Let's see a simple way to model complex numbers in C#.

First of all, let's create a simple console project in .NET Core and add a class file into the solution. We choose Z as the name of the class to match the mathematical notation of complex numbers:

class Z 
{
   public double x;
   public double y;
}

We've added two variables, x and y of type double.

To express z = 5 + 10i we will have to type:

z = new Z() { x = 5, y = 10 };

It would be handier if we could type:

var z = new Z(5,10);

So let's provide a constructor to Z class:

public Z(double x, double y) 
{
   this.x = x; this.y = y;
}

Now we can use the shorter (and handier) form:

var z = new Z(-1 , 2);

What if we wanted to present the complex number in a mathematical way?

Let's override ToString() method.

public override string ToString()
{
   string sign = y >= 0 ? "+" : "";
   return $"{x}{sign}{y}i";
}

Now, this is possible:

Console.WriteLine(new Z(-5, -4));

The ouput will be:

-5-4i