using System;

​

public static class MyExtensions

{

    public static string ToSignedPercent(this double input)

    {

        return $"{(input >= 0 ? "+" : "")}{input.ToString("P1")}";

    }   

}

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine((0.12345).ToSignedPercent());

        Console.WriteLine((-0.6789).ToSignedPercent());

    }

}