blocks|key|149157|text|以下是同步调用的示例，但您可以使用await-sync轻松更改为异步调用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|149158|var+pairs+=+new+List<KeyValuePair<string,+string>>
++++++++++++{
++++++++++++++++new+KeyValuePair<string,+string>("login",+"abc")
++++++++++++};

var+content+=+new+FormUrlEncodedContent(pairs);

var+client+=+new+HttpClient+{BaseAddress+=+new+Uri("http://localhost:6740")};

++++//+call+sync
var+response+=+client.PostAsync("/api/membership/exist",+content).Result;+
if+(response.IsSuccessStatusCode)
{
}|code-block|syntax|javascript|149159|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Below is example to call synchronously but you can easily change to async by using await-sync:

<pre><code>var pairs = new List&lt;KeyValuePair&lt;string, string&gt;&gt;
 {
 new KeyValuePair&lt;string, string&gt;("login", "abc")
 };

var content = new FormUrlEncodedContent(pairs);

var client = new HttpClient {BaseAddress = new Uri("http://localhost:6740")};

 // call sync
var response = client.PostAsync("/api/membership/exist", content).Result; 
if (response.IsSuccessStatusCode)
{
}
</code></pre>

blocks|key|367406|text|你可以这样做|type|unstyled|depth|inlineStyleRanges|entityRanges|data|367407|HttpWebRequest+req+=+(HttpWebRequest)WebRequest.Create("http://localhost:6740/api/Membership/exist");

req.Method+=+"POST";
req.ContentType+=+"application/x-www-form-urlencoded";+++++++++
req.ContentLength+=+6;

StreamWriter+streamOut+=+new+StreamWriter(req.GetRequestStream(),+System.Text.Encoding.ASCII);
streamOut.Write(strRequest);
streamOut.Close();
StreamReader+streamIn+=+new+StreamReader(req.GetResponse().GetResponseStream());
string+strResponse+=+streamIn.ReadToEnd();
streamIn.Close();|code-block|syntax|javascript|367408|然后，strReponse应该包含And服务返回的值|367409|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

You could do something like this 

<pre><code>HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://localhost:6740/api/Membership/exist");

req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded"; 
req.ContentLength = 6;

StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
streamOut.Write(strRequest);
streamOut.Close();
StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
string strResponse = streamIn.ReadToEnd();
streamIn.Close();
</code></pre>

And then strReponse should contain the values returned by your webservice

blocks|key|367414|text|using+System;
using+System.Collections.Generic;
using+System.Net.Http;

class+Program
{
++++static+void+Main(string[]+args)
++++{
++++++++Task.Run(()+=>+MainAsync());
++++++++Console.ReadLine();
++++}

++++static+async+Task+MainAsync()
++++{
++++++++using+(var+client+=+new+HttpClient())
++++++++{
++++++++++++client.BaseAddress+=+new+Uri("http://localhost:6740");
++++++++++++var+content+=+new+FormUrlEncodedContent(new[]
++++++++++++{
++++++++++++++++new+KeyValuePair<string,+string>("",+"login")
++++++++++++});
++++++++++++var+result+=+await+client.PostAsync("/api/Membership/exists",+content);
++++++++++++string+resultContent+=+await+result.Content.ReadAsStringAsync();
++++++++++++Console.WriteLine(resultContent);
++++++++}
++++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|367415|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>using System;
using System.Collections.Generic;
using System.Net.Http;

class Program
{
 static void Main(string[] args)
 {
 Task.Run(() =&gt; MainAsync());
 Console.ReadLine();
 }

 static async Task MainAsync()
 {
 using (var client = new HttpClient())
 {
 client.BaseAddress = new Uri("http://localhost:6740");
 var content = new FormUrlEncodedContent(new[]
 {
 new KeyValuePair&lt;string, string&gt;("", "login")
 });
 var result = await client.PostAsync("/api/Membership/exists", content);
 string resultContent = await result.Content.ReadAsStringAsync();
 Console.WriteLine(resultContent);
 }
 }
}
</code></pre>

blocks|key|367435|text|在ASP.NET的网站上有一篇关于你的问题的文章。我希望它能帮助你。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|367436|如何使用asp+net调用api|367437|http://www.asp.net/web-api/overview/advanced/calling-a-web-api-from-a-net-client|offset|length|367438|以下是文章的POST部分中的一小部分|367439|以下代码发送一个POST请求，其中包含一个JSON格式的Product实例：|367440|//+HTTP+POST
var+gizmo+=+new+Product()+{+Name+=+"Gizmo",+Price+=+100,+Category+=+"Widget"+};
response+=+await+client.PostAsJsonAsync("api/products",+gizmo);
if+(response.IsSuccessStatusCode)
{
++++//+Get+the+URI+of+the+created+resource.
++++Uri+gizmoUrl+=+response.Headers.Location;
}|code-block|syntax|javascript|367441|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|28|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Y|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Z|8|@]|9|@$F|10|G|11|1|12]]|A|$]]|$1|H|3|I|5|6|7|13|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|14|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|15|8|@]|9|@]|A|$O|P]]|$1|Q|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|E]]]]

There is an article about your question on asp.net's website. I hope it can help you. 

How to call an api with asp net

<a href="http://www.asp.net/web-api/overview/advanced/calling-a-web-api-from-a-net-client" rel="noreferrer">http://www.asp.net/web-api/overview/advanced/calling-a-web-api-from-a-net-client</a>

Here is a small part from the POST section of the article

The following code sends a POST request that contains a Product instance in JSON format:

<pre><code>// HTTP POST
var gizmo = new Product() { Name = "Gizmo", Price = 100, Category = "Widget" };
response = await client.PostAsJsonAsync("api/products", gizmo);
if (response.IsSuccessStatusCode)
{
 // Get the URI of the created resource.
 Uri gizmoUrl = response.Headers.Location;
}
</code></pre>

blocks|key|149205|text|Here我找到了这篇文章，它是使用JsonConvert.SerializeObject()和StringContent()向HttpClient.PostAsync数据发送post请求|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|149206|static+async+Task+Main(string[]+args)
{
++++var+person+=+new+Person();
++++person.Name+=+"John+Doe";
++++person.Occupation+=+"gardener";

++++var+json+=+Newtonsoft.Json.JsonConvert.SerializeObject(person);
++++var+data+=+new+System.Net.Http.StringContent(json,+Encoding.UTF8,+"application/json");

++++var+url+=+"https://httpbin.org/post";
++++using+var+client+=+new+HttpClient();

++++var+response+=+await+client.PostAsync(url,+data);

++++string+result+=+response.Content.ReadAsStringAsync().Result;
++++Console.WriteLine(result);
}|code-block|syntax|javascript|149207|entityMap|0|LINK|mutability|MUTABLE|url|http://zetcode.com/csharp/httpclient/^0|H|T|1B|F|1R|K|0|4|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

<a href="http://zetcode.com/csharp/httpclient/" rel="noreferrer">Here</a> I found this article which is send post request using <code>JsonConvert.SerializeObject()</code> &amp; <code>StringContent()</code> to <code>HttpClient.PostAsync</code> data
<pre><code>static async Task Main(string[] args)
{
 var person = new Person();
 person.Name = &quot;John Doe&quot;;
 person.Occupation = &quot;gardener&quot;;

 var json = Newtonsoft.Json.JsonConvert.SerializeObject(person);
 var data = new System.Net.Http.StringContent(json, Encoding.UTF8, &quot;application/json&quot;);

 var url = &quot;https://httpbin.org/post&quot;;
 using var client = new HttpClient();

 var response = await client.PostAsync(url, data);

 string result = response.Content.ReadAsStringAsync().Result;
 Console.WriteLine(result);
}
</code></pre>

How can I create using C# and HttpClient the following POST request:
<img src="https://i.stack.imgur.com/yp8jr.png" alt="User-Agent: Fiddler Content-type: application/x-www-form-urlencoded Host: localhost:6740 Content-Length: 6">

I need such a request for my WEB API service:

<pre><code>[ActionName("exist")]
[HttpPost]
public bool CheckIfUserExist([FromBody] string login)
{ 
 return _membershipProvider.CheckIfExist(login);
}
</code></pre>

.NET HttpClient. How to POST string value?

JSON

如何使用C#和HttpClient创建以下POST请求：​我的WEB API服务需要这样一个请求：[ActionName("exist")][HttpPost]public bool CheckIfUserExist([FromBody] string login){               return _mem...

问.NET HttpClient.如何发布字符串值？
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