添加项目文件。

Message/MyMessage.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.Messaging.Messages;
+
+namespace PMSWPF.Message
+{
+    public class MyMessage : ValueChangedMessage<int>
+    {
+        private int count;
+
+        public MyMessage(int value) : base(value)
+        {
+        }
+
+        public int Count { get => count; set => count = value; }
+    }
+}