Add custom hashcode to SolidBrush.

Dashboard.Drawing/SolidBrush.cs

@@ -1,4 +1,5 @@
-using System.Drawing;
+using System;
+using System.Drawing;
 
 namespace Dashboard.Drawing
 {
@@ -22,6 +23,11 @@ namespace Dashboard.Drawing
         {
             Color = color;
         }
+
+        public override int GetHashCode()
+        {
+            return HashCode.Combine(Kind, Color);
+        }
     }
 
     public class SolidBrushExtension : DrawExtension