Console const if null String NullReferenceException readonly static String Literal String switch string.Intern string.IsNullOrEmpty

Empty strings

The C# string.Empty field is an empty string literal. It is not the same as an empty string literal constant—there is a subtle difference.

We compare ways to create, and test, empty strings. An understanding of string literals is helpful here. There are some minor performance differences.

In this example we use string.Empty in a C# program. The string.Empty field is initialized to the empty string literal at runtime by .NET.

And This means string.Empty cannot be used in a switch. But we can test with string.Empty in if-statements.

Part 1 We assign to string.Empty. This is mostly equivalent to "". We test the variable against the constant "", which returns true.

Part 2 The program tests the Length of the Empty string. Its length is 0, so the string Empty is printed.

Part 3 Here we use a switch statement on string.Empty. We cannot use string.Empty as a case, as it is not a constant.

using System;

// Part 1: initialize string to empty string field.
string value = string.Empty;
if (value == "")
{
    Console.WriteLine("Empty");
}

// Part 2: test against Length.
if (value.Length == 0)
{
    Console.WriteLine("Empty");
}

// Part 3: switch on the string constant.
// ... We cannot have string.Empty as a case.
switch (value)
{
    case "":
        {
            Console.WriteLine("Empty");
            break;
        }
}Empty
Empty
Empty

Has characters

An important point is to focus on the intention of code. If we want to see if a string has characters, IsNullOrEmpty is a good choice.

Tip Suppose we have a slow part of our code, and we want to avoid running if unless a string has characters.

Info With IsNullOrEmpty, code is easier to understand and less likely to have mistakes in the logic that cause the slow path to be run.

using System;

string animal = "";
if (!string.IsNullOrEmpty(animal))
{
    // Do something that might be slow.
    Console.WriteLine(animal.ToUpper());
}
else
{
    Console.WriteLine("SKIPPED");
}
SKIPPED

Benchmark, empty `string`

An empty string has 0 chars. These strings can be checked using a clear and efficient method. Using the fastest method can help many programs.

Version 1 We test an empty string against the empty string literal. If both strings are literals, this would be faster.

Version 2 This code uses string.IsNullOrEmpty, and it has slightly different logic (it handles null).

Version 3 The third version uses the Length property. The Length check throws a NullReferenceException when the string is null.

Result For testing strings that are not constant literals (and are not null), using Length is the fastest. This was last tested in .NET 7.

using System;
using System.Diagnostics;

// Get empty string.
string input = "a".Replace("a", "");

int count = 0;
const int m = 100000000;
// Version 1: test against empty literal.
Stopwatch s1 = Stopwatch.StartNew();
for (int i = 0; i < m; i++)
{
    if (input == "")
    {
        count++;
    }
}
s1.Stop();
// Version 2: use IsNullOrEmpty.
Stopwatch s2 = Stopwatch.StartNew();
for (int i = 0; i < m; i++)
{
    if (string.IsNullOrEmpty(input))
    {
        count++;
    }
}
s2.Stop();
// Version 3: test Length.
Stopwatch s3 = Stopwatch.StartNew();
for (int i = 0; i < m; i++)
{
    if (input.Length == 0)
    {
        count++;
    }
}
s3.Stop();
Console.WriteLine(s1.ElapsedMilliseconds);
Console.WriteLine(s2.ElapsedMilliseconds);
Console.WriteLine(s3.ElapsedMilliseconds); 70 ms    == ""
251 ms   IsNullOrEmpty
 33 ms   Length == 0

Benchmark, `string.Empty`

When you specify "" instead string.Empty, the C# compiler itself will know the value of the string data. The runtime is not a part of this decision.

Info You can run a benchmark where string.Empty is much slower than "", by using a conditional that is removed by the C# compiler.

Version 1 This code tests the string.Empty read only field against the null literal in a tight loop.

Version 2 Here we test the empty string literal against null in a tight loop. The constant comparison can be removed at compile-time.

Result Certain C# compiler optimizations are effective only with "", not string.Empty.

using System;
using System.Diagnostics;

const int _max = 100000000;
var s1 = Stopwatch.StartNew();
// Version 1: test string.Empty.
for (int i = 0; i < _max; i++)
{
    if (string.Empty == null)
    {
        throw new Exception();
    }
}
s1.Stop();
var s2 = Stopwatch.StartNew();
// Version 2: test string literal.
for (int i = 0; i < _max; i++)
{
    if ("" == null)
    {
        throw new Exception();
    }
}
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"));0.57 ns    string.Empty
0.28 ns    ""

Implementation

Here we look inside .NET and see how the Empty field is initialized. In the String class, look at the static constructor.

And The Empty field is assigned to the constant "". This constant is a string literal instance.

static String()
{
    Empty = "";
    //
    // More initialization omitted
    //
}
public static readonly string Empty;

Switch

Because string.Empty is initialized in the static constructor for the String class, it cannot be determined when you compile your program. It is a readonly field.

Important The case statements in switches can only contain const strings. So string.Empty, a readonly field, cannot be used.

And Complex switch statements are implemented as a Dictionary, providing constant lookup times.

Notes, equality operator

If you look at == in the IL, it is called op_Equality. This operator compiles to a method call (like any other). The == is just syntactic sugar.

An empty string can be checked in many ways. We choose which one depending on clarity and convention (what is used already). The string.Empty field can be used.