Response status code does not indicate success: 401 (Unauthorized).

在使用ASP.NET5调用IBM Bluemix上的REST API服务时出现此异常；

Response status code does not indicate success: 401 (Unauthorized).

at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at dotnethelloworld.Controllers.ValuesController.<Get>d__1.MoveNext()

相关代码：

// GET api/values/txt
[HttpGet("{txt}")]
public async Task<string> Get(string txt)
{
String retString = txt;
try
{
//构造url,source为要翻译文本的语言，target指定要翻译为什么语言
String uri = "https://gateway.watsonplatform.net/language-translation/api/v2/translate?";
uri += "source=en&target=es&text=" + txt;

System.Net.Http.HttpClientHandler handler = new System.Net.Http.HttpClientHandler();
NetworkCredential myCred = new NetworkCredential("用户名", "密码", "gateway.watsonplatform.net");
CredentialCache myCache = new CredentialCache();
myCache.Add(new Uri(uri), "Basic", myCred);
handler.Credentials = myCache;
System.Net.Http.HttpClient httpClient = new System.Net.Http.HttpClient(handler);

retString = await httpClient.GetStringAsync(uri);
return "{\"text\":\"" + retString + "\"}";
}
catch (Exception ex)
{
return ex.Message;
//throw ex;
}
}

MSDN中关于NetworkCredential 类的使用示例：

NetworkCredential myCred = new NetworkCredential(
SecurelyStoredUserName,SecurelyStoredPassword,SecurelyStoredDomain);

CredentialCache myCache = new CredentialCache();

myCache.Add(new Uri("www.contoso.com"), "Basic", myCred);
myCache.Add(new Uri("app.contoso.com"), "Basic", myCred);

WebRequest wr = WebRequest.Create("www.contoso.com");
wr.Credentials = myCache;

只是它是用的WebRequest，而我是用的HttpClient，却不生效，没有找到具体原因；但可以通过另一种方法来解决用户验证的问题。

解决方法：

设置httpClient.DefaultRequestHeaders.Authorization 属性，代码如下：

System.Net.Http.HttpClient httpClient = new System.Net.Http.HttpClient();
//将服务凭证转换为Base64编码格式
byte[] auth = Encoding.UTF8.GetBytes("用户名:密码");
String auth64 = Convert.ToBase64String(auth);
//创建并指定服务凭证，认证方案为Basic
httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", auth64);

retString = await httpClient.GetStringAsync(uri);