C#实现以文件流的形式返回本地文件或远程文件路径_Asp.net

filestream和fileinfo只能处理本地文件路径，无法直接处理http url。以下是几种实现远程pdf返回给前端的解决方案：

方案1：使用httpclient下载远程文件（推荐）

[httpget]
public async task<httpresponsemessage> getreportfile()
{  
    try
    {
        string orderno = httpcontext.current.request.querystring["orderno"];
        string filepath = "d:\\1.pdf";
        
        if(!string.isnullorempty(orderno))
        {            
            filepath = "http://www.a.com/1.pdf";
            
            // 使用httpclient下载远程文件
            using (var httpclient = new httpclient())
            {
                // 设置超时时间
                httpclient.timeout = timespan.fromseconds(30);
                
                // 下载文件内容
                var response = await httpclient.getasync(filepath);
                response.ensuresuccessstatuscode();
                
                var content = await response.content.readasbytearrayasync();
                
                // 创建返回结果
                var result = new httpresponsemessage(httpstatuscode.ok);
                result.content = new bytearraycontent(content);
                result.content.headers.contenttype = new mediatypeheadervalue("application/pdf");
                result.content.headers.contentdisposition = new contentdispositionheadervalue("inline")
                {
                    filename = "document.pdf"
                };
                
                return result;
            }
        }
        else
        {
            // 处理本地文件
            fileinfo foundfileinfo = new fileinfo(filepath);
            filestream fs = new filestream(filepath, filemode.open, fileaccess.read, fileshare.readwrite);
            
            var result = new httpresponsemessage(httpstatuscode.ok);
            result.content = new streamcontent(fs);
            result.content.headers.contenttype = new mediatypeheadervalue("application/pdf");
            result.content.headers.contentdisposition = new contentdispositionheadervalue("inline")
            {
                filename = foundfileinfo.name
            };
            
            return result;
        }
    }
    catch (exception ex)
    {
        logger.error(ex, ex.message);
        return new httpresponsemessage(httpstatuscode.badrequest);
    }
}

方案2：流式传输（节省内存）【推荐】

[httpget]
public async task<httpresponsemessage> getreportfile()
{  
    try
    {
        string orderno = httpcontext.current.request.querystring["orderno"];
        string filepath = "d:\\1.pdf";
        
        if(!string.isnullorempty(orderno))
        {            
            filepath = "http://www.a.com/1.pdf";
            
            using (var httpclient = new httpclient())
            {
                httpclient.timeout = timespan.fromseconds(30);
                
                // 流式下载
                var response = await httpclient.getasync(filepath, httpcompletionoption.responseheadersread);
                response.ensuresuccessstatuscode();
                
                var stream = await response.content.readasstreamasync();
                
                var result = new httpresponsemessage(httpstatuscode.ok);
                result.content = new streamcontent(stream);
                result.content.headers.contenttype = new mediatypeheadervalue("application/pdf");
                result.content.headers.contentdisposition = new contentdispositionheadervalue("inline")
                {
                    filename = "document.pdf"
                };
                
                return result;
            }
        }
        else
        {
            // 本地文件处理...
        }
    }
    catch (exception ex)
    {
        logger.error(ex, ex.message);
        return new httpresponsemessage(httpstatuscode.badrequest);
    }
}

方案3：添加缓存和错误处理

[httpget]
public async task<httpresponsemessage> getreportfile()
{  
    try
    {
        string orderno = httpcontext.current.request.querystring["orderno"];
        string filepath = "d:\\1.pdf";
        
        if(!string.isnullorempty(orderno))
        {            
            filepath = "http://www.a.com/1.pdf";
            
            using (var httpclient = new httpclient())
            {
                // 添加user-agent头，有些服务器需要
                httpclient.defaultrequestheaders.add("user-agent", "mozilla/5.0");
                httpclient.timeout = timespan.fromseconds(30);
                
                // 先获取头部信息检查文件是否存在
                var headresponse = await httpclient.sendasync(new httprequestmessage(httpmethod.head, filepath));
                
                if (!headresponse.issuccessstatuscode)
                {
                    return new httpresponsemessage(httpstatuscode.notfound)
                    {
                        content = new stringcontent("远程文件未找到")
                    };
                }
                
                // 获取文件名（从content-disposition或url中提取）
                string filename = "document.pdf";
                if (headresponse.content.headers.contentdisposition != null)
                {
                    filename = headresponse.content.headers.contentdisposition.filename ?? filename;
                }
                
                // 下载文件
                var getresponse = await httpclient.getasync(filepath, httpcompletionoption.responseheadersread);
                getresponse.ensuresuccessstatuscode();
                
                var result = new httpresponsemessage(httpstatuscode.ok);
                result.content = new streamcontent(await getresponse.content.readasstreamasync());
                result.content.headers.contenttype = new mediatypeheadervalue("application/pdf");
                result.content.headers.contentdisposition = new contentdispositionheadervalue("inline")
                {
                    filename = filename
                };
                
                // 添加缓存头（可选）
                result.headers.cachecontrol = new cachecontrolheadervalue()
                {
                    maxage = timespan.fromhours(1)
                };
                
                return result;
            }
        }
        else
        {
            // 本地文件处理...
        }
    }
    catch (httprequestexception httpex)
    {
        logger.error(httpex, "网络请求错误");
        return new httpresponsemessage(httpstatuscode.badgateway);
    }
    catch (taskcanceledexception timeoutex)
    {
        logger.error(timeoutex, "请求超时");
        return new httpresponsemessage(httpstatuscode.requesttimeout);
    }
    catch (exception ex)
    {
        logger.error(ex, ex.message);
        return new httpresponsemessage(httpstatuscode.internalservererror);
    }
}