Best practices in developing ASP.NET applications

string.IsNullOrEmpty:   312 ms
Length:                 140 ms  [fastest]

(string != null && string.Length !=0)

String.IsNullOrEmpty(string)

if (!String.IsNullOrEmpty(someString)) {
//do something with someString
...
}

if (someString != null && someString.Length != 0) {
//do something with someString
...
}