System.String.IsNullOrEmpty method
This article provides supplementary remarks to the reference documentation for this API.
IsNullOrEmpty is a convenience method that enables you to simultaneously test whether a String is null
or its value is String.Empty. It is equivalent to the following code:
bool TestForNullOrEmpty(string s)
{
bool result;
result = s == null || s == string.Empty;
return result;
}
string s1 = null;
string s2 = "";
Console.WriteLine(TestForNullOrEmpty(s1));
Console.WriteLine(TestForNullOrEmpty(s2));
// The example displays the following output:
// True
// True
result = s Is Nothing OrElse s = String.Empty
let testForNullOrEmpty (s: string): bool =
s = null || s = String.Empty
let s1 = null
let s2 = ""
printfn "%b" (testForNullOrEmpty s1)
printfn "%b" (testForNullOrEmpty s2)
// The example displays the following output:
// true
// true
You can use the IsNullOrWhiteSpace method to test whether a string is null
, its value is String.Empty, or it consists only of white-space characters.
What is a null string?
A string is null
if it has not been assigned a value (in C++ and Visual Basic) or if it has explicitly been assigned a value of null
. Although the composite formatting feature can gracefully handle a null string, as the following example shows, attempting to call one if its members throws a NullReferenceException.
String s = null;
Console.WriteLine("The value of the string is '{0}'", s);
try
{
Console.WriteLine("String length is {0}", s.Length);
}
catch (NullReferenceException e)
{
Console.WriteLine(e.Message);
}
// The example displays the following output:
// The value of the string is ''
// Object reference not set to an instance of an object.
Module Example
Public Sub Main()
Dim s As String
Console.WriteLine("The value of the string is '{0}'", s)
Try
Console.WriteLine("String length is {0}", s.Length)
Catch e As NullReferenceException
Console.WriteLine(e.Message)
End Try
End Sub
End Module
' The example displays the following output:
' The value of the string is ''
' Object reference not set to an instance of an object.
let (s: string) = null
printfn "The value of the string is '%s'" s
try
printfn "String length is %d" s.Length
with
| :? NullReferenceException as ex -> printfn "%s" ex.Message
// The example displays the following output:
// The value of the string is ''
// Object reference not set to an instance of an object.
What is an empty string?
A string is empty if it is explicitly assigned an empty string ("") or String.Empty. An empty string has a Length of 0. The following example creates an empty string and displays its value and its length.
String s = "";
Console.WriteLine("The length of '{0}' is {1}.", s, s.Length);
// The example displays the following output:
// The length of '' is 0.
Dim s As String = ""
Console.WriteLine("The length of '{0}' is {1}.", s, s.Length)
' The example displays the following output:
' The length of '' is 0.
let s = ""
printfn "The length of '%s' is %d." s s.Length
// The example displays the following output:
// The length of '' is 0.