Is there any reason why I cannot deconstruct dynamic values into typed variables?

[jshirota]

Is there any reason why I cannot deconstruct dynamic values into typed variables?

using System;

var cat = new Cat();

// This is OK.
(dynamic x, dynamic y) = cat;
var name = (string)x;
var age = (int)y;

// This is not.  Why?
// (string name, int age) = cat;

Console.WriteLine($"{name} {age}");

public class Cat
{
    // Some dynamic data from somewhere.
    private dynamic[] data = { "Leo", 8 };

    public void Deconstruct(out dynamic name, out dynamic age)
    {
        name = data[0];
        age = data[1];
    }
}

Originally posted by @jshirota in #4892

[333fred]

Think of the assignment as turning into cat.Deconstruct(out Type name, out Type age). If that doesn't work (it doesn't here), then the deconstruction assignment won't either.

[svick]

@333fred I don't think that answer is satisfying. For example if you have a class C with void Deconstruct(out int i, out int j), you can do (long x, long y) = c;, even though c.Deconstruct(out long x, out long y); won't compile. See SharpLab.

[333fred]

int->long is an implicit numeric conversion, and is a standard conversion. dynamic->int is not a standard conversion, and therefore cannot have the same substitution applied.

[jnm2]

Yeah, I want to know the answer to @FaustVX's point too. Why is it that assignment and out don't convert the same way? Is it for the same reason that C# doesn't allow properties to be passed to ref/out parameters, where an implicit temporary variable would have to be introduced?

[FaustVX]

@jnm2
[On Sharplab], the implicit call to Deconstruct already introduce temporary variable.

[jshirota]

I really don't see any consistent pattern here from the user's point of view.

C1.GetNumber(out long a);
C1.GetNumber(out dynamic b);
C2.GetNumber(out long c);
C2.GetNumber(out dynamic d);        // OK.
new C3().GetNumber(out long e);
new C3().GetNumber(out dynamic f);
new C4().GetNumber(out long g);
new C4().GetNumber(out dynamic h);  // OK.
(long i, _) = new C5();             // OK.
(dynamic j, _) = new C5();          // OK.
(long k, _) = new C6();
(dynamic l, _) = new C6();          // OK.

class C1 { public static void GetNumber(out int x) => x = 1; }
class C2 { public static void GetNumber(out dynamic x) => x = 1; }
class C3 { public void GetNumber(out int x) => x = 1; }
class C4 { public void GetNumber(out dynamic x) => x = 1; }
class C5 { public void Deconstruct(out int x, out int y) => (x, y) = (1, 2); }
class C6 { public void Deconstruct(out dynamic x, out dynamic y) => (x, y) = (1, 2); }