Manhattan distance between A and B (a.k.a L1-norm) is
Sum(abs(a[i] - b[i]))
When Euclidian one between A and B (a.k.a L2-norm) is
Sqrt(Sum((a[i] - b[i])**2))
where a[i], b[i] are corresponding coordinates of A and B points. So the code could be
private double Manhattan_H()
{
double xd = this.x - this._goalNode.x;
double yd = this.y - this._goalNode.y;
return Math.Abs(xd) + Math.Abs(yd);
}