blocks|key|172450|text|是，您需要将引用添加到|type|unstyled|depth|inlineStyleRanges|entityRanges|data|172451|System.Net.Http.Formatting.dll|code-block|syntax|javascript|172452|这可以在扩展程序集区域中找到。|172453|实现此目的的一个好方法是将NuGet+package+Microsoft.AspNet.WebApi.Client添加到您的项目中。|offset|length|style|CODE|172454|entityMap|0|LINK|mutability|MUTABLE|url|https://www.nuget.org/packages/Microsoft.AspNet.WebApi.Client/^0|0|0|0|R|U|D|18|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|Z|8|@$K|10|L|11|M|N]]|9|@$K|12|L|13|1|14]]|A|$]]|$1|O|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

Yes, you need to add a reference to 

<pre><code>System.Net.Http.Formatting.dll
</code></pre>

This can be found in the extensions assemblies area.

A good way of achieving this is by adding the <a href="https://www.nuget.org/packages/Microsoft.AspNet.WebApi.Client/" rel="noreferrer">NuGet package <code>Microsoft.AspNet.WebApi.Client</code></a> to your project.

blocks|key|173699|text|缺少的引用是System.Net.Http.Formatting.dll。但更好的解决方案是添加NuGet包Microsoft.AspNet.WebApi.Client，以确保格式化dll的版本与我的项目中的System.Net.Http的.NET框架版本一起工作。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|173700|entityMap^0|6|U|1I|U|2W|F|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

The missing reference is the <code>System.Net.Http.Formatting.dll</code>. But the better solution is to add the NuGet package <code>Microsoft.AspNet.WebApi.Client</code> to ensure the version of the formatting dll worked with the .NET framework version of <code>System.Net.Http</code> in my project.

blocks|key|2991118|text|您可以使用internet上提供的包装器之一，而不是编写如此多的代码来进行简单的调用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2991119|我写了一个叫WebApiClient的，可以在NuGet上找到...看看这个！|2991120|https://www.nuget.org/packages/WebApiRestService.WebApiClient/|offset|length|2991121|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|1Q|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Q|8|@]|9|@$F|R|G|S|1|T]]|A|$]]|$1|H|3|-4|5|6|7|U|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|E]]]]

Instead of writing this amount of code to make a simple call, you could use one of the wrappers available over the internet.

I've written one called WebApiClient, available at NuGet... check it out!

<a href="https://www.nuget.org/packages/WebApiRestService.WebApiClient/" rel="nofollow noreferrer">https://www.nuget.org/packages/WebApiRestService.WebApiClient/</a>

blocks|key|172478|text|PostAsJsonAsync不再位于System.Net.Http.dll+(.NET+4.5.2)中。您可以添加对System.Net.Http.Formatting.dll的引用，但这实际上属于较旧的版本。我在我们的TeamCity+build服务器上遇到了这个问题，这两者不能协同工作。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|172479|或者，您可以用PostAsync调用替换PostAsJsonAsync，这只是新dll的一部分。替换|172480|var+response+=+client.PostAsJsonAsync("api/AgentCollection",+user).Result;|code-block|syntax|javascript|172481|通过以下方式：|172482|var+response+=+client.PostAsync("api/AgentCollection",+new+StringContent(
+++new+JavaScriptSerializer().Serialize(user),+Encoding.UTF8,+"application/json")).Result;|172483|请注意，JavaScriptSerializer位于名称空间System.Web.Script.Serialization中。|172484|您必须在csproj：System.Web.Extensions.dll中添加一个程序集引用|172485|请参阅https://code.msdn.microsoft.com/windowsapps/How-to-use-HttpClient-to-b9289836|172486|entityMap|0|LINK|mutability|MUTABLE|url|https://code.msdn.microsoft.com/windowsapps/How-to-use-HttpClient-to-b9289836^0|0|F|J|J|1N|U|0|7|9|K|F|0|0|0|0|4|K|U|V|0|B|P|0|3|25|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1B|8|@$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|1G|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1H|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|1I|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1J|8|@$9|1K|A|1L|B|C]|$9|1M|A|1N|B|C]]|D|@]|E|$]]|$1|S|3|T|5|6|7|1O|8|@$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|U|3|V|5|6|7|1R|8|@]|D|@$9|1S|A|1T|1|1U]]|E|$]]|$1|W|3|-4|5|6|7|1V|8|@]|D|@]|E|$]]]|X|$Y|$5|Z|10|11|E|$12|13]]]]

<code>PostAsJsonAsync</code> is no longer in the <code>System.Net.Http.dll</code> (.NET 4.5.2). You can add a reference to <code>System.Net.Http.Formatting.dll</code>, but this actually belongs to an older version. I ran into problems with this on our TeamCity build server, these two wouldn't cooperate together. 

Alternatively, you can replace <code>PostAsJsonAsync</code>with a <code>PostAsync</code> call, which is just part of new dll. 
Replace

<pre><code>var response = client.PostAsJsonAsync("api/AgentCollection", user).Result;
</code></pre>

With:

<pre><code>var response = client.PostAsync("api/AgentCollection", new StringContent(
 new JavaScriptSerializer().Serialize(user), Encoding.UTF8, "application/json")).Result;
</code></pre>

Note that <code>JavaScriptSerializer</code> is in the namespace: <code>System.Web.Script.Serialization</code>.

You will have to add an assembly reference in your csproj: <code>System.Web.Extensions.dll</code>

See <a href="https://code.msdn.microsoft.com/windowsapps/How-to-use-HttpClient-to-b9289836" rel="noreferrer">https://code.msdn.microsoft.com/windowsapps/How-to-use-HttpClient-to-b9289836</a>

blocks|key|172490|text|我知道这个回复太晚了，我也有同样的问题，我添加了System.Net.Http.Formatting.Extension+Nuget，在这里和那里检查之后，我发现Nuget已经添加了，但是System.Net.Http.Formatting.dll没有添加到引用中，我只是重新安装了Nuget|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|172491|entityMap^0|O|10|2M|U|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

I know this reply is too late, I had the same issue and i was adding the <code>System.Net.Http.Formatting.Extension</code> Nuget, after checking here and there I found that the Nuget is added but the <code>System.Net.Http.Formatting.dll</code> was not added to the references, I just reinstalled the Nuget

blocks|key|2778631|text|正如已经讨论过的，这种方法从.NET+4.5.2开始不再可用。要在Jeroen+K's+answer上展开，您可以创建一个扩展方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2778632|public+static+async+Task<HttpResponseMessage>+PostAsJsonAsync<TModel>(this+HttpClient+client,+string+requestUrl,+TModel+model)
{
++++var+serializer+=+new+JavaScriptSerializer();
++++var+json+=+serializer.Serialize(model);
++++var+stringContent+=+new+StringContent(json,+Encoding.UTF8,+"application/json");
++++return+await+client.PostAsync(requestUrl,+stringContent);
}|code-block|syntax|javascript|2778633|现在可以调用client.PostAsJsonAsync("api/AgentCollection",+user)了。|style|CODE|2778634|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/34594919/2441442^0|X|H|0|0|0|6|1F|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|Z|8|@$A|10|B|11|K|L]]|9|@]|C|$]]|$1|M|3|-4|5|6|7|12|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

As already debatted, this method isn't available anymore since .NET 4.5.2. To expand on <a href="https://stackoverflow.com/a/34594919/2441442">Jeroen K's answer</a> you can make an extension method:

<pre><code>public static async Task&lt;HttpResponseMessage&gt; PostAsJsonAsync&lt;TModel&gt;(this HttpClient client, string requestUrl, TModel model)
{
 var serializer = new JavaScriptSerializer();
 var json = serializer.Serialize(model);
 var stringContent = new StringContent(json, Encoding.UTF8, "application/json");
 return await client.PostAsync(requestUrl, stringContent);
}
</code></pre>

Now you are able to call <code>client.PostAsJsonAsync("api/AgentCollection", user)</code>.

blocks|key|172517|text|我在一个项目中也遇到了这个问题，我刚刚从源代码控制中签出了这个问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|172518|症状是上述错误和引用System.Net.Http.Formatting的黄色警告三角形|offset|length|style|CODE|172519|为了修复这个问题，我删除了损坏的引用，然后使用NuGet安装了最新版本的Microsoft.AspNet.WebApi.Client。|172520|entityMap^0|0|A|Q|0|10|U|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@$D|Q|E|R|F|G]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|K|$]]

I had this issue too on a project I'd just checked out from source control.

The symptom was the error described above and a yellow warning triangle on a reference to <code>System.Net.Http.Formatting</code>

To fix this, I removed the broken reference and then used NuGet to install the latest version of <code>Microsoft.AspNet.WebApi.Client</code>.

blocks|key|2991228|text|尝试在项目中安装NuGet包:+Microsoft.AspNet.WebApi.Client：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2991229|Install-Package+Microsoft.AspNet.WebApi.Client|code-block|syntax|javascript|2991230|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|$]]

Try to install in your project the NuGet Package: Microsoft.AspNet.WebApi.Client:

<pre><code>Install-Package Microsoft.AspNet.WebApi.Client
</code></pre>

blocks|key|172566|text|如果你正在玩Blazor并得到这个错误，你需要添加包Microsoft.AspNetCore.Blazor.HttpClient。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|172567|entityMap^0|Q|12|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

If you're playing around in Blazor and get the error, you need to add the package <code>Microsoft.AspNetCore.Blazor.HttpClient</code>.

blocks|key|173793|text|只需使用注释中的提示扩展Jeroen's+answer：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|173794|var+content+=+new+StringContent(
++++JsonConvert.SerializeObject(user),+
++++Encoding.UTF8,+
++++MediaTypeNames.Application.Json);

var+response+=+await+client.PostAsync("api/AgentCollection",+content);|code-block|syntax|javascript|173795|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/34594919/48729^0|C|F|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

Just expanding <a href="https://stackoverflow.com/a/34594919/48729">Jeroen's answer</a> with the tips in comments:

<pre><code>var content = new StringContent(
 JsonConvert.SerializeObject(user), 
 Encoding.UTF8, 
 MediaTypeNames.Application.Json);

var response = await client.PostAsync("api/AgentCollection", content);
</code></pre>

blocks|key|2778766|text|对我来说，经过多次尝试，我找到了解决方案，它正在替换|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2778767|HttpClient|code-block|syntax|javascript|2778768|使用|2778769|System.Net.Http.HttpClient|2778770|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

For me I found the solution after a lot of try which is replacing
<pre><code>HttpClient
</code></pre>
with
<pre><code>System.Net.Http.HttpClient
</code></pre>

blocks|key|176268|text|好了，现在是2020年世界末日，你可以在NuGet包System.Net.Http.Json中找到这些方法。但要注意它在内部使用System.Text.Json。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|176269|如果您确实需要找出哪个API驻留在哪里，那么只需使用https://apisof.net/。|176270|entityMap|0|LINK|mutability|MUTABLE|url|https://apisof.net/^0|Q|K|1S|G|0|Q|J|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@]|D|@$9|V|A|W|1|X]]|E|$]]|$1|H|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

Ok, it is apocalyptical 2020 now, and you can find these methods in NuGet package <code>System.Net.Http.Json</code>. But beware that it uses <code>System.Text.Json</code> internally.
And if you really need to find out which API resides where, just use <a href="https://apisof.net/" rel="nofollow noreferrer">https://apisof.net/</a>

blocks|key|2991328|text|如果你已经在使用Newtonsoft.Json了，试试这个：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2991329|++//+Alternative+using+WebApi.Client+5.2.7
++////var+response+=+await+Client.PutAsJsonAsync(
++////++++"api/AgentCollection",+user
++////++++requestListDto)

++var+response+=+await+Client.PostAsync("api/AgentCollection",+new+StringContent(
++++JsonConvert.SerializeObject(user),+Encoding.UTF8,+"application/json"));|code-block|syntax|javascript|2991330|性能比JavaScriptSerializer更好。看看这里，https://www.newtonsoft.com/json/help/html/Introduction.htm|offset|length|2991331|entityMap|0|LINK|mutability|MUTABLE|url|https://www.newtonsoft.com/json/help/html/Introduction.htm^0|0|0|V|1M|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

If you are already using Newtonsoft.Json try this:
<pre><code> // Alternative using WebApi.Client 5.2.7
 ////var response = await Client.PutAsJsonAsync(
 //// &quot;api/AgentCollection&quot;, user
 //// requestListDto)

 var response = await Client.PostAsync(&quot;api/AgentCollection&quot;, new StringContent(
 JsonConvert.SerializeObject(user), Encoding.UTF8, &quot;application/json&quot;));
</code></pre>
Performance are better than JavaScriptSerializer. Take a look here <a href="https://www.newtonsoft.com/json/help/html/Introduction.htm" rel="nofollow noreferrer">https://www.newtonsoft.com/json/help/html/Introduction.htm</a>

I am trying to call a web API from my web application. I am using .Net 4.5 and while writing the code I am getting the error <code>HttpClient</code> does not contain a definition <code>PostAsJsonAsync</code> method.

Below is the code:

<pre><code>HttpClient client = new HttpClient();
client.BaseAddress = new Uri("http://localhost:51093/");
client.DefaultRequestHeaders.Accept.Add(
 new MediaTypeWithQualityHeaderValue("application/json"));
var user = new Users();
user.AgentCode = 100;
user.Remarks = "Test";
user.CollectionDate = System.DateTime.Today;
user.RemittanceDate = System.DateTime.Today;
user.TotalAmount = 1000;
user.OrgBranchID = 101;

var response = client.PostAsJsonAsync("api/AgentCollection", user).Result;
</code></pre>

and I am getting the error message: 

<blockquote>
 Error: 'System.Net.Http.HttpClient' does not contain a definition for
 'PostAsJsonAsync' and No extension method 'PostAsJsonAsync' accepting a first argument of
 type 'System.Net.Http.HttpClient' could be found (are you missing
 a using directive or an assembly reference?)
</blockquote>

Please have a look and advice me.

HttpClient not supporting PostAsJsonAsync method C#

我正在尝试从我的web应用程序调用web API。我使用的是.Net 4.5，在编写代码时，我得到了一个错误：HttpClient不包含定义PostAsJsonAsync方法。代码如下：HttpClient client = new HttpClient();client.BaseAddress = new Uri("...

问HttpClient不支持PostAsJsonAsync方法C#
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