无法通过 HttpClient 或 WebClient 下载文件

Question

大家好，感谢您的关注。我写了一个返回 FileStreamResult 或 NotModified 状态代码的操作，它在 Postman 中有效，但如果我使用 C# 代码，它就不起作用。

        [HttpGet("{id}")]
        public async Task<IActionResult> Get(Guid id, [FromServices] IContentTypeProvider contentTypeProvider)
        {
            string? hash = Request.Headers.IfNoneMatch;
            long userId = long.Parse(User.FindFirstValue(ClaimTypes.NameIdentifier));
            FileDto fileDto = await _fileService.GetMetadataAsync(id, userId);
            if (fileDto.Hash == hash)
            {
                return StatusCode((int)HttpStatusCode.NotModified);
            }

            Stream stream = await _fileService.GetStreamAsync(id, userId);
            string? contentType;
            if (contentTypeProvider.TryGetContentType(fileDto.Name, out contentType) is not true)
            {
                contentType = "application/octet-stream";
            }

            Response.Headers.CacheControl = "public";
            Response.Headers.ETag = fileDto.Hash;
            return new FileStreamResult(stream, contentType)
            {
                FileDownloadName = fileDto.Name,
            };
        }

邮递员的结果：

HttpClient（和WebClient）的行为很奇怪：

1. 返回200状态码但内容为空

2. Etag 为空，但如果将响应序列化为 JSON，则表明 Etag 存在于响应标头中。

序列化响应：

{
    "Version": "1.1",
    "Content": {
        "Headers": [
            {
                "Key": "Content-Length",
                "Value": [
                    "0"
                ]
            },
            {
                "Key": "Content-Type",
                "Value": [
                    "image/jpeg"
                ]
            },
            {
                "Key": "Content-Disposition",
                "Value": [
                    "attachment; filename=shrek.jpg; filename*=UTF-8\u0027\u0027shrek.jpg"
                ]
            }
        ]
    },
    "StatusCode": 200,
    "ReasonPhrase": "OK",
    "Headers": [
        {
            "Key": "Date",
            "Value": [
                "Fri, 21 Apr 2023 11:36:24 GMT"
            ]
        },
        {
            "Key": "Server",
            "Value": [
                "Kestrel"
            ]
        },
        {
            "Key": "Cache-Control",
            "Value": [
                "public"
            ]
        },
        {
            "Key": "ETag",
            "Value": [
                "DA39A3EE5E6B4B0D3255BFEF95601890AFD80709"
            ]
        }
    ],
    "TrailingHeaders": [],
    "RequestMessage": {
        "Version": "1.1",
        "VersionPolicy": 0,
        "Content": null,
        "Method": {
            "Method": "GET"
        },
        "RequestUri": "https://localhost:201/api/files/ad71424e-3ac1-47cd-b36d-107deadce4f2",
        "Headers": [
            {
                "Key": "Authorization",
                "Value": [
                    "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJodHRwOi8vc2NoZW1hcy54bWxzb2FwLm9yZy93cy8yMDA1LzA1L2lkZW50aXR5L2NsYWltcy9uYW1lIjoic2Vjb25kVXNlckRvd25sb2FkRmlsZUB3aXRoQWNjZXNzRmlsZS50ZXN0Iiwic3ViIjoiMiIsImp0aSI6ImRhZDUxMTNlLWFiOWYtNDRlMS1iMTI2LTE0OWZjYzRhNDk2OCIsImh0dHA6Ly9zY2hlbWFzLm1pY3Jvc29mdC5jb20vd3MvMjAwOC8wNi9pZGVudGl0eS9jbGFpbXMvcm9sZSI6IkVtYWlsIENvbmZpcm1lZCIsImV4cCI6MTY4MjA3Nzg4NSwiaXNzIjoiaHR0cHM6Ly9sb2NhbGhvc3Q6MjAxIiwiYXVkIjoiaHR0cHM6Ly9sb2NhbGhvc3Q6MjAxIn0.MBiMYbOuZkEkwJKySNTBqczPPjEQfGyVi_G1jPw6vnk"
                ]
            }
        ],
        "Properties": {},
        "Options": {}
    },
    "IsSuccessStatusCode": true
}

为什么 Postman 工作但 HttpClient（WebClient 也是）不工作？也许我错过了什么？