string pattern1 = @"(?i)/plus/download.php\?open[^""\s]+?""";

------解决方案--------------------

Htmlstr.Text = strRegex;
应该是
Htmlstr.Text = m[0].Value;
或者类似的……

------解决方案--------------------
      string str = "/plus/download.php?open=2&id=114&uhash=67aed3395bb5d02c3eb3980e\"";
       var s = Regex.Match(str, @"(?is)/plus/download\.php\?open[^""]+(?="")").Value;
       Response.Write(s);
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结果
/plus/download.php?open=2&id=114&uhash=67aed3395bb5d02c3eb3980e  
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