request.DefaultRequestHeaders.Authorization = 
    new AuthenticationHeaderValue(
        "Basic", Convert.ToBase64String(
            System.Text.ASCIIEncoding.ASCII.GetBytes(
               $"{yourusername}:{yourpwd}")));

6年过去了，但为了帮助别人，我加了这个。

https://www.codeproject.com/Tips/996401/Authenticate-WebAPIs-with-Basic-and-Windows-Authen

var authenticationBytes = Encoding.ASCII.GetBytes("<username>:<password>");
using (HttpClient confClient = new HttpClient())
{
  confClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", 
         Convert.ToBase64String(authenticationBytes));
  confClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue(Constants.MediaType));  
  HttpResponseMessage message = confClient.GetAsync("<service URI>").Result;
  if (message.IsSuccessStatusCode)
  {
    var inter = message.Content.ReadAsStringAsync();
    List<string> result = JsonConvert.DeserializeObject<List<string>>(inter.Result);
  }
}

使用基本授权和Json参数。

using (HttpClient client = new HttpClient())
{
    var request_json = "your json string";

    var content = new StringContent(request_json, Encoding.UTF8, "application/json");

    var authenticationBytes = Encoding.ASCII.GetBytes("YourUsername:YourPassword");

    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic",
            Convert.ToBase64String(authenticationBytes));
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

    var result = await client.PostAsync("YourURL", content);

    var result_string = await result.Content.ReadAsStringAsync();
}

方法是这样的，

httpClient.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("Bearer", "Your Oauth token");

我在设置不记名代币

httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);

它在一个端点上起作用，但在另一个端点上不起作用。问题是我在"持票人"上用了小写b。改变后，现在它工作的两个api的我正在击中。如果你甚至不把它看作是寻找针的干草堆之一，这是一件很容易错过的事情。

一定要有“持票人”，要有资本。

使用c# HttpClient设置基本身份验证。下面的代码为我工作。

   using (var client = new HttpClient())
        {
            var webUrl ="http://localhost/saleapi/api/";
            var uri = "api/sales";
            client.BaseAddress = new Uri(webUrl);
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            client.DefaultRequestHeaders.ConnectionClose = true;

            //Set Basic Auth
            var user = "username";
            var password = "password";
            var base64String =Convert.ToBase64String( Encoding.ASCII.GetBytes($"{user}:{password}"));
            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic",base64String);

            var result = await client.PostAsJsonAsync(uri, model);
            return result;
        }