如果你在安装NuGet Microsoft.AspNet.WebApi.Client后没有找到它，在解决方案的packages文件夹中手动添加一个引用:

\Microsoft.AspNet.WebApi.Client.5.2.6\lib\net45\System.Net.Http.Formatting.dll

不要陷入将旧的引用添加到System.Net.Http.Formatting.dll NuGet的陷阱

如果你在安装NuGet Microsoft.AspNet.WebApi.Client后没有找到它，在解决方案的packages文件夹中手动添加一个引用:

\Microsoft.AspNet.WebApi.Client.5.2.6\lib\net45\System.Net.Http.Formatting.dll

不要陷入将旧的引用添加到System.Net.Http.Formatting.dll NuGet的陷阱

2021更新:看起来在。net 5中删除了该方法。或者，您可以使用ReadFromJsonAsync<>()从System.Net.Http.Json.HttpContentJsonExtensions。它解决了问题。

虽然我也有同样的问题，但这个帖子中的答案并没有完全帮助我解决问题。因此，我决定把我的研究结果写在这篇文章里。要修复此问题，请执行以下步骤:

使用NuGet将Microsoft.AspNet.WebApi.Client包添加到项目中。而在ASP。在Visual Studio IDE的Tools > NuGet Package Manager > Package Manager Console中打开包管理器控制台，并将Microsoft.AspNet.WebApi.Client包添加到解决方案中。

Install-Package Microsoft.AspNet.WebApi.Client -Version 5.2.7

安装后，检查系统上是否存在扩展DLL。作为第一步的结果，System.Net.Http.Formatting.dll文件应该出现在如下所示的目录中。

{root-solution-directory}\packages\Microsoft.AspNet.WebApi.Client.{package-version}\lib\net45\

Manually add the reference to the relevant project. Right click on the "References" section in the ASP.NET project within the solution click on the "Add Reference..." section. Since the file System.Net.Http.Formatting.dll is an extension, it will not be listed when searched directly like other items in the Microsoft.AspNet.WebApi.Client package. Therefore, to add the DLL file manually, click the "Browse..." button at the bottom of the "Reference Manager" window. Select the System.Net.Http.Formatting.dll file in the directory shown in the second step and check the checkbox to include the DLL file in the project. Include the System.Net.Http namespace in the project to use the features provided by this DLL in the project; using System.Net.Http.Formatting; declaration is available within the HttpContentExtensions static class.

using System.Net.Http;

可选:您可以通过安装System.Net.Http.Formatting.Extension或WebApiDoodle.Net.Http.Formatting包并遵循上面的步骤来实现类似的解决方案。

如果你已经在使用Newtonsoft。Json，不想安装Microsoft.AspNet.WebApi.Client:

 var myInstance = JsonConvert.DeserializeObject<MyClass>(
   await response.Content.ReadAsStringAsync());

你可以写扩展方法:

public static async Task<Tout> ReadAsAsync<Tout>(this System.Net.Http.HttpContent content) {
    return Newtonsoft.Json.JsonConvert.DeserializeObject<Tout>(await content.ReadAsStringAsync());
}