NegativeStringLength.cs

using System;
using System.Runtime.InteropServices;

class Program
{
	[StructLayout(LayoutKind.Explicit)]
	struct Union
	{
		[FieldOffset(0)]
		public string str;
		[FieldOffset(0)]
		public FakeString fakeStr;
	}

	class FakeString
	{
		public int length;
	}


	static void Main()
	{
		var union = default(Union);
		union.str = "Hello, World!";
		union.fakeStr.length = -42;

		Console.WriteLine("Hello, World!".Length); // -42
	}
}