官方文档

2.1-2.2

2.2-3.0

3.0-3.1

迁移

打开后缀为.csproj文件(如果使用vs,右键项目-编辑项目文件)，迁移之前的代码。

根据官方文档添加的包

    <FrameworkReference Include="Microsoft.AspNetCore.App" />
    <PackageReference Include="Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore" Version="3.1.0" />
    <PackageReference Include="Microsoft.AspNetCore.Identity.EntityFrameworkCore" Version="3.1.0" />
    <PackageReference Include="Microsoft.AspNetCore.Identity.UI" Version="3.1.0" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="3.1.0" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="3.1.0" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="3.1.0" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation" Version="3.1.0" Condition="'$(Configuration)' == 'Debug'" />

修改后

 右键项目重新生成，出现了一个错误，经过查找资料，我把<FrameworkReference Include="Microsoft.AspNetCore.App" />这个包去掉了，就解决了。

 修改Program.cs

 修改Startup.cs

using Microsoft.Extensions.Hosting;
#删除
public void ConfigureServices(IServiceCollection services)
{
   services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
     app.UseMvc(routes =>
     {
        routes.MapRoute(
         name: "default",
         template: "{controller=Home}/{action=Index}/{id?}");
     });
}

#添加
public void ConfigureServices(IServiceCollection services)
{
   services.AddControllersWithViews();
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseRouting();
    app.UseAuthorization();
    app.UseEndpoints(endpoints =>
    {
          endpoints.MapControllerRoute(
             name: "default",
             pattern: "{controller=Home}/{action=Index}/{id?}");
     });
}

重新生成项目，生成文件成功。