplexFunction delegates and get them implied. But I haven't
managed it. I tried

let rec apply_between start_date end_date (var : #BaseClass<'a, 'b>) fn =
    if start_date < end_date then     
        var.AddFunction(start_date, new 'b (fn))
        apply_between (start_date + 1) end_date var fn
        
But get an error "Calls to object constructors on type parameters can not be given arguments".

Is there someway I can move the construction of the delegate inside apply_between in some
generic way? (Or since there's only a couple of options for this set up some explicit switch
on the types?)

Many Thanks
Ian