Migrated the project to dotnet core 3.0

src/NotifyIconWpf/Interop/IconState.cs

@@ -0,0 +1,22 @@
+namespace Hardcodet.Wpf.TaskbarNotification.Interop
+{
+    /// <summary>
+    /// The state of the icon - can be set to
+    /// hide the icon.
+    /// </summary>
+    public enum IconState
+    {
+        /// <summary>
+        /// The icon is visible.
+        /// </summary>
+        Visible = 0x00,
+
+        /// <summary>
+        /// Hide the icon.
+        /// </summary>
+        Hidden = 0x01,
+
+        // The icon is shared - currently not supported, thus commented out.
+        //Shared = 0x02
+    }
+}