null operator Sort String Compare, CompareTo string.Intern string.IsNullOrEmpty StringComparison

`String.Equals`

This method compares strings. It differs from the Compare and CompareTo methods: Equals() tests strings for equality. A StringComparison can be used to ignore case.

The Equals method is invoked with the method name Equals or with the equality operator. Most C# programs use the "==" operator syntax.

`Equals` example

Here we compare strings and test equality. The various parts of this code example code not all do the same thing, but each part does compare strings in some way.

Part 1 We use instance Equals on the first string—so it must not be null. If the string is null, we will get an exception.

Part 2 We use instance Equals by calling equals on the second string instead of the first string.

Part 3 The most common method for string comparisons is the equality operator.

using System;

string a = "a" + 1;
string b = "a" + 1;

// Part 1: use instance Equals() method.
if (a.Equals(b))
{
    Console.WriteLine("a.Equals(b) = true");
}

// Part 2: use instance method on other string.
if (b.Equals(a))
{
    Console.WriteLine("b.Equals(a) = true");
}

// Part 3: use op_Equality.
if (a == b)
{
    Console.WriteLine("a == b = true");
}a.Equals(b) = true
b.Equals(a) = true
a == b = true

Compare

Next, we review the Compare methods. You can see the result of the string.Compare method being tested for a value of zero.

Tip When string.Compare returns zero, the strings are equal. It returns 1 or -1 depending on the sorted order of the string parameters.

Info This means that if string.Compare ever returns 1 or -1, the strings are not equal.

using System;

string a = "a" + 1;
string b = "a" + 1;

// Use Compare methods.
// ... This returns zero if the strings are equal.
if (string.Compare(a, b) == 0)
{
    Console.WriteLine("string.Compare(a, b) = 0");
}

// ... CompareOrdinal returns 0 if numeric values equal.
if (string.CompareOrdinal(a, b) == 0)
{
    Console.WriteLine("string.CompareOrdinal(a, b) = 0");
}

// ... Use instance Compare method.
if (a.CompareTo(b) == 0)
{
    Console.WriteLine("a.CompareTo(b) = 0");
}string.Compare(a, b) = 0
string.CompareOrdinal(a, b) = 0
a.CompareTo(b) = 0

Benchmark

To test the code for Equals methods, I ran a benchmark of 2 strings that are equal but don't share a string pool reference. This avoids the intern pool.

Result The performance of Equals and the equality operator are about the same. The clearest code is the best option here.

Empty strings

You will find that, as FxCop recommends, testing against Length is usually the best way. The string.IsNullOrEmpty method deserves a special mention—it is fast and safe.

Cases

You can easily compare strings in such a way that their case differences are ignored. We use values of the StringComparison enum.

A summary

We used the string.Equals and Compare methods. The string.Equals and equal operators have the same use normally. Compare and CompareTo are useful for sorting.

String.Equals