使用RazorEngine生成HTML邮件模板时遇System.CodeDom加载失败问题

问题描述 #

在ASP.NET Core MVC项目中使用RazorEngine的Engine.Razor.RunCompile()或TemplateService.Parse()方法生成HTML邮件模板时，触发以下运行时错误：

System.IO.FileNotFoundException: 'Could not load file or assembly 'System.CodeDom, Version=0.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. The system cannot find the file specified.'

即使是极简示例也会触发错误：

using RazorEngine;
using RazorEngine.Templating;

string template = "Hello @Model.Name, welcome to RazorEngine!";
var result = Engine.Razor.RunCompile(template, "templateKey", null, new { Name = "World" });

用户已安装RazorEngine包，参考官方文档及相关方案未解决，测试代码如下：

using Microsoft.AspNetCore.Mvc;
using OperationalForecast.Models;
using OperationalForecast.Models.EmailTemplateModels;
using OperationalForecast.Services;
using RazorEngine;
using RazorEngine.Configuration;
using RazorEngine.Templating;
using System.Diagnostics;
using System.Reflection;
using System.Text;

namespace OperationalForecast.Controllers
{
    public class HomeController : Controller
    {
        private readonly ILogger<HomeController> _logger;
        private IDistrictServices districtlist { get; set; }
        private IEmailServices emailservices { get; set; }

        public HomeController(ILogger<HomeController> logger, IDistrictServices dataservice, IEmailServices emailservices)
        {
            _logger = logger;
            districtlist = dataservice;
            this.emailservices = emailservices;
        }

        public IActionResult Index()
        {
            return View(districtlist.GetAllDistricts());
        }


        [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
        public IActionResult Error()
        {
            return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
        }

    
        public string SendTestEmail2() // 测试模板渲染的方法
        {
            string template = "Hello @Model.Name, welcome to RazorEngine!";
            var result = Engine.Razor.RunCompile(template, "templateKey", null, new { Name = "World" });

            var approvaltemplate = new ApprovalEmailTemplate {district_name ="Akron", hire_date=new DateTime(2024,1,1), class_size=10, forecasted_need=5 };
            string forparsing = @"class size is @Model.class_size";
            var templateService = new TemplateService();
            var emailhtmlbody = Engine.Razor.RunCompile(forparsing, "CreatedNewHireClassApproval", null, approvaltemplate);

            EmailModel newEmail = new EmailModel { ToEmail = "tristenhannah@usicllc.com", Subject = "test", Body = emailhtmlbody, IsHtml = true };
            
            return emailservices.SendEmail(newEmail).ToString();
        }
    }
}

解决方案 #

方案一：修复RazorEngine依赖问题 #

RazorEngine是针对.NET Framework开发的旧库，在.NET Core环境下需要手动补充缺失的依赖：

1. 安装System.CodeDom NuGet包：
   在NuGet包管理器中搜索并安装System.CodeDom，或使用命令行：

   Install-Package System.CodeDom
   

2. 如果项目是.NET 5+，需确保项目文件（.csproj）中包含以下引用：

   <ItemGroup>
     <PackageReference Include="System.CodeDom" Version="7.0.0" />
   </ItemGroup>
   

方案二：替换为ASP.NET Core原生Razor视图渲染（推荐） #

RazorEngine对.NET Core的兼容性较差，官方原生的Razor视图引擎更稳定可靠，步骤如下：

1. 创建邮件模板视图
   在项目中创建Views/EmailTemplates目录，添加ApprovalEmail.cshtml模板文件：

   @model ApprovalEmailTemplate
   <html>
     <body>
       <p>地区：@Model.district_name</p>
       <p>入职日期：@Model.hire_date.ToString("yyyy-MM-dd")</p>
       <p>班级规模：@Model.class_size</p>
       <p>预计需求：@Model.forecasted_need</p>
     </body>
   </html>
   

2. 封装视图渲染服务
   创建一个服务类用于渲染Razor视图为字符串：

   using Microsoft.AspNetCore.Mvc;
   using Microsoft.AspNetCore.Mvc.Razor;
   using System.IO;
   using System.Threading.Tasks;
   
   public class RazorViewRenderer
   {
       private readonly IRazorViewEngine _viewEngine;
       private readonly ITempDataProvider _tempDataProvider;
       private readonly IServiceProvider _serviceProvider;
   
       public RazorViewRenderer(IRazorViewEngine viewEngine, ITempDataProvider tempDataProvider, IServiceProvider serviceProvider)
       {
           _viewEngine = viewEngine;
           _tempDataProvider = tempDataProvider;
           _serviceProvider = serviceProvider;
       }
   
       public async Task<string> RenderViewToStringAsync<TModel>(string viewName, TModel model)
       {
           var actionContext = new ActionContext(new DefaultHttpContext { RequestServices = _serviceProvider }, new System.Routing.RouteData(), new ActionDescriptor());
   
           using var sw = new StringWriter();
           var viewResult = _viewEngine.FindView(actionContext, viewName, false);
   
           if (!viewResult.Success)
           {
               throw new FileNotFoundException("视图未找到", viewName);
           }
   
           var viewDictionary = new ViewDataDictionary<TModel>(new EmptyModelMetadataProvider(), new ModelStateDictionary())
           {
               Model = model
           };
   
           var viewContext = new ViewContext(
               actionContext,
               viewResult.View,
               viewDictionary,
               new TempDataDictionary(actionContext.HttpContext, _tempDataProvider),
               sw,
               new HtmlHelperOptions()
           );
   
           await viewResult.View.RenderAsync(viewContext);
           return sw.ToString();
       }
   }
   

3. 注册服务并使用
   在Program.cs中注册服务：

   builder.Services.AddScoped<RazorViewRenderer>();
   

   在HomeController中修改测试方法：

   private readonly RazorViewRenderer _razorViewRenderer;
   
   // 构造函数注入
   public HomeController(ILogger<HomeController> logger, IDistrictServices dataservice, IEmailServices emailservices, RazorViewRenderer razorViewRenderer)
   {
       _logger = logger;
       districtlist = dataservice;
       this.emailservices = emailservices;
       _razorViewRenderer = razorViewRenderer;
   }
   
   public async Task<string> SendTestEmail2()
   {
       var approvaltemplate = new ApprovalEmailTemplate 
       { 
           district_name = "Akron", 
           hire_date = new DateTime(2024,1,1), 
           class_size = 10, 
           forecasted_need = 5 
       };
   
       // 渲染原生Razor视图
       var emailhtmlbody = await _razorViewRenderer.RenderViewToStringAsync("EmailTemplates/ApprovalEmail", approvaltemplate);
   
       EmailModel newEmail = new EmailModel 
       { 
           ToEmail = "tristenhannah@usicllc.com", 
           Subject = "test", 
           Body = emailhtmlbody, 
           IsHtml = true 
       };
       
       return emailservices.SendEmail(newEmail).ToString();
   }
   

内容的提问来源于stack exchange，提问作者Tristen Hannah