horror.cs

void Main()
{
	string input = Console.ReadLine();
	
	input.EatHorror("abcdefg");
	
	input.Dump("Eaten");
	(input + " - Appended").Dump("Fix?");
	"hello".Dump("Try entering \"hello\"");
}

public static class Ext
{
	// Unspeakable things
	public static unsafe void EatHorror(this string s, string r)
	{
		fixed (char* c = string.Intern(s)) for (int i = 0; i < Math.Min(s.Length, r.Length); c[i] = r[i++]) ;
	}
}

Example where input was hello.