MATLAB angle() to C# conversion

Question

I want to transfer to C#, a function that computes the phasor angle of an expression from MATLAB, angle(). I found that angle(x+yi)=atan2(y,x) but here comes my problem, I have a square root that depending on the values I give it is either positive or negative. But, in MATLAB if the sqrt function gets a negative, it returns an imaginary, unlike C# where it returns NaN.

So, how can I make the two codes give the same result?

ie MATLAB:

angle(a*1i-sqrt(expression))

C#:

Mathf.Atan2(a,-sqrt(expression))(what i do, and i think is wrong)

Answer 1

You could do the same thing Matlab does and use Complex math:

using System.Numerics;

public static class Foo
{
    public static double GetAngle(double a, double expression)
    {
        Complex cA = new Complex(0, a);
        Complex sqrt = Complex.Sqrt(expression);
        Complex result = cA - sqrt;
        return result.Phase;
    }
}

If you don't want to do that, you can see, that sqrt(expression) is a number on the (positive) imaginary axis if expression is negative meaning that a*i-sqrt(Expression) == (a-sqrt(abs(expression)))*i the phase of which is either pi/2 or 3*pi/2:

public static class Foo
{
    public static double GetAngle(double a, double expression)
    {
        if (expression < 0.0)
        {
            double result = a - Math.Sqrt(-expression);
            if (result > 0.0)
                return Math.PI * 0.5;
            else if (result < 0.0)
                return Math.PI * 1.5;
            else
                return 0.0;
        }
        else
            return Math.Atan2(a, -Math.Sqrt(expression));
    }
}