StringIsNullOrEmptysstring

String.IsNullOrEmpty is a static method in the .NET framework, defined on the System.String class. It returns true when the input string is either null or has a length of zero, and false otherwise. The method is commonly used to validate input, guard against null references, and simplify conditional logic that would otherwise require separate null and empty checks.

A related method, String.IsNullOrWhiteSpace, extends the check by treating strings that consist only of whitespace characters as empty as well. This makes it useful for trimming user input or validating form fields where non-visible characters should not count as meaningful data.

Usage examples:

- string s = null; String.IsNullOrEmpty(s) // true

- string t = ""; String.IsNullOrEmpty(t) // true

- string u = " "; String.IsNullOrEmpty(u) // false

- String.IsNullOrWhiteSpace(u) // true

Performance and behavior notes:

- String.IsNullOrEmpty performs a straightforward null and length check (O(1)) and does not allocate a new string.

- String.IsNullOrWhiteSpace may inspect each character to determine if whitespace-only, making it O(n) in the length of the string. It uses Unicode whitespace definitions to decide what counts as whitespace.

See also:

- String.IsNullOrWhiteSpace

- Common null/empty checks in other programming languages, such as (s == null || s.Length == 0) patterns.