C# functiion by passing parameters : Calculate a c# function by passing parameters 

My function is something like this:
Result<double> Evaluate(string var, Result<double> height, Result<double> perm)
Var can be anything user wants to enter. For example, if the user enters FC then the calculation would be like height * perm and return the var.
If user enters SC the calculation is height/ perm and result is stored in var and returns the result