Visual Studio Warning - Prefix 'S_' or 'T_' is not expected

Question

Whenever I create many constants for different purposes in a project, I tend to categorize them by putting a one letter prefix and an underscore in front of the names. Now, I chose the prefix T_ for some constants and noticed that the compiler gave me a warning:

IDE1006: Naming rule violation: Prefix 'T_' is not expected

That made me really curious, since I have no naming rule configured that says anything about that prefix, so I experimented a little. This warning only shows for the letters S and T, both uppercase and lowercase, so other prefixes like A_, b_ or Z_ are allowed. I tried this with local variables, properties, constants, methods and classes, and for all those cases, this rule applies: No uppercase or lowercase T_ or S_ at the start of an identifier. I know there are many cases I didn't test, but I'm pretty sure this is consistent across all sorts of identifiers in C#.

Now the question is, why is that? Are the prefixes T_, t_, S_, s_ used for anything specific internally, or is it a general naming convention that I simply don't know anything about?

I already googled for the specific message about the prefix, or the prefix in general, and I also took a very brief look at the documentation of IDE1006. None of those things yielded any answers.

Since it was requested, here's my .editorconfig:

[*.cs]

# IDE0039: Use local function
dotnet_diagnostic.IDE0039.severity = silent

As you can see, there is nothing naming related here.

And here's my .csproj:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

And finally, the .editorconfig file generated from my Code Style settings (naming conventions at the very bottom):

# Remove the line below if you want to inherit .editorconfig settings from higher directories
root = true

# C# files
[*.cs]

#### Core EditorConfig Options ####

# Indentation and spacing
indent_size = 4
indent_style = tab
tab_width = 4

# New line preferences
end_of_line = crlf
insert_final_newline = false

#### .NET Coding Conventions ####

# Organize usings
dotnet_separate_import_directive_groups = true
dotnet_sort_system_directives_first = true
file_header_template = unset

# this. and Me. preferences
dotnet_style_qualification_for_event = false:warning
dotnet_style_qualification_for_field = false
dotnet_style_qualification_for_method = false:warning
dotnet_style_qualification_for_property = false:warning

# Language keywords vs BCL types preferences
dotnet_style_predefined_type_for_locals_parameters_members = true:warning
dotnet_style_predefined_type_for_member_access = true:warning

# Parentheses preferences
dotnet_style_parentheses_in_arithmetic_binary_operators = always_for_clarity:suggestion
dotnet_style_parentheses_in_other_binary_operators = always_for_clarity:suggestion
dotnet_style_parentheses_in_other_operators = never_if_unnecessary:suggestion
dotnet_style_parentheses_in_relational_binary_operators = always_for_clarity:suggestion

# Modifier preferences
dotnet_style_require_accessibility_modifiers = for_non_interface_members

# Expression-level preferences
dotnet_style_coalesce_expression = true:warning
dotnet_style_collection_initializer = true:warning
dotnet_style_explicit_tuple_names = true:warning
dotnet_style_namespace_match_folder = true
dotnet_style_null_propagation = true:warning
dotnet_style_object_initializer = true:warning
dotnet_style_operator_placement_when_wrapping = beginning_of_line
dotnet_style_prefer_auto_properties = true:warning
dotnet_style_prefer_compound_assignment = true:warning
dotnet_style_prefer_conditional_expression_over_assignment = true:warning
dotnet_style_prefer_conditional_expression_over_return = true:warning
dotnet_style_prefer_inferred_anonymous_type_member_names = true:warning
dotnet_style_prefer_inferred_tuple_names = true:warning
dotnet_style_prefer_is_null_check_over_reference_equality_method = true:warning
dotnet_style_prefer_simplified_boolean_expressions = true:warning
dotnet_style_prefer_simplified_interpolation = true

# Field preferences
dotnet_style_readonly_field = true:warning

# Parameter preferences
dotnet_code_quality_unused_parameters = all:warning

# Suppression preferences
dotnet_remove_unnecessary_suppression_exclusions = none

# New line preferences
dotnet_style_allow_multiple_blank_lines_experimental = true
dotnet_style_allow_statement_immediately_after_block_experimental = true

#### C# Coding Conventions ####

# var preferences
csharp_style_var_elsewhere = false:warning
csharp_style_var_for_built_in_types = false:warning
csharp_style_var_when_type_is_apparent = false:warning

# Expression-bodied members
csharp_style_expression_bodied_accessors = true:warning
csharp_style_expression_bodied_constructors = true:warning
csharp_style_expression_bodied_indexers = true:warning
csharp_style_expression_bodied_lambdas = true:warning
csharp_style_expression_bodied_local_functions = true:warning
csharp_style_expression_bodied_methods = true:warning
csharp_style_expression_bodied_operators = true:warning
csharp_style_expression_bodied_properties = true:warning

# Pattern matching preferences
csharp_style_pattern_matching_over_as_with_null_check = true:warning
csharp_style_pattern_matching_over_is_with_cast_check = true:warning
csharp_style_prefer_extended_property_pattern = true
csharp_style_prefer_not_pattern = true:warning
csharp_style_prefer_pattern_matching = true:warning
csharp_style_prefer_switch_expression = true:warning

# Null-checking preferences
csharp_style_conditional_delegate_call = true:warning

# Modifier preferences
csharp_prefer_static_local_function = true:warning
csharp_preferred_modifier_order = public,private,protected,internal,static,extern,new,virtual,abstract,sealed,override,readonly,unsafe,volatile,async

# Code-block preferences
csharp_prefer_braces = false:warning
csharp_prefer_simple_using_statement = true:warning
csharp_style_namespace_declarations = file_scoped:warning

# Expression-level preferences
csharp_prefer_simple_default_expression = true:warning
csharp_style_deconstructed_variable_declaration = true
csharp_style_implicit_object_creation_when_type_is_apparent = true:warning
csharp_style_inlined_variable_declaration = true:warning
csharp_style_prefer_index_operator = true:warning
csharp_style_prefer_local_over_anonymous_function = true:warning
csharp_style_prefer_null_check_over_type_check = true:warning
csharp_style_prefer_range_operator = true:warning
csharp_style_prefer_tuple_swap = true
csharp_style_throw_expression = true:warning
csharp_style_unused_value_assignment_preference = discard_variable:warning
csharp_style_unused_value_expression_statement_preference = discard_variable

# 'using' directive preferences
csharp_using_directive_placement = outside_namespace:warning

# New line preferences
csharp_style_allow_blank_line_after_colon_in_constructor_initializer_experimental = false:warning
csharp_style_allow_blank_lines_between_consecutive_braces_experimental = true
csharp_style_allow_embedded_statements_on_same_line_experimental = true

#### C# Formatting Rules ####

# New line preferences
csharp_new_line_before_catch = false
csharp_new_line_before_else = false
csharp_new_line_before_finally = false
csharp_new_line_before_members_in_anonymous_types = true
csharp_new_line_before_members_in_object_initializers = true
csharp_new_line_before_open_brace = none
csharp_new_line_between_query_expression_clauses = true

# Indentation preferences
csharp_indent_block_contents = true
csharp_indent_braces = false
csharp_indent_case_contents = true
csharp_indent_case_contents_when_block = false
csharp_indent_labels = one_less_than_current
csharp_indent_switch_labels = true

# Space preferences
csharp_space_after_cast = true
csharp_space_after_colon_in_inheritance_clause = true
csharp_space_after_comma = true
csharp_space_after_dot = false
csharp_space_after_keywords_in_control_flow_statements = true
csharp_space_after_semicolon_in_for_statement = true
csharp_space_around_binary_operators = before_and_after
csharp_space_around_declaration_statements = false
csharp_space_before_colon_in_inheritance_clause = true
csharp_space_before_comma = false
csharp_space_before_dot = false
csharp_space_before_open_square_brackets = false
csharp_space_before_semicolon_in_for_statement = false
csharp_space_between_empty_square_brackets = false
csharp_space_between_method_call_empty_parameter_list_parentheses = false
csharp_space_between_method_call_name_and_opening_parenthesis = false
csharp_space_between_method_call_parameter_list_parentheses = false
csharp_space_between_method_declaration_empty_parameter_list_parentheses = false
csharp_space_between_method_declaration_name_and_open_parenthesis = false
csharp_space_between_method_declaration_parameter_list_parentheses = false
csharp_space_between_parentheses = false
csharp_space_between_square_brackets = false

# Wrapping preferences
csharp_preserve_single_line_blocks = true
csharp_preserve_single_line_statements = false

#### Naming styles ####

# Naming rules

dotnet_naming_rule.constant_should_be_capslock.severity = warning
dotnet_naming_rule.constant_should_be_capslock.symbols = constant
dotnet_naming_rule.constant_should_be_capslock.style = capslock

dotnet_naming_rule.local_should_be_camel_case.severity = warning
dotnet_naming_rule.local_should_be_camel_case.symbols = local
dotnet_naming_rule.local_should_be_camel_case.style = camel_case

dotnet_naming_rule.interface_should_be_begins_with_i.severity = warning
dotnet_naming_rule.interface_should_be_begins_with_i.symbols = interface
dotnet_naming_rule.interface_should_be_begins_with_i.style = begins_with_i

dotnet_naming_rule.types_should_be_pascal_case.severity = warning
dotnet_naming_rule.types_should_be_pascal_case.symbols = types
dotnet_naming_rule.types_should_be_pascal_case.style = pascal_case

dotnet_naming_rule.non_field_members_should_be_pascal_case.severity = warning
dotnet_naming_rule.non_field_members_should_be_pascal_case.symbols = non_field_members
dotnet_naming_rule.non_field_members_should_be_pascal_case.style = pascal_case

# Symbol specifications

dotnet_naming_symbols.interface.applicable_kinds = interface
dotnet_naming_symbols.interface.applicable_accessibilities = public, internal, private, protected, protected_internal
dotnet_naming_symbols.interface.required_modifiers = 

dotnet_naming_symbols.types.applicable_kinds = class, struct, interface, enum
dotnet_naming_symbols.types.applicable_accessibilities = public, internal, private, protected, protected_internal
dotnet_naming_symbols.types.required_modifiers = 

dotnet_naming_symbols.non_field_members.applicable_kinds = property, event, method
dotnet_naming_symbols.non_field_members.applicable_accessibilities = public, internal, private, protected, protected_internal
dotnet_naming_symbols.non_field_members.required_modifiers = 

dotnet_naming_symbols.constant.applicable_kinds = *
dotnet_naming_symbols.constant.applicable_accessibilities = *
dotnet_naming_symbols.constant.required_modifiers = const

dotnet_naming_symbols.local.applicable_kinds = *
dotnet_naming_symbols.local.applicable_accessibilities = local
dotnet_naming_symbols.local.required_modifiers = 

# Naming styles

dotnet_naming_style.pascal_case.required_prefix = 
dotnet_naming_style.pascal_case.required_suffix = 
dotnet_naming_style.pascal_case.word_separator = 
dotnet_naming_style.pascal_case.capitalization = pascal_case

dotnet_naming_style.begins_with_i.required_prefix = I
dotnet_naming_style.begins_with_i.required_suffix = 
dotnet_naming_style.begins_with_i.word_separator = 
dotnet_naming_style.begins_with_i.capitalization = pascal_case

dotnet_naming_style.capslock.required_prefix = 
dotnet_naming_style.capslock.required_suffix = 
dotnet_naming_style.capslock.word_separator = 
dotnet_naming_style.capslock.capitalization = all_upper

dotnet_naming_style.camel_case.required_prefix = 
dotnet_naming_style.camel_case.required_suffix = 
dotnet_naming_style.camel_case.word_separator = 
dotnet_naming_style.camel_case.capitalization = camel_case

---

PS: I know using underscores in identifiers is sort of frowned upon, the reason I use them is because I name my constants in capslock, which, I think, is also frowned upon? I do that because I prefer having a clear indication of what's a constant and what isn't.

Answer 1

I'm pretty sure this is a bug with the naming rule code, and I've opened an issue with Roslyn here.

The problem happens when you have a naming rule which applies to your consts (although I suspect the exact type of symbol doesn't matter). The code which checks that a name meets a rule is here. It's fine up to this section:

name = StripCommonPrefixes(name.Substring(Prefix.Length), out var prefix);

if (prefix != string.Empty)
{
    // name started with specified prefix, but has at least one additional common prefix 
    // Example: specified prefix "test_", actual prefix "test_m_"
    failureReason = Prefix == string.Empty ?
        string.Format(CompilerExtensionsResources.Prefix_0_is_not_expected, prefix) :
        string.Format(CompilerExtensionsResources.Prefix_0_does_not_match_expected_prefix_1, prefix, Prefix);
    return false;
}

(Prefix_0_is_not_expected is the "Prefix '{0}' is not expected" error message you're seeing, which is how I found this code.)

StripCommonPrefixes looks like this:

public static string StripCommonPrefixes(string name, out string prefix)
{
    var index = 0;
    while (index + 1 < name.Length)
    {
        switch (char.ToLowerInvariant(name[index]))
        {
            case 'm':
            case 's':
            case 't':
                if (index + 2 < name.Length && name[index + 1] == '_')
                {
                    index += 2;
                    continue;
                }

                break;

            case '_':
                index++;
                continue;

            default:
                break;
        }

        // If we reach this point, the current iteration did not strip any additional characters
        break;
    }

    prefix = name.Substring(0, index);
    return name.Substring(index);
}

If it finds that the name starts with m_, s_ or t_ (in any casing) with any number of leading underscores, it determines that it's a "prefix". Since we didn't configure a prefix, and it found one, we get a warning.

This is why you found the rule only fires with S_ and T_ (you missed M_).

However, I don't see how this code can reliably determine what is a prefix and what isn't given that our name is supposed to contain underscores, which is why I raised it as an issue.

When this code was originally introduced, it just stripped common prefixes from the name. This was changed in Jan 2019, apparently with unintended consequences.

Answer 2

Are the prefixes T_, t_, S_, s_ used for anything specific

Quote the coding conventions manual:

When working with static fields that are private or internal, use the s_ prefix and for thread static use t_.

public class DataService
{
   private static IWorkerQueue s_workerQueue;

   [ThreadStatic]
   private static TimeSpan t_timeSpan;
}