C#

decimal average = lValues.Skip(1)
    .Zip(lValues, (second, first) => second == 0 ? 1 : (second - first) / second)
    .Average();

// Input:    { 100, 0, 120, 118, 126, 102 }
// % Change: { 1, 1, -0.0169491525423728813559322034, 0.0634920634920634920634920635, -0.2352941176470588235294117647 }
// Output:   0.3622497586605263574356296191

Values.Select(
  p =>
    new {avg = Values.Skip(1).Where(x => index < Values.Count).Select(x => (x - Values[index++])/x).Average()})
  .Select(x => new {x, last = Values.Last()})
  .Select(@t => new {@t, aver = @t.last + (@t.last*@t.x.avg)})
  .Select(@t => new {value1 = @t.aver, value2 = @t.aver + (@t.aver*@t.@t.x.avg)}).First();

Values.Select(
  p =>
    new
    {
      avg = Values.Skip(1).Take(Values.Count - 1)
        .Zip(Values, (second, first) => (second - first)/second)
        .Average()
    })
  .Select(x => new {x, last = Values.Last()})
  .Select(@t => new {@t, aver = @t.last + (@t.last*@t.x.avg)})
  .Select(@t => new {value1 = @t.aver, value2 = @t.aver + (@t.aver*@t.@t.x.avg)}).First();

Quote:

Hiding the existence of the setter with an interface already protects against unintentional abuse, so I would say that's enough.

0x01AA wrote:

But like in c++ one can cast the interface and have afterwards full access (in c++ this means a dirty const_cast)

Quote:

you do not trust your users then you cannot directly expose the data at all.

public interface IReadOnlySpectrum
{
    double this[int lambda] { get; }
}
public interface IWritableSpectrum : IReadOnlySpectrum
{
    new double this[int lambda] { get; set; }
}
public class Spectrum : IWritableSpectrum {...}

echo off
cd %~dp0

rem The following two path you Need to adjust to your needs
set regasm_tool= "C:\Windows\Microsoft.NET\Framework\v4.0.30319\RegAsm.exe"
set assembly= "C:\YourProject\YourComVisible.dll"

rem unregister previous version
%regasm_tool% %assembly%  /u

rem Register actual Version and create a YourComVisible.tlb
%regasm_tool% %assembly%  /codebase /tlb

pause

public interface ISomething
{
    void SomeMethod();     // Existing method signature.
    void NewMethod();      // New method signature to add. This would be nice but is a breaking change!
}

// Syntax here is arbitrary and not representative
// of any working implementation.

public interface ISomething
{
    void SomeMethod();     // Existing method signature.
}


// Now extend the above interface...
public extension interface ISomethingEx : ISomething
{
    void NewMethod()
    {
        ... some default implementation;
    }
}