WebProxy class in Portable Class Library

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C#

proxy

I'm building an Portable Class Library project. I seem not support WebProxy ( I'm preferenced the System.Net, i added Using System.Net )

cookieContainer = new CookieContainer();
handler = new HttpClientHandler
{
CookieContainer = cookieContainer,
UseCookies = true,
UseDefaultCredentials = false,
UseProxy = true,
Proxy = new WebProxy("http://127.0.0.1:8888"), // It raised the error here: Error 1 The type or namespace name 'WebProxy' could not be found (are you missing a using directive or an assembly reference?)
}; 

How to slove this ?