使用PowerShell的[System.Net.Http.HttpClient]发送multipart/form-data遇400错误

我正在开发用于与WebEx API交互的自定义函数Invoke-WebExRequest，该函数大部分功能正常，但无法处理multipart/form-data类型的数据。

已成功的场景 #

1. 用Invoke-WebRequest手动构造multipart请求体

手动构造纯文本格式的请求体，指定包含边界的-ContentType参数后，使用Invoke-WebRequest可成功发送multipart/form-data数据：

$GUID = New-Guid

$Body = @"
------------$($GUID)------------
Content-Disposition: form-data; name="roomId"

$($RoomId)
------------$($GUID)------------
Content-Disposition: form-data; name="html"

<h3>Testing...</h3>
------------$($GUID)------------
"@

$ContentType = "multipart/form-data; boundary=----------$($GUID)------------"
$Result = Invoke-WebRequest -Headers     $Headers `
                            -Uri         $URI `
                            -Method      'Post' `
                            -Body        $Body `
                            -ContentType $ContentType
$GUID = New-Guid

2. 用[System.Net.Http.HttpClient]发送JSON请求

仅发送JSON格式文本消息时，[System.Net.Http.HttpClient]能正常工作：

$Body = @{
           "roomId" = $RoomId
           "text"   = 'This is just a test'
         }

$Client  = [System.Net.Http.HttpClient]::new()
$Content = [System.Net.Http.HttpRequestMessage]::new()
$Content.Headers.Add('Authorization',"Bearer $($Token)")
$Content.RequestUri = $URI
$Content.Method = 'Post'

$UTF8 = [System.Text.Encoding]::UTF8
$Body = $($Body | ConvertTo-Json)
$Body = [System.Net.Http.StringContent]::new($Body,$UTF8,'application/json')
$Content.Content = $Body
$Client.SendAsync($Content).GetAwaiter().GetResult()

遇到的问题 #

尝试用[System.Net.Http.HttpClient]发送multipart/form-data时，代码无报错但返回400 - BadRequest，尝试的代码如下：

$Body = @"
------------$($GUID)------------
Content-Disposition: form-data; name="roomId"

$($RoomId)
------------$($GUID)------------
Content-Disposition: form-data; name="html"

<h3>Testing with [System.Net.Http.HttpClient]</h3>
------------$($GUID)------------
"@

$Client  = [System.Net.Http.HttpClient]::new()
$Content = [System.Net.Http.HttpRequestMessage]::new()
$Content.Headers.Add('Authorization',"Bearer $($Token)")
$Content.RequestUri = $URI
$Content.Method = 'Post'

$UTF8 = [System.Text.Encoding]::UTF8
$Body = [System.Net.Http.StringContent]::new($Body,$UTF8,'multipart/form-data')
$Content.Content = $Body
$Client.SendAsync($Content).GetAwaiter().GetResult()

解决方案 #

问题出在手动构造multipart请求体后，用StringContent传递时未正确设置ContentType的边界参数，且HttpClient提供了专门的MultipartFormDataContent类处理这类请求，无需手动拼接字符串，可避免格式错误：

$Client = [System.Net.Http.HttpClient]::new()
$Content = [System.Net.Http.MultipartFormDataContent]::new()

# 添加roomId字段
$roomIdContent = [System.Net.Http.StringContent]::new($RoomId)
$Content.Add($roomIdContent, "roomId")

# 添加html字段
$htmlContent = [System.Net.Http.StringContent]::new("<h3>Testing with [System.Net.Http.HttpClient]</h3>")
$Content.Add($htmlContent, "html")

# 创建请求消息
$request = [System.Net.Http.HttpRequestMessage]::new([System.Net.Http.HttpMethod]::Post, $URI)
$request.Headers.Add('Authorization', "Bearer $($Token)")
$request.Content = $Content

# 发送请求
$response = $Client.SendAsync($request).GetAwaiter().GetResult()
$response.EnsureSuccessStatusCode() # 自动抛出非2xx状态码的异常

说明 #

1. MultipartFormDataContent会自动生成正确的边界值并设置到ContentType头中，避免手动拼接时可能出现的换行、分隔符格式错误；
2. 若需上传文件，只需用ByteArrayContent或StreamContent包装文件内容，再添加到MultipartFormDataContent即可。

内容的提问来源于stack exchange，提问作者James W