输入如下代码。请注意，C＃区分大小写

enter code like below. Please note that C# is case sensitive

string ext = System.IO.Path.GetExtension(fileupload1.filename);

请检查您的代码中的任何语法错误可能是因为在visual studio中无法使用的智能

Please check for any syntax error in your code may be because of that intelli-sense not working in visual studio