Using `is` operator with value type tuples gives error

Question

I am trying to check if an object variable is (int, int) and if so I will use the casted variable so I have tried the codes below:

//this one gives the error
public void MyMethodWithIs(object val)
{
    if(val is (int id, int name) pair)
    {
        ConsoleWriteLine($"{pair.id}, {pair.name}");
    }
}

//This one works
public void MyMethodWithAs(object val)
{
    var pair = val as (int id, int name)?;
    if(pair!=null)
    {
        ConsoleWriteLine($"{pair.id}, {pair.name}");
    }
}

The MyMethodWithIs method gives the error below in the editor:

No suitable deconstruct instance or extension method was found for type

My Question

Why one works fine but the other gives an error at all? I think MyMethodWithIs more readable and suitable to use for my case but I can't use it due to giving an error.

Reorganization

AFAIK value tuples don't store item name information, so there is probably no way to check if the items are id and name. I am guessing it might work without names if (val is (int, int) pair) – Coan

@Coan I also tried it, but the same error appears. – Reorganization

if (val is ValueTuple<int,string> pair) works though. Somewhat related: #44706998 – Sikata

Answer 1

Using C# 8's pattern matching capabilities, you can write this:

if (val is (int id, int name))
{
    Console.WriteLine($"id: {id}; name: {name}");
}

However this boxes id and name, which is surprising. There's a proposal to optimize this.

Below C# 8, you can write this:

if (val is ValueTuple<int, int> pair)
{
    Console.WriteLine($"id: {pair.Item1}; name: {pair.Item2}");
}

You can of course make it a bit clearer:

if (val is ValueTuple<int, int> pair)
{
    var (id, name) = pair;
    Console.WriteLine($"id: {id}; name: {name}");
}

It looks like you can also deconstruct the ValueTuple inline:

if (val is ValueTuple<int, int>(var id, var name))
{
    Console.WriteLine($"id: {id}; name: {name}");
}

... which is somewhat horrific, but appears to be legal.

I might have expected val is (int, int) pair) to work, but it seems nobody's designed this case (yet).

Bakker

The first code block gives the same error. The second one works but I don't want to use Item1, Item2 etc. – Reorganization

@Reorganization It works in sharplab - might be new in C# 8. sharplab.io/… – Bakker

@Reorganization found another way, added to my answer – Bakker

I think my editor does not let me to use c#8 yet (I think because it is preview). Your updated answer is probably the closest I can get for now. – Reorganization