C# 以文件流的形式返回本地文件或远程文件

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);
    }
}

重要注意事项：

1. 异步方法：将方法改为async Task<HttpResponseMessage>以支持异步操作
2. 资源释放：确保正确释放HttpClient和流资源
3. 超时处理：为远程请求设置合理的超时时间
4. 错误处理：添加针对网络请求的特定错误处理
5. 内存考虑：对于大文件，使用流式传输避免内存溢出

推荐使用方案2的流式传输，因为它内存效率更高，特别适合处理大文件。