Question

Given the following flags,

  [Flags]
    public enum Operations
    {
        add = 1,
        subtract = 2,
        multiply = 4,
        divide = 8,
        eval = 16,
    }

How could I implement an IF condition to perform each operation? In my attempt, the first condition is true for add, eval, which is correct. However the first condition is also true for subtract, eval, which is incorrect.

        public double Evaluate(double input)
    {
        if ((operation & (Operations.add & Operations.eval)) == (Operations.add & Operations.eval))
            currentResult += input;
        else if ((operation & (Operations.subtract & Operations.eval)) == (Operations.subtract & Operations.eval))
            currentResult -= input;
        else
            currentResult = input;

        operation = null;

        return currentResult;
    }

I cannot see what the problem is.

Answer 1

Change your inner & to |:

if ((operation & (Operations.add | Operations.eval)) == (Operations.add | Operations.eval))

This is equivalent to:

if( ((operation & Operations.add)==Operations.add) &&
    ((operation & Operations.eval)==Operations.eval))

which might be more readable. You might also want to consider an Extension like this:

public static bool HasFlag(this Operations op, Operations checkflag)
{
    return (op & checkflag)==checkflag;
}

then you can do this:

if(operation.HasFlag(Operations.add) && Operations.HasFlag(Operations.eval))

which might be even more readable. Finally you could create this extension for even more fun:

public static bool HasAllFlags(this Operations op, params Operations[] checkflags)
{
    foreach(Operations checkflag in checkflags)
    {
        if((op & checkflag)!=checkflag)
            return false;
    }
    return true;
}

Then your expression could turn into:

if(operation.HasAllFlags(Operations.add, Operations.eval))