asp.net 去除字符串右侧的最后一个字符

string  str  =   " HTML,Asp/Aspx,MySQL/Cgi/Java/Php, " ;
        str  =  str.Remove(str.LastIndexOf( " , " ), 1 );
        Response.Write(str); 

string  a  =   " 1,3,5, " ;
        Response.Write(a.TrimEnd( ' , ' ));