在.net4.5框架下提供一个使用c#下载大文件的webapi接口

在.NET 4.5框架下，你可以创建一个Web API服务来处理大文件的下载。这里是一个简单的示例，展示如何设计一个支持分块下载的API接口：

using System;
using System.IO;
using System.Threading.Tasks;
using System.Web.Http;

namespace YourNamespace.Controllers
{
    public class DownloadController : ApiController
    {
        [HttpGet]
        public async Task<HttpResponseMessage> DownloadLargeFile(string fileId)
        {
            if (string.IsNullOrEmpty(fileId))
                return Request.CreateResponse(HttpStatusCode.BadRequest, "Invalid file ID");

            // 假设你有一个存储大文件的数据库或文件系统
            string filePath = GetFilePathFromDatabase(fileId);
            if (!System.IO.File.Exists(filePath))
                return Request.CreateResponse(HttpStatusCode.NotFound, "File not found");

            var contentLength = new FileInfo(filePath).Length;
            var response = Request.CreateResponse(HttpStatusCode.OK);
            response.Content.Headers.ContentLength = contentLength;

            // 使用FileStreamResult或StreamContent进行流式传输
            using (var fileStream = File.OpenRead(filePath))
            {
                response.Content = new StreamContent(fileStream);
                response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
                response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
                {
                    FileName = Path.GetFileName(filePath)
                };

                // 为了支持断点续传，可以设置Range头来获取部分数据
                if (Request.Headers.Range != null)
                {
                    await HandlePartialDownload(fileStream, contentLength, response);
                }
                else
                {
                    await response.Content.ReadAsStreamAsync();
                }

                return response;
            }
        }

        private async Task HandlePartialDownload(Stream fileStream, long totalSize, HttpResponseMessage response)
        {
            // Parse Range header and seek to the appropriate position
            byte[] rangeStartBytes = Convert.ToInt64(response.Headers.RequestRange.Start, 10).ToByteArray();
            long startByte = BitConverter.ToInt64(rangeStartBytes, 0);
            
            // Check for valid range and adjust end byte accordingly
            long endByte;
            if (rangeStartBytes.Length == 8 && !response.Headers.RequestRange.HasRangeEnd)
            {
                endByte = totalSize - 1;
            }
            else
            {
                byte[] rangeEndBytes = Convert.ToInt64(response.Headers.RequestRange.End.ToString(), 10).ToByteArray();
                endByte = Math.Min(BitConverter.ToInt64(rangeEndBytes, 0), totalSize - 1);
            }

            // Seek to the starting position and send only requested data
            fileStream.Seek(startByte, SeekOrigin.Begin);
            response.Content.Headers.ContentRange = new ContentRangeHeaderValue(startByte + 1, endByte, totalSize);

            byte[] buffer = new byte[4096];
            int bytesRead;
            while ((bytesRead = await fileStream.ReadAsync(buffer, 0, buffer.Length)) > 0)
            {
                await response.Content.WriteAsync(buffer, 0, bytesRead);
            }
        }
    }
}

这个`DownloadLargeFile`方法首先验证请求的文件ID，然后读取文件并设置响应头。如果请求包含Range头，它将处理断点续传。注意，这只是一个基本示例，实际生产环境中可能需要考虑错误处理、文件版本控制、并发访问限制等问题。