Добавьте файлы проекта.

CairoObjective/DrawObjects/Line.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace CairoObjective.DrawObjects
+{
+    public class Line
+    {
+        public double X1, X2, Y1, Y2;
+
+        public Line(double x1, double y1, double x2, double y2)
+        {
+            X1 = x1;
+            X2 = x2;
+            Y1 = y1;
+            Y2 = y2;
+        }
+    }
+}