HTTP HEAD request with HttpClient in .NET 4.5 and C#

问题

Is it possible to create a HTTP HEAD request with the new HttpClient in .NET 4.5? The only methods I can find are GetAsync, DeleteAsync, PutAsync and PostAsync. I know that the HttpWebRequest-class is able to do that, but I want to use the modern HttpClient.

回答1:

Use the SendAsync method with an instance of HttpRequestMessage that was constructed using HttpMethod.Head .

GetAsync, PostAsync, etc are convenient wrappers around SendAsync; the less common HTTP methods such as HEAD, OPTIONS, etc, don't get a wrapper.

回答2:

You may also do as follows to fetch just the headers:

this.GetAsync($"http://url.com", HttpCompletionOption.ResponseHeadersRead).Result;

回答3:

I needed to do this, to get TotalCount of ATMs that I was returning from my Web API's GET Method.

When I tried @Smig's answer I got the following Response from my Web API.

MethodNotAllowed : Pragma: no-cache X-SourceFiles: =?UTF-8?B?dfdsf Cache-Control: no-cache Date: Wed, 22 Mar 2017 20:42:57 GMT Server: Microsoft-IIS/10.0 X-AspNet-Version: 4.0.30319 X-Powered-By: ASP.NET

Had to built upon @Smig's answer to get this working successfully. I found out that the Web API methods needs to explicitly allow the Http HEAD verb by specifying it in the Action method as an Attribute.

Here's the complete code with inline explanation by way of code comments. I've removed the sensitive code.

In my Web Client:

        HttpClient client = new HttpClient();

        // set the base host address for the Api (comes from Web.Config)
        client.BaseAddress = new Uri(ConfigurationManager.AppSettings.Get("ApiBase"));
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add( 
          new MediaTypeWithQualityHeaderValue("application/json"));

        // Construct the HEAD only needed request. Note that I am requesting
        //  only the 1st page and 1st record from my API's endpoint.
        HttpRequestMessage request = new HttpRequestMessage(
          HttpMethod.Head, 
          "api/atms?page=1&pagesize=1");

        HttpResponseMessage response = await client.SendAsync(request);

        // FindAndParsePagingInfo is a simple helper I wrote that parses the 
        // json in the Header and populates a PagingInfo poco that contains 
        // paging info like CurrentPage, TotalPages, and TotalCount, which 
        // is the total number of records in the ATMs table.
        // The source code is pasted separately in this answer.
        var pagingInfoForAtms = HeaderParser.FindAndParsePagingInfo(response.Headers);

        if (response.IsSuccessStatusCode)
            // This for testing only. pagingInfoForAtms.TotalCount correctly
            //  contained the record count
            return Content($"# of ATMs {pagingInfoForAtms.TotalCount}");

            // if request failed, execution will come through to this line 
            // and display the response status code and message. This is how
            //  I found out that I had to specify the HttpHead attribute.
            return Content($"{response.StatusCode} : {response.Headers.ToString()}");
        }

In the Web API.

    // Specify the HttpHead attribute to avoid getting the MethodNotAllowed error.
    [HttpGet, HttpHead]
    [Route("Atms", Name = "AtmsList")]
    public IHttpActionResult Get(string sort="id", int page = 1, int pageSize = 5)
    {
        try
        {
            // get data from repository
            var atms =  _atmRepository.GetAll().AsQueryable().ApplySort(sort);
            // ... do some code to construct pagingInfo etc.
            // .......
            // set paging info in header.
            HttpContext.Current.Response.Headers.Add(
              "X-Pagination", JsonConvert.SerializeObject(paginationHeader));
            // ...
            return Ok(pagedAtms));
        }
        catch (Exception exception)
        {
            //... log and return 500 error
        }
    }

FindAndParsePagingInfo Helper method for parsing the paging header data.

public static class HeaderParser
{
public static PagingInfo FindAndParsePagingInfo(HttpResponseHeaders responseHeaders)
{
    // find the "X-Pagination" info in header
    if (responseHeaders.Contains("X-Pagination"))
    {
        var xPag = responseHeaders.First(ph => ph.Key == "X-Pagination").Value;

        // parse the value - this is a JSON-string.
        return JsonConvert.DeserializeObject<PagingInfo>(xPag.First());
    }

    return null;
}

public static string GetSingleHeaderValue(HttpResponseHeaders responseHeaders, 
    string keyName)
{
    if (responseHeaders.Contains(keyName))
        return responseHeaders.First(ph => ph.Key == keyName).Value.First();

    return null;
}

}

来源：https://stackoverflow.com/questions/16416699/http-head-request-with-httpclient-in-net-4-5-and-c-sharp