C# code: public class Class1 { public delegate void CancelHandler(Object sender); public CancelHandler onCancel; public void Verify() { if (onCancel == null) { //do something; } } } Usually , the null is converted to option type in F# , but in this situation , that’s a little difficult to make it works. F# code: type public Class1() = [] val mutable public onCancel : System.Object -> unit member public this.Verify() = if box(this.onCancel) = null then ()//do something