Add polygon shape

StructureHelperCommon/Models/Shapes/IPolygonShape.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperCommon.Models.Shapes
+{
+    public interface IPolygonShape : IShape
+    {
+        IReadOnlyList<IVertex> Vertices { get; }
+        bool IsClosed { get; set; }
+
+        IVertex AddVertex(IVertex vertex);
+        IVertex InsertVertex(int index, IVertex vertex);
+        IVertex AddVertexBefore(IVertex existing, IVertex vertex);
+        IVertex AddVertexAfter(IVertex existing, IVertex vertex);
+
+        void RemoveVertex(IVertex vertex);
+        void Clear();
+    }
+
+}