Maximum by row or column

(Sorry for my poor english)
For a post-treatement of finit element model, I need a function to get the maximum and minimum for a selection of values.
I have a matrix A (dimensions nn) that define the differents selections (0 value not to be considered, 1 value to be considered), and a matrix B (dimensions 1n) with the complete list of value.
I think there is to way to solve my problem.
1* : create a copy of the function multiply which give the maximum instead of the sum ( Cij=max(AikBkj) instead of Cij=sum(AikBkj) )
2* : a function that give the maximum (or minimum) by row or column for a given matrix

I work with VisualStudio Express 2013 under Windows7Pro 64bit. I have installed MathNet using NuGet packages manager.
I’d like to try creating this function using copy of existing functions (multiply or maximum), but I never open the source code of mathnet.numerics before, I even don’t know where to find it.

Best regards

Xavier Ducloyer

I have download the source from GitHub, now I’m looking for the original function multiply(matrixmatrix, not scalarmatrix) or maximum to create a modified version.

You use F# or C#? I’ll write an example

Thank you for your help.
I prefer the C#.
It is not an emergency, I’m on holyday so I will not work on this professional subject before 3 weeks.