Please try the following. In Java you need to pass an array to the toArray
method, but that's not correct in C# (.NET).
//C# class definitions, constructors, fields, methods etc here.
//sphbasis is a ArrayList object.
public SphericalHarmonicDecomposition[] getSphericalHarmonicBasis() {
return (SphericalHarmonicDecomposition[])(sphbasislist.ToArray());
}