.NET Lambda注解框架中解析multipart/form-data及实现ParseMultipartFormData函数

以下是适配APIGatewayHttpApiV2ProxyRequest的ParseMultipartFormData函数实现，用于解析multipart/form-data格式的请求体：

using System;
using System.Collections.Generic;
using System.IO;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;

private Dictionary<string, byte[]> ParseMultipartFormData(string base64Body, string contentType)
{
    var formData = new Dictionary<string, byte[]>();
    
    // 解码base64编码的请求体字节流
    var bodyBytes = Convert.FromBase64String(base64Body);
    using var bodyStream = new MemoryStream(bodyBytes);
    
    // 从Content-Type头中提取multipart边界标识
    var boundary = GetBoundary(contentType);
    if (string.IsNullOrEmpty(boundary))
    {
        throw new InvalidOperationException("无法从Content-Type头中解析multipart边界");
    }
    
    var reader = new MultipartReader(boundary, bodyStream);
    MultipartSection section;
    
    // 遍历所有multipart分段
    while ((section = reader.ReadNextSectionAsync().Result) != null)
    {
        var contentDisposition = ContentDispositionHeaderValue.Parse(section.ContentDisposition);
        var fieldName = contentDisposition.Name.Trim('"');
        
        // 读取当前分段的内容到字节数组
        using var contentStream = new MemoryStream();
        section.Body.CopyTo(contentStream);
        formData.Add(fieldName, contentStream.ToArray());
    }
    
    return formData;
}

// 辅助方法：从Content-Type头中提取边界字符串
private string GetBoundary(string contentType)
{
    var headerElements = contentType.Split(';');
    foreach (var element in headerElements)
    {
        var keyValue = element.Trim().Split('=');
        if (keyValue.Length == 2 && keyValue[0].Equals("boundary", StringComparison.OrdinalIgnoreCase))
        {
            return keyValue[1].Trim('"');
        }
    }
    return null;
}

关键说明 #

• 由于APIGatewayHttpApiV2ProxyRequest的Body是base64编码格式，所以第一步需要解码为字节流
• 通过Content-Type头中的boundary参数识别multipart分段的分隔符
• 函数返回Dictionary<string, byte[]>，键为表单字段名，值为字段对应的字节内容（文件或普通文本都以字节数组存储）
• 如果需要区分文件和普通文本字段，可以通过contentDisposition.FileName判断：存在该值则为文件，否则可通过Encoding.UTF8.GetString()将字节数组转为字符串

调用示例 #

[LambdaFunction()]
[HttpApi(LambdaHttpMethod.Post, "/create")]
public async Task<IActionResult> AddEntity([FromBody] APIGatewayHttpApiV2ProxyRequest request)
{
    var contentType = request.Headers["content-type"];

    if (contentType.Contains("multipart/form-data"))
    {
        var formData = ParseMultipartFormData(request.Body, contentType);
        var fileContent = formData["file"]; // 处理文件内容，例如上传至S3
        
        // 处理普通文本字段示例
        // var username = Encoding.UTF8.GetString(formData["username"]);
    }

    return Ok(new { message = "Data added successfully" });
}

内容的提问来源于stack exchange，提问作者Karthick J