如何在ASP.NET WebForm中用WebClient调用ASP.NET Core WebAPI的IFormFile上传接口？

问题描述 #

在ASP.NET Core 3.1中开发了文件上传API，使用Postman或.NET Core项目的HttpClient调用正常，但在ASP.NET WebForm中使用WebClient调用该接口失败，需实现WebForm端正常调用并获取包含上传文件路径和名称的响应。

API核心代码如下：

[HttpPost("Upload")]
public IActionResult Upload([FromForm] IFormFile fileUpload)
{
    ResponseViewModel responseModel = new ResponseViewModel();
    try
    {       
        if (fileUpload == null)
        {
            responseModel.Message = "Please select file.";
            return BadRequest(responseModel);
        }

        //file upload to temp folder
        string tempFilPathWithName = Path.GetTempFileName();
        using (FileStream file = System.IO.File.OpenWrite(tempFilPathWithName))
        {
            fileUpload.CopyTo(file);
        }
        UploadFile uploadFile = new UploadFile()
        {
            FileName = fileUpload.FileName,
            TempFilPathWithName = tempFilPathWithName
        };

        responseModel.Model = uploadFile;
        responseModel.Message = "File successfully uploaded";
        return Ok(responseModel);
    }
    catch (Exception ex)
    {
        responseModel.ExceptionMessage = ex.ToString();
        responseModel.Message = "File upload failed.";
    }
    return BadRequest(responseModel);
}

public class ResponseViewModel
{   
    public object Model { get; set; }
    public string ExceptionMessage { get; set; }
    public string Message { get; set; } 
}

public class UploadFile
{
    public string FileName { get; set; }
    public string TempFilPathWithName { get; set; }
}

解决方案：WebForm 端正确的 WebClient 调用代码 #

WebClient调用失败通常是因为未正确构造multipart/form-data格式的请求内容，以下是可正常工作的调用代码：

using System;
using System.IO;
using System.Net;
using System.Text;
using System.Web.Script.Serialization;

protected void UploadBtn_Click(object sender, EventArgs e)
{
    if (FileUpload1.HasFile)
    {
        string apiUrl = "http://your-api-domain/api/Upload";
        string fileName = Path.GetFileName(FileUpload1.PostedFile.FileName);

        using (WebClient client = new WebClient())
        {
            // 构造multipart/form-data边界
            string boundary = "----WebKitFormBoundary" + DateTime.Now.Ticks.ToString("x");
            client.Headers.Add("Content-Type", $"multipart/form-data; boundary={boundary}");

            using (MemoryStream ms = new MemoryStream())
            {
                // 写入文件内容部分
                byte[] boundaryBytes = Encoding.ASCII.GetBytes($"\r\n--{boundary}\r\n");
                byte[] headerBytes = Encoding.ASCII.GetBytes($"Content-Disposition: form-data; name=\"fileUpload\"; filename=\"{fileName}\"\r\nContent-Type: application/octet-stream\r\n\r\n");
                
                ms.Write(boundaryBytes, 0, boundaryBytes.Length);
                ms.Write(headerBytes, 0, headerBytes.Length);
                
                // 写入文件流
                FileUpload1.PostedFile.InputStream.CopyTo(ms);
                
                // 写入结束边界
                byte[] endBoundaryBytes = Encoding.ASCII.GetBytes($"\r\n--{boundary}--\r\n");
                ms.Write(endBoundaryBytes, 0, endBoundaryBytes.Length);
                
                ms.Position = 0;
                
                // 发送请求并获取响应
                byte[] responseBytes = client.UploadData(apiUrl, "POST", ms.ToArray());
                string responseString = Encoding.UTF8.GetString(responseBytes);
                
                // 解析响应为ResponseViewModel对象
                JavaScriptSerializer serializer = new JavaScriptSerializer();
                ResponseViewModel responseModel = serializer.Deserialize<ResponseViewModel>(responseString);
                
                // 处理响应结果
                if (responseModel.Message == "File successfully uploaded")
                {
                    UploadFile uploadFile = serializer.ConvertToType<UploadFile>(responseModel.Model);
                    Label1.Text = $"上传成功！文件名：{uploadFile.FileName}，临时路径：{uploadFile.TempFilPathWithName}";
                }
                else
                {
                    Label1.Text = $"上传失败：{responseModel.Message}，错误信息：{responseModel.ExceptionMessage}";
                }
            }
        }
    }
    else
    {
        Label1.Text = "请选择要上传的文件";
    }
}

// WebForm项目中需定义对应的模型类
public class ResponseViewModel
{
    public object Model { get; set; }
    public string ExceptionMessage { get; set; }
    public string Message { get; set; }
}

public class UploadFile
{
    public string FileName { get; set; }
    public string TempFilPathWithName { get; set; }
}

关键注意事项 #

• 正确构造multipart/form-data请求：WebClient默认不会自动处理文件上传的表单格式，必须手动构造边界、请求头和文件内容，确保匹配API的[FromForm]接收要求。
• 响应解析：API返回JSON格式数据，需用JavaScriptSerializer（或Newtonsoft.Json）将响应字符串反序列化为对应模型类，才能提取文件路径和名称。
• 跨域与权限：若API和WebForm部署在不同域名下，需确保API已配置CORS允许WebForm所在域名的请求，否则会触发跨域错误。

内容的提问来源于stack exchange，提问作者Prashant Girase