BRO1106: Use string.Empty for empty strings

"" should be written as string.Empty. Replaces StyleCop SA1122.

Example

Before

public class Person
{
    public string Name { get; set; } = "";

    public bool HasName() => Name != "";
}

After

public class Person
{
    public string Name { get; set; } = string.Empty;

    public bool HasName() => Name != string.Empty;
}

Why

string.Empty states the intent, while "" can look like a string that is missing its text, or one where a space was lost. One way of writing it also makes empty strings easy to search for.

Not reported

string.Empty is not a compile-time constant, so wherever C# requires a constant the literal stays:

• const fields and locals, including a "" inside a longer constant expression (const string X = "" + "a")
• attribute arguments: [Obsolete("")]
• parameter default values: void M(string s = "")
• case "" labels and patterns: s is "", "" => ... in a switch expression (the arm's result, _ => "", is replaced)

Interpolated ($"") and UTF-8 (""u8) literals are different kinds of literal and aren't reported either.

Configuration

[*.cs]
# string_empty (default): string.Empty, like StyleCop
# literal: ""
stylebro_empty_string_style = string_empty

With literal, the rule reports the other way around: string.Empty (also written String.Empty or System.String.Empty) becomes "", everywhere, since a literal is valid in every context.

// Before (stylebro_empty_string_style = literal)
public string Name { get; set; } = string.Empty;
public bool HasName() => Name != String.Empty;

// After
public string Name { get; set; } = "";
public bool HasName() => Name != "";

Not reported in the literal style: a member named Empty on another type, nameof(string.Empty) (nameof("") doesn't compile), and an access with a comment inside it (string /* x */ .Empty). A verbatim @"" is left to BRO1138, which removes the @.

Compared with StyleCop

Same results as StyleCop. The literal style is the opposite of StyleCop's rule (Roslynator's RCS1078 has the same choice).