Length

Every C# string object has a Length property. Every character (no matter its value) is counted in this property. Length is cached on strings.

Length gets the character count in the string instance. The string cannot be null. It is possible to avoid exceptions with help from Length.

An example

The string class has a Length property, which returns the number of characters in its internal buffer. We do not need to iterate through every character of a string.

using System;

// Part 1: an example string.
string a = "One example";
Console.WriteLine("LENGTH: " + a.Length);

// Part 2: an empty example string.
string b = "";
Console.WriteLine("LENGTH: " + b.Length);

// Part 3: a constant string.
const string c = "Three";
Console.WriteLine("LENGTH: " + c.Length);

// Part 4: a literal string.
Console.WriteLine("LENGTH: " + "Four".Length);
LENGTH: 11
LENGTH: 0
LENGTH: 5
LENGTH: 4

Null

We must first test for null strings before calling the Length property. This is because you cannot access members on null reference types. The IsNullOrEmpty method is ideal for this.

using System;

class Program
{
    static void Main()
    {
        F(null);
        F("cat");
        F("book");
        F("");
    }

    static void F(string a)
    {
        if (string.IsNullOrEmpty(a))
        {
            // String is null or empty.
            Console.WriteLine(true);
        }
        else
        {
            // Print length of string.
            Console.WriteLine(a.Length);
        }
    }
}
True
3
4
True

Error, cannot be assigned

We cannot assign to Length. We can only read the value of Length and change the string reference in other ways. This is an important aspect of the C# string type.

class Program
{
    static void Main()
    {
        string value = "test";
        value.Length = 10;
    }
}
error CS0200: Property or indexer 'string.Length' cannot be assigned to -- it is read only

Performance, hoisting length

In earlier versions of the .NET Framework, hoisting the Length and using a local helped speedup programs.

The Length property finds the number of chars in a string. It is precomputed, so is fast to repeatedly access. It returns its result in constant time.