I've tried Excel Add-in (from Excel) and Open Solver where most, but not all of the Solver code is available. The MS Solver is a 'black box'; I have no visibility into what's under the hood. As I enter more datasets into the mix to optimize parameters, both versions fail with the OpenSolver failing sooner, i.e., with fewer datasets in the optimization mix.
So what I would want is either the complete VBA code for the built-in Excel solver or the complete VBA code for a superior iterative solver.
o Why exactly do you need the source code?
o Solver is a commercial product and the Excel add in is only a subset of it. You can have a two-week free trial at www.solver.com to see how it performs with your problem.
o If you are a company user, the yearly fee can be afforded. I never heard of a very powerful, complete and free solver.