C#

using System;
using System.Collections;

namespace TestIt
{
	class Class1
	{
		[STAThread]
		static void Main(string[] args)
		{
			SomeStruct val1 = new SomeStruct(1.0);
			SomeStruct val2 = new SomeStruct(1.0);
			SomeStruct val3 = new SomeStruct(1.00000000009999);
			SomeStruct val4 = new SomeStruct(0.99999999999);

			Console.WriteLine("Starting . . .");
			if (val1.Equals(val2))
				Console.WriteLine("val1 == val2");
			if (val2.Equals(val3))
				Console.WriteLine("val2 == val3");
			if (val1.Equals(val3))
				Console.WriteLine("val1 == val3");
			if (val1.Equals(val4))
				Console.WriteLine("val1 == val4");
			if (val3.Equals(val4))
				Console.WriteLine("val3 == val4"); // problem, this doesn't happen
			Console.WriteLine("Done");
		}

		public struct SomeStruct
		{
			private static double accuracy = 0.0000000001;
			public double Value;

			public SomeStruct(double val)
			{
				Value = val;
			}

			public override bool Equals(object obj)
			{
				if (obj is SomeStruct)
					return (Math.Abs(Value - ((SomeStruct)obj).Value) < accuracy);
				return false;
			}
		}
	}
}