Asp.Net Core 2.0实现HttpResponse中繁切换

随笔背景：因为项目中有个简单的功能是需要实现中文简体到繁体的切换，数据库中存储的源数据都是中文简体的，为了省事就想着通过HttpHeader的方式来控制Api返回对应的繁体数据。

实现方式：通过Asp.Net Core 中的中间件来拦截HttpResponse，然后通过转换字符编码来实现中文繁体切换。

实现代码如下：

HttpContextMiddleware 中间件

public class HttpContextMiddleware
    {
        private readonly RequestDelegate _next;
        public HttpContextMiddleware(RequestDelegate next)
        {
            _next = next;
        }

        public async Task Invoke(HttpContext context)
        {
            var originalBodyStream = context.Response.Body;
            
            using (var responseBody = new MemoryStream())
            {
                context.Response.Body = responseBody;
                await _next(context);
                var result = await FormatResponse(context.Response);
                if (context.Request.Headers.Keys.Contains(Constants.HttpHeaderLanguage))
                {
                    var lang = context.Request.Headers.GetCommaSeparatedValues(Constants.HttpHeaderLanguage).GetValue(0).ToString();           
                    if (lang == "zh-tw")
                    {
                        var traditionresult = ConvertHelper.ToTraditional(result);
                        byte[] array = Encoding.UTF8.GetBytes(traditionresult);
                        MemoryStream stream = new MemoryStream(array);
                        try
                        {
                            await stream.CopyToAsync(originalBodyStream);
                        }
                        catch (Exception ex)
                        {

                            throw ex;
                        }
                    }
                    else
                    {
                        try
                        {
                            await responseBody.CopyToAsync(originalBodyStream);
                        }
                        catch (Exception ex)
                        {

                            throw ex;
                        }
                    }
                }
                else
                {
                    await responseBody.CopyToAsync(originalBodyStream);
                }

            }
        }

        private async Task<string> FormatResponse(HttpResponse response)
        {
            response.Body.Seek(0, SeekOrigin.Begin);
            var text = await new StreamReader(response.Body).ReadToEndAsync();
            response.Body.Seek(0, SeekOrigin.Begin);

            return $"{text}";
        }
    }

Startup.cs

 public void Configure(IApplicationBuilder app, IHostingEnvironment env)
       {
            app.UseDefaultFiles();
            app.UseAuthentication();
            app.UseStaticFiles();
            //在Mvc之前注入到管道中
            app.UseMiddleware<HttpContextMiddleware>();
            app.UseMvc();    
        }

ConvertHelper 中文简体繁体转换工具类

public static  class ConvertHelper
  {
        private const int LOCALE_SYSTEM_DEFAULT = 0x0800;
        private const int LCMAP_SIMPLIFIED_CHINESE = 0x02000000;
        private const int LCMAP_TRADITIONAL_CHINESE = 0x04000000;

        [DllImport("kernel32", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern int LCMapString(int Locale, int dwMapFlags, string lpSrcStr, int cchSrc, [Out] string lpDestStr, int cchDest);

        /// <summary>
        /// 将字符转换成简体中文
        /// </summary>
        /// <param name="source">输入要转换的字符串</param>
        /// <returns>转换完成后的字符串</returns>
        public static string ToSimplified(string source)
        {
            String target = new String(' ', source.Length);
            int ret = LCMapString(LOCALE_SYSTEM_DEFAULT, LCMAP_SIMPLIFIED_CHINESE, source, source.Length, target, source.Length);
            return target;
        }

        /// <summary>
        /// 将字符转换为繁体中文
        /// </summary>
        /// <param name="source">输入要转换的字符串</param>
        /// <returns>转换完成后的字符串</returns>
        public static string ToTraditional(string source)
        {
            String target = new String(' ', source.Length);
            int ret = LCMapString(LOCALE_SYSTEM_DEFAULT, LCMAP_TRADITIONAL_CHINESE, source, source.Length, target, source.Length);
            return target;
        }
    }

以上源代码就是所有关键代码了，中间件注入以后，不用再在action里或controller里单独拦截，经测试，会拦截所有api的响应结果。这里有个插曲是我之前尝试过使用ResultFilter来拦截，但没法做到在响应后拦截响应结果。不知出于什么原因，最后我放弃了Filter的方式，选择了这种中间件的拦截方式。

本文参考如下：

https://elanderson.net/2017/02/log-requests-and-responses-in-asp-net-core/

本文如有不对的地方，欢迎指正！愿与君共勉。

最后，感谢您的阅读！