string webURL = "http://localhost/api/maintenance.php";
WebClient wc = new WebClient();
wc.Headers.Add("user-agent", "Only a Header!");
byte[] rawByteArray = wc.DownloadData(webURL);
string webContent = Encoding.UTF8.GetString(rawByteArray);

if (webContent.ToUpper().Contains("SAY_HELLO"))
     MessageBox.Show("hello world");
else if (webContent.ToUpper().Contains("SAY_HELLOME"))
     MessageBox.Show("hello me");

最好不要使用WebClient，它现在已经过时了，请参考下面的。

[Obsolete(Obsoletions.WebRequestMessage, DiagnosticId = Obsoletions.WebRequestDiagId, UrlFormat = Obsoletions.SharedUrlFormat)]
public WebClient()
{
    // We don't know if derived types need finalizing, but WebClient doesn't.
    if (GetType() == typeof(WebClient))
    {
        GC.SuppressFinalize(this);
    }
}

相反，最好使用HttpClient，它还提供异步方法。

string webURL = "http://localhost/api/maintenance.php";
HttpClient wc = new HttpClient();
wc.DefaultRequestHeaders.Add("user-agent", "Only a Header!");
byte[] rawByteArray = await wc.GetByteArrayAsync(webURL);
string webContent = Encoding.UTF8.GetString(rawByteArray);

if (webContent.ToUpper().Contains("SAY_HELLO"))
     MessageBox.Show("hello world");
else if (webContent.ToUpper().Contains("SAY_HELLOME"))
     MessageBox.Show("hello me");

您也可以使用string webContent = await wc.GetStringAsync(webURL);。不需要先取得字节数组，然后再转换成字串。注意：我还没有测试代码，但它应该工作。