添加项目文件。

MessageService.WebApi/DesignTimeDbContextFactory.cs

@@ -0,0 +1,18 @@
+using IM.InitCommon;
+using MessageService.Infrastructure;
+using Microsoft.EntityFrameworkCore.Design;
+
+namespace MessageService.WebApi
+{
+    public class DesignTimeDbContextFactory : IDesignTimeDbContextFactory<MessageDbContext>
+    {
+        public MessageDbContext CreateDbContext(string[] args)
+        {
+            // 1. 复用你写好的配置工厂，提取连接字符串
+            var optionsBuilder = DbContextOptionsBuilderFactory.Create<MessageDbContext>();
+
+            // 2. 🌟 关键补刀：把假的 Mediator 传进去，满足构造函数的要求！
+            return new MessageDbContext(optionsBuilder.Options, null);
+        }
+    }
+}