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How do I implement an F# high order function in C#?

public ICommand RequestAccount = 
 new DelegateCommand(FuncConvert.ToFSharpFunc( _ => Debug.WriteLine() ), 
 FuncConvert.ToFSharpFunc( _ => return true )
 );

Error CS0411 The type arguments for method
'FuncConvert.ToFSharpFunc(Action)' cannot be inferred from the
usage. Try specifying the type arguments explicitly.

Based on the error, I'm not aware of how to express the type parameters explicitly. Hence, I don't think C# understands what a unit is that's to be returned on the first lambda.

DelegateCommand

type DelegateCommand (action:(obj -> unit), canExecute:(obj -> bool)) =
 let event = new DelegateEvent<EventHandler>()
 interface ICommand with
 [<CLIEvent>]
 member this.CanExecuteChanged = event.Publish
 member this.CanExecute arg = canExecute(arg)
 member this.Execute arg = action(arg

If you are in control of both the C# and the F# part of the code, then I would not try to create F# functions explicitly from C# - that will just make your C# code ugly. You can easily add a static method that will take Func and Action delegates and provide a C#-friendly interface:

type DelegateCommand (action:(obj -> unit), canExecute:(obj -> bool)) =
 let event = new DelegateEvent<EventHandler>()
 interface ICommand with
 [<CLIEvent>]
 member this.CanExecuteChanged = event.Publish
 member this.CanExecute arg = canExecute(arg)
 member this.Execute arg = action(arg)
 static member Create(action:Action<obj>, canExecute:Func<obj, bool>) = 
 DelegateCommand(action.Invoke, canExecute.Invoke)

Now you can use DelegateCommand.Create from C# in a nice way:

DelegateCommand.Create(
 (o => Console.WriteLine(o)),
 (o => true) )

For the record, I also do not quite see the value of defining DelegateCommand in F# and using that from C# if you are not doing anything else on the F# side - it seems like a simple type that could as well be defined in C# (i.e. you are not gaining much by doing that in F#).

Try to explicitly specify the arguments type

public ICommand RequestAccount =
 new DelegateCommand(FuncConvert.ToFSharpFunc<object>(obj => Debug.WriteLine(obj)),
 FuncConvert.ToFSharpFunc<object, bool>(_ => true));