using System;

public class Program

{

    public static void Main()

    {

        object retVal1 = ConvertTo("48".AsSpan(), typeof(int));

        object retVal2 = ConvertToOld("48".AsSpan(), typeof(int));

        object retVal3 = ConvertTo2("48".AsSpan(), typeof(int));

        Console.WriteLine(retVal1.GetType().Name);

        Console.WriteLine(retVal2.GetType().Name);

        Console.WriteLine(retVal3.GetType().Name);

    }

    public static object ConvertTo(ReadOnlySpan<char> key, Type keyType)

        => keyType switch

        {

            var t when t == typeof(int) => int.Parse(key),

            var t when t == typeof(double) => double.Parse(key),

            _ => throw new ArgumentException($"Key type is not supported: {keyType.Name}")

        };

    public static object ConvertTo2(ReadOnlySpan<char> key, Type keyType)

        => keyType switch

        {

            var t when t == typeof(int) => ConvertToInt32(key),

            var t when t == typeof(double) => ConvertToDouble(key),

            _ => throw new ArgumentException($"Key type is not supported: {keyType.Name}")

        };

    public static object ConvertToOld(ReadOnlySpan<char> key, Type keyType)

    {

        switch (keyType) 

        {

            case var t when t == typeof(int):

                return int.Parse(key);

            case var t when t == typeof(double):

                return double.Parse(key);

        }

        return null;

    }

    private static int ConvertToInt32(scoped ReadOnlySpan<char> key)

    {

        return int.Parse(key);

    }

    private static double ConvertToDouble(scoped ReadOnlySpan<char> key)

    {

        return double.Parse(key);

    }

}