IsNullOrEmpty

The string.IsNullOrEmpty method tests strings. It checks for string references that are null or have no data. IsNullOrWhiteSpace tests for whitespace.

Strings are reference types and can be equal to null. Often our programs want to handle empty, null, or whitespace strings in special ways. These 2 methods are ideal here.

IsNullOrEmpty

Here, we look at examples of IsNullOrEmpty. The 3 parts in the code samples how the IsNullOrEmpty method matching a null string, an empty string, and then a normal string.

using System;

// Part 1: test with IsNullOrEmpty.
string s = null;
if (string.IsNullOrEmpty(s) == true)
{
    // True.
    Console.WriteLine("Null or empty");
}
else
{
    Console.WriteLine("Not null and not empty");
}

// Part 2: test with IsNullOrEmpty again.
string y = "";
if (string.IsNullOrEmpty(y) == true)
{
    // True.
    Console.WriteLine("Null or empty");
}
else
{
    Console.WriteLine("Not null and not empty");
}

// Part 3: test with IsNullOrEmpty without writing true.
string z = "sam";
if (string.IsNullOrEmpty(z))
{
    // False.
    Console.WriteLine("Null or empty");
}
else
{
    Console.WriteLine("Not null and not empty");
}
Null or empty
Null or empty
Not null and not empty

IsNullOrWhiteSpace

This method tests an entire string. It returns true if the string contains only whitespace characters or is null.

using System;

class Program
{
    static void Main()
    {
        bool a = string.IsNullOrWhiteSpace("   ");
        bool b = string.IsNullOrWhiteSpace("\n");
        bool c = string.IsNullOrWhiteSpace("\r\n\v ");
        bool d = string.IsNullOrWhiteSpace(null);
        bool e = string.IsNullOrWhiteSpace("");
        bool f = string.IsNullOrWhiteSpace(string.Empty);
        bool g = string.IsNullOrWhiteSpace("dotnetperls");
        Console.WriteLine(a);
        Console.WriteLine(b);
        Console.WriteLine(c);
        Console.WriteLine(d);
        Console.WriteLine(e);
        Console.WriteLine(f);
        Console.WriteLine(g);
    }
}
True
True
True
True
True
True
False

Benchmark, IsNullOrEmpty

We compare ways of checking strings. The string is tested against null before checking its length—this avoids a null reference exception.

using System;
using System.Diagnostics;

const int _max = 100000000;
string tested = "test";
var s1 = Stopwatch.StartNew();
// Version 1: call IsNullOrEmpty.
for (int i = 0; i < _max; i++)
{
    if (string.IsNullOrEmpty(tested))
    {
        return;
    }
}
s1.Stop();
var s2 = Stopwatch.StartNew();
// Version 2: use inlined tests.
for (int i = 0; i < _max; i++)
{
    if (tested == null || tested.Length == 0)
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
1.34 ns    string.IsNullOrEmpty
0.84 ns    null, Length

Notes, IsNullOrEmpty

This method detects null and empty strings in a single call. It is static—this means it can be called to test a null string, and no exception is raised.

Internals, IsNullOrEmpty

How is IsNullOrEmpty implemented? It simply tests for null and then a Length equal to 0. It provides no magic optimizations (as testing also shows).

IsNullOrEmpty, a string method, gives us a way to check whether a string is acceptable to save or use. For performance it may be better to use manual null checks.