使用这样的：

  VAR的结果=的String.Compare（AA，AA，StringComparison.OrdinalIgnoreCase）;
 

的String.Compare方法（字符串，字符串，布尔）

How to convert the string to uppercase before performing a compare, or is it possible to compare the string by ignoring the case

 if (Convert.ToString(txt_SecAns.Text.Trim()).ToUpper() == 
     Convert.ToString(hidden_secans.Value).ToUpper())

use this:

var result = String.Compare("AA", "aa", StringComparison.OrdinalIgnoreCase);

String.Compare Method (String, String, Boolean)