C# check if Twitch steam is online or not

API
2

You should use HttpClient, for example:

class SomeClass {
        static HttpClientHandler hcHandle = new HttpClientHandler();

        static async Task<bool> IsOnline(string channel)
        {
            using (var hc = new HttpClient(hcHandle, false))
                    // false here prevents disposing the handler, which should live for the duration of the program and be shared by all requests that use the same handler properties
            {
                hc.DefaultRequestHeaders.Add("Client-ID", "your client id");
                hc.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", "your oauth token, should you have one (you should, but not required)");
                hc.DefaultRequestHeaders.UserAgent.ParseAdd("this would be good practice to set to your own");
                hc.Timeout = TimeSpan.FromSeconds(5); // good idea to set to something reasonable

                using (var response = await hc.GetAsync($"https://api.twitch.tv/helix/streams?user_login={channel}"))
                {
                    response.EnsureSuccessStatusCode(); // throws, if fails, can check response.StatusCode yourself if you prefer
                    string jsonString = await response.Content.ReadAsStringAsync();
                    // TODO: parse json and return true, if the returned array contains the stream
                }
            }
        }
}