Why is this an issue?

Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.

On the other hand, constants can be referenced from many places, but only need to be updated in a single place.

Noncompliant code example

public class Foo
{
    private string name = "foobar"; // Noncompliant

    public string DefaultName { get; } = "foobar"; // Noncompliant

    public Foo(string value = "foobar") // Noncompliant
    {
        var something = value ?? "foobar"; // Noncompliant
    }
}

Compliant solution

public class Foo
{
    private const string Foobar = "foobar";

    private string name = Foobar;

    public string DefaultName { get; } = Foobar;

    public Foo(string value = Foobar)
    {
        var something = value ?? Foobar;
    }
}

Exceptions

The following are ignored:

• literals with fewer than 5 characters
• literals matching one of the parameter names
• literals used in attributes