file.cs

using System;
using System.Collections.Generic;

public static class C {
    public static void M() {
        var xs = new Dictionary<int, string>();
        foreach ((int id, string key) in xs) {
            // ...
            // int a = 0, b = 1;
            // ...
            // GC.KeepAlive((b, a) = (1, 2));
        }
        
        foreach (var (id, key) in xs) {
        	// ...
            // id++;
        }
    }
    
    public static void Deconstruct<T1, T2>(this KeyValuePair<T1, T2> kv, out T1 item1, out T2 item2) {
        item1 = kv.Key;
        item2 = kv.Value;
    }
}
        
namespace System {
    public struct ValueTuple<T1, T2> {
        public T1 Item1;
        public T2 Item2;
        
        public ValueTuple(T1 item1, T2 item2) : this() { }
    }
}