Question

I tried to catch an Exception but the compiler gives warning: This type test or downcast will always hold

let testFail () =
    try
        printfn "Ready for failing..."
        failwith "Fails"
    with
    | :? System.ArgumentException -> ()
    | :? System.Exception -> ()

The question is: how to I do it without the warning? (I believe there must be a way to do this, otherwise there should be no warning)

Like C#

try
{
    Console.WriteLine("Ready for failing...");
    throw new Exception("Fails");
}
catch (Exception)
{
}

Answer 1

C#:

void testFail()
{
    try
    {
        Console.WriteLine("Ready for failing...");
        throw new Exception("Fails");
    }
    catch (ArgumentException)
    {
    }
    catch
    {
    }
}

F# equivalent:

let testFail () =
    try
        printfn "Ready for failing..."
        failwith "Fails"
    with
    | :? System.ArgumentException -> ()
    | _ -> ()

---

C#:

void testFail()
{
    try
    {
        Console.WriteLine("Ready for failing...");
        throw new Exception("Fails");
    }
    catch (ArgumentException ex)
    {
    }
    catch (Exception ex)
    {
    }
}

F# equivalent:

let testFail () =
    try
        printfn "Ready for failing..."
        failwith "Fails"
    with
    | :? System.ArgumentException as ex -> ()
    | ex -> ()

---

C#:

void testFail()
{
    try
    {
        Console.WriteLine("Ready for failing...");
        throw new Exception("Fails");
    }
    catch
    {
    }
}

F# equivalent:

let testFail () =
    try
        printfn "Ready for failing..."
        failwith "Fails"
    with
    | _ -> ()

---

As Joel noted, you would not want to use catch (Exception) in C# for the same reason you don’t use | :? System.Exception -> in F#.