using System;

using System.Runtime.CompilerServices;

using System.Globalization;

​

using NUnit.Framework;

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//https://stackoverflow.com/questions/73531166/how-to-reconstruct-the-return-shape-of-a-generic-method-which-has-generic-return

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#nullable enable

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public class Program

{

    public static void Main(string[] args)

    {

        //???

        var test1 = Transfer<(int? Result, string? Error)>(new () { Result = "10101", Error = null });

        var test2 = Transfer<(bool? Result, string? Error)>(new () { Result = "true", Error = null });

        var test3 = Transfer<(object? Result, string? Error)>(new () { Result = "Hello", Error = null });

        var test4 = Transfer<Tuple<string, string>>(new () { Result = "Hello", Error = null });

​

        test1.Dump();

        Assert.AreEqual(test1.Result, 10101);

        test2.Dump();

        Assert.AreEqual(test2.Result, true);

        test3.Dump();

        Assert.AreEqual(test3.Result, "Hello");

        test4.Dump();

​

        Assert.Throws<ArgumentException>(() =>  Transfer<(object? Result, string? Error, string? dummy)>(new () { Result = "Hello", Error = null }));

    }

​

    public static U Transfer<U>(MessageBundle bundle)

    {

        if (!typeof(ITuple).IsAssignableFrom(typeof(U))  // Assert it's a tuple

            || !(typeof(U).IsGenericType && typeof(U).GetGenericArguments() is var arguments && arguments.Length == 2) // Assert it has exactly two generic parameters

            || arguments[1] != typeof(string)) // Assert that the second parameter is a string

            throw new ArgumentException(string.Format("Unexpected type {0}", typeof(U)));

        var tuple = (U)Activator.CreateInstance(typeof(U), 

                                                bundle.Result == null ? null : Convert.ChangeType(bundle.Result, Nullable.GetUnderlyingType(arguments[0]) ?? arguments[0]), 

                                                bundle.Error)!;

        return tuple;

    }

}

​

​

public class MessageBundle

{

    public string? Error { get; set; }

    public object? Result { get; set; }

}

​