Необходимо предоставить текст запроса, если задано ContentLength>0 или SendChunked==true. Сделайте это, вызвав [Begin]GetRequestStream перед вызовом [Begin]GetResponse.

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You must provide a request body if you set ContentLength>0 or SendChunked==true. Do this by calling [Begin]GetRequestStream before [Begin]GetResponse.

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