Code annotation attributes

CanBeNullAttribute

Indicates that the value of the marked element could be null sometimes, so checking for null is required before its usage.

[CanBeNull] object Test() => null;

void UseTest() {
  var p = Test();
  var s = p.ToString(); // Warning: Possible 'System.NullReferenceException'
}

For more information, see Value and nullability analysis.

NotNullAttribute

Indicates that the value of the marked element can never be null.

[NotNull] object Foo() {
  return null; // Warning: Possible 'null' assignment
}

For more information, see Value and nullability analysis.

ItemNotNullAttribute

Can be applied to symbols of types derived from IEnumerable as well as to symbols of Task and Lazy classes to indicate that the value of a collection item, of the Task.Result property or of the Lazy.Value property can never be null.

public void Foo([ItemNotNull]List<string> books)
{
  foreach (var book in books) {
    if (book != null) // Warning: Expression is always true
     Console.WriteLine(book.ToUpper());
  }
}

For more information, see Value and nullability analysis.

ItemCanBeNullAttribute

Can be applied to symbols of types derived from IEnumerable as well as to symbols of Task and Lazy classes to indicate that the value of a collection item, of the Task.Result property or of the Lazy.Value property can be null.

public void Foo([ItemCanBeNull]List<string> books)
{
  foreach (var book in books)
  {
    // Warning: Possible 'System.NullReferenceException'
    Console.WriteLine(book.ToUpper());
  }
}

For more information, see Value and nullability analysis.

StringFormatMethodAttribute

Indicates that the marked method builds string by the format pattern and (optional) arguments. The parameter, which contains the format string, should be given in the constructor. The format string should be in System.String.Format(System.IFormatProvider,System.String,System.Object[])-like form.

[StringFormatMethod("message")]
void ShowError(string message, params object[] args) { /* do something */ }

void Foo() {
  ShowError("Failed: {0}"); // Warning: Non-existing argument in format string
}

For more information, see In custom string formatting methods.

StructuredMessageTemplateAttribute

Indicates that the marked parameter is a message template where placeholders are to be replaced by the following arguments in the order in which they appear

void LogInfo([StructuredMessageTemplate]string message, params object[] args) { /* do something */ }

void Foo() {
  LogInfo("User created: {username}"); // Warning: Non-existing argument in format string
}

ValueProviderAttribute

Use this annotation to specify a type that contains static or const fields with values for the annotated property/field/parameter. The specified type will be used to improve completion suggestions.

namespace TestNamespace
{
  public class Constants
  {
    public static int INT_CONST = 1;
    public const string STRING_CONST = "1";
  }

  public class Class1
  {
    [ValueProvider("TestNamespace.Constants")] public int myField;
    public void Foo([ValueProvider("TestNamespace.Constants")] string str) { }

    public void Test()
    {
      Foo(/*try completion here*/);//
      myField = /*try completion here*/
    }
  }
}

ValueRangeAttribute

Indicates that the integral value falls into the specified interval. It's allowed to specify multiple non-intersecting intervals. Values of interval boundaries are inclusive.

void Foo([ValueRange(0, 100)] int value) {
  if (value == -1) { // Warning: Expression is always 'false'
    ...
  }
}

NonNegativeValueAttribute

Indicates that the integral value never falls below zero.

void Foo([NonNegativeValue] int value) {
  if (value == -1) { // Warning: Expression is always 'false'
    ...
  }
}

For more information, see Analysis of integer values (integral arithmetics).

InvokerParameterNameAttribute

Indicates that the function argument should be a string literal and match one of the parameters of the caller function. For example, ReSharper annotates the parameter of System.ArgumentNullException.

void Foo(string param) {
  if (param == null)
    throw new ArgumentNullException("par"); // Warning: Cannot resolve symbol
}

NotifyPropertyChangedInvocatorAttribute

Indicates that the method is contained in a type that implements System.ComponentModel.INotifyPropertyChanged interface and this method is used to notify that some property value changed.

The method should be non-static and conform to one of the supported signatures:

• NotifyChanged(string)

• NotifyChanged(params string[])

• NotifyChanged{T}(Expression{Func{T}})

• NotifyChanged{T,U}(Expression{Func{T,U}})

• SetProperty{T}(ref T, T, string)

public class Foo : INotifyPropertyChanged {
  public event PropertyChangedEventHandler PropertyChanged;

  [NotifyPropertyChangedInvocator]
  protected virtual void NotifyChanged(string propertyName) { ... }

  string _name;

  public string Name {
    get { return _name; }
    set { _name = value; NotifyChanged("LastName"); /* Warning */ }
  }
}

ContractAnnotationAttribute

Describes dependency between method input and output.

For more information, see Contract annotations.

LocalizationRequiredAttribute

Indicates whether the marked element should be localized.

[LocalizationRequiredAttribute(true)]
class Foo {
  string str = "my string"; // Warning: Localizable string
}

CannotApplyEqualityOperatorAttribute

Indicates that the value of the marked type (or its derivatives) cannot be compared using '==' or '!=' operators and Equals() should be used instead. However, using '==' or '!=' for comparison with null is always permitted.

[CannotApplyEqualityOperator]
class NoEquality { }

class UsesNoEquality {
  void Test() {
    var ca1 = new NoEquality();
    var ca2 = new NoEquality();
    if (ca1 != null) { // OK
      bool condition = ca1 == ca2; // Warning
    }
  }
}

BaseTypeRequiredAttribute

When applied to a target attribute, specifies a requirement for any type marked with the target attribute to implement or inherit specific type or types.

[BaseTypeRequired(typeof(IComponent)] // Specify requirement
class ComponentAttribute : Attribute { }

[Component] // ComponentAttribute requires implementing IComponent interface
class MyComponent : IComponent { }

UsedImplicitlyAttribute

Indicates that the marked symbol is used implicitly (e.g. via reflection, in external library), so this symbol will be ignored by usage-checking inspections.
You can use ImplicitUseKindFlags and ImplicitUseTargetFlags to configure how this attribute is applied.

[UsedImplicitly]
public class TypeConverter {}

public class SummaryData
{
  [UsedImplicitly(ImplicitUseKindFlags.InstantiatedWithFixedConstructorSignature)]
  public SummaryData() {}
}

[UsedImplicitly(ImplicitUseTargetFlags.WithInheritors | ImplicitUseTargetFlags.Default)]
public interface IService {}

MeansImplicitUseAttribute

Can be applied to attributes, type parameters, and parameters of a type assignable from System.Type. When applied to an attribute, the decorated attribute behaves the same as UsedImplicitlyAttribute. When applied to a type parameter or to a parameter of type System.Type, indicates that the corresponding type is used implicitly.

PublicAPIAttribute

This attribute is intended to mark publicly available API, which should not be removed and so is treated as used.

InstantHandleAttribute

Tells the code analysis engine if the parameter is completely handled when the invoked method is on stack. If the parameter is a delegate, indicates that delegate is executed while the method is executed. If the parameter is an enumerable, indicates that it is enumerated while the method is executed.

PureAttribute

Indicates that a method does not make any observable state changes. The same as System.Diagnostics.Contracts.PureAttribute.

[Pure] int Multiply(int x, int y) => x * y;

void M() {
  Multiply(123, 42); // Warning: Return value of pure method is not used
}

MustUseReturnValueAttribute

Indicates that the return value of the method invocation must be used.

Methods decorated with this attribute (in contrast to pure methods) might change state, but make no sense without using their return value.
Similarly to PureAttribute, this attribute will help to detect usages of the method when the return value is not used. Optionally, you can specify a message to use when showing warnings, e.g.

[MustUseReturnValue("Use the return value to...")]

RequireStaticDelegateAttribute

This annotation allows to enforce allocation-less usage patterns of delegates for performance-critical APIs. When this annotation is applied to the parameter of delegate type, IDE checks the input argument of this parameter: * When lambda expression or anonymous method is passed as an argument, IDE verifies that the passed closure has no captures of the containing local variables and the compiler is able to cache the delegate instance to avoid heap allocations. Otherwise the warning is produced. * IDE warns when method name or local function name is passed as an argument as this always results in heap allocation of the delegate instance.

In C# 9.0 code IDE would also suggest to annotate the anonymous function with 'static' modifier to make use of the similar analysis provided by the language/compiler.

For more information, see Code Inspection: Lambda expression/anonymous method should not have captures of the containing context.

ProvidesContextAttribute

Indicates the type member or parameter of some type, that should be used instead of all other ways to get the value of that type. This annotation is useful when you have some "context" value evaluated and stored somewhere, meaning that all other ways to get this value must be consolidated with existing one.

class Foo {
  [ProvidesContext] IBarService _barService = ...;

  void ProcessNode(INode node) {
    DoSomething(node, node.GetGlobalServices().Bar);
    //              ^ Warning: use value of '_barService' field
  }
}

PathReferenceAttribute

Indicates that a parameter is a path to a file or a folder within a web project. Path can be relative or absolute, starting from web root (~).

SourceTemplateAttribute

An extension method marked with this attribute is processed by code completion as a 'Source Template'. When the extension method is completed over some expression, its source code is automatically expanded like a template at call site.

Template method body can contain valid source code and/or special comments starting with '$'. Text inside these comments is added as source code when the template is applied. Template parameters can be used either as additional method parameters or as identifiers wrapped in two '$' signs. Use the MacroAttribute attribute to specify macros for parameters.

[SourceTemplate]
public static void forEach<T>(this IEnumerable<T> xs) {
  foreach (var x in xs) {
     //$ $END$
  }
}

MacroAttribute

Allows specifying a macro for a parameter of a SourceTemplateAttribute.

You can apply the attribute on the whole method or on any of its additional parameters. The macro expression is defined in the Expression property. When applied on a method, the target template parameter is defined in the Target property. To apply the macro silently for the parameter, set the Editable property value = -1.

[SourceTemplate, Macro(Target = "item", Expression = "suggestVariableName()")]
public static void forEach<T>(this IEnumerable<T> collection) {
  foreach (var item in collection) {
    //$ $END$
  }
}

[SourceTemplate]
public static void something(this Entity x, [Macro(Expression = "guid()", Editable = -1)] string newguid) {
  /*$ var $x$Id = "$newguid$" + x.ToString();
  x.DoSomething($x$Id); */
}

AspMvcActionAttribute

ASP.NET MVC attribute. If applied to a parameter, indicates that the parameter is an MVC action. If applied to a method, the MVC action name is calculated implicitly from the context. Use this attribute for custom wrappers similar to System.Web.Mvc.Html.ChildActionExtensions.RenderAction(HtmlHelper, String).

AspMvcAreaAttribute

ASP.NET MVC attribute. Indicates that the marked parameter is an MVC area. Use this attribute for custom wrappers similar to System.Web.Mvc.Html.ChildActionExtensions.RenderAction(HtmlHelper, String).

AspMvcControllerAttribute

ASP.NET MVC attribute. If applied to a parameter, indicates that the parameter is an MVC controller. If applied to a method, the MVC controller name is calculated implicitly from the context. Use this attribute for custom wrappers similar to System.Web.Mvc.Html.ChildActionExtensions.RenderAction(HtmlHelper, String, String).

AspMvcMasterAttribute

ASP.NET MVC attribute. Indicates that the marked parameter is an MVC Master. Use this attribute for custom wrappers similar to System.Web.Mvc.Controller.View(String, String).

AspMvcModelTypeAttribute

ASP.NET MVC attribute. Indicates that the marked parameter is an MVC model type. Use this attribute for custom wrappers similar to System.Web.Mvc.Controller.View(String, Object).

AspMvcPartialViewAttribute

ASP.NET MVC attribute. If applied to a parameter, indicates that the parameter is an MVC partial view. If applied to a method, the MVC partial view name is calculated implicitly from the context. Use this attribute for custom wrappers similar to System.Web.Mvc.Html.RenderPartialExtensions.RenderPartial(HtmlHelper, String).

AspMvcSuppressViewErrorAttribute

ASP.NET MVC attribute. Allows disabling inspections for MVC views within a class or a method.

AspMvcDisplayTemplateAttribute

ASP.NET MVC attribute. Indicates that a parameter is an MVC display template. Use this attribute for custom wrappers similar to System.Web.Mvc.Html.DisplayExtensions.DisplayForModel(HtmlHelper, String).

AspMvcEditorTemplateAttribute

ASP.NET MVC attribute. Indicates that the marked parameter is an MVC editor template. Use this attribute for custom wrappers similar to System.Web.Mvc.Html.EditorExtensions.EditorForModel(HtmlHelper, String).

AspMvcTemplateAttribute

ASP.NET MVC attribute. Indicates that the marked parameter is an MVC template. Use this attribute for custom wrappers similar to System.ComponentModel.DataAnnotations.UIHintAttribute(System.String).

AspMvcViewAttribute

ASP.NET MVC attribute. If applied to a parameter, indicates that the parameter is an MVC view component. If applied to a method, the MVC view name is calculated implicitly from the context. Use this attribute for custom wrappers similar to System.Web.Mvc.Controller.View(Object).

AspMvcViewComponentAttribute

ASP.NET MVC attribute. If applied to a parameter, indicates that the parameter is an MVC view component name.

AspMvcViewComponentViewAttribute

ASP.NET MVC attribute. If applied to a parameter, indicates that the parameter is an MVC view component view. If applied to a method, the MVC view component view name is default.

AspMvcActionSelectorAttribute

ASP.NET MVC attribute. When applied to a parameter of an attribute, indicates that this parameter is an MVC action name.

[ActionName("Foo")]
public ActionResult Login(string returnUrl) {
  ViewBag.ReturnUrl = Url.Action("Foo"); // OK
  return RedirectToAction("Bar"); // Error: Cannot resolve action
}

RazorSectionAttribute

Razor attribute. Indicates that the marked parameter or method is a Razor section. Use this attribute for custom wrappers similar to System.Web.WebPages.WebPageBase.RenderSection(String).

CollectionAccessAttribute

Indicates how method, constructor invocation, or property access over collection type affects the contents of the collection. When applied to a return value of a method indicates if the returned collection is created exclusively for the caller (CollectionAccessType.UpdatedContent) or can be read/updated from outside (CollectionAccessType.Read | CollectionAccessType.UpdatedContent) Use CollectionAccessType to specify the access type.

Using this attribute only makes sense if all collection methods are marked with this attribute.

public class MyStringCollection : List<string>
{
  [CollectionAccess(CollectionAccessType.Read)]
  public string GetFirstString()
  {
    return this.ElementAt(0);
  }
}
class Test
{
  public void Foo()
  {
    // Warning: Contents of the collection is never updated
    var col = new MyStringCollection();
    string x = col.GetFirstString();
  }
}

AssertionMethodAttribute

Indicates that the marked method is assertion method, i.e. it halts the control flow if one of the conditions is satisfied. To set the condition, mark one of the parameters with AssertionConditionAttribute attribute.

AssertionConditionAttribute

Indicates the condition parameter of the assertion method. The method itself should be marked by AssertionMethodAttribute attribute. The mandatory argument of the attribute is the assertion type.

TerminatesProgramAttribute

Indicates that the marked method unconditionally terminates control flow execution. For example, it could unconditionally throw exception.

LinqTunnelAttribute

Indicates that the method is a pure LINQ method, with postponed enumeration (like Enumerable.Select, .Where). This annotation allows inference of [InstantHandle] annotation for parameters of delegate type by analyzing LINQ method chains.

NoEnumerationAttribute

Indicates that IEnumerable passed as a parameter is not enumerated. Use this annotation to suppress the 'Possible multiple enumeration of IEnumerable' inspection.

static void ThrowIfNull<T>([NoEnumeration] T v, string n) where T : class
{
  // custom check for null but no enumeration
}

void Foo(IEnumerable<string> values)
{
  ThrowIfNull(values, nameof(values));
  var x = values.ToList(); // No warnings about multiple enumeration
}

RegexPatternAttribute

Indicates that the marked parameter, field, or property is a regular expression pattern.

For more information, see Regular expressions assistance.

LanguageInjectionAttribute

Indicates that the marked parameter, field, or property is accepting a string literal containing code fragment in a language specified by the InjectedLanguage.

void Foo([LanguageInjection(InjectedLanguage.CSS, Prefix = "body{", Suffix = "}")] string cssProps)
{
  // cssProps should only contains a list of CSS properties
}

For more information, see Use [LanguageInjectionAttribute].

NoReorderAttribute

Prevents the Member Reordering feature from tossing members of the marked class.

The attribute must be mentioned in your member reordering patterns.

For more information, see Rearrange members with file and type layout patterns.

XamlItemsControlAttribute

XAML attribute. Indicates the type that has ItemsSource property and should be treated as ItemsControl-derived type, to enable inner items DataContext type resolve.

XamlItemBindingOfItemsControlAttribute

XAML attribute. Indicates the property of some BindingBase-derived type, that is used to bind some item of ItemsControl-derived type. This annotation will enable the DataContext type resolve for XAML bindings for such properties.

Property should have the tree ancestor of the ItemsControl type or marked with the XamlItemsControlAttribute attribute.

XamlItemStyleOfItemsControlAttribute

XAML attribute. Indicates the property of some Style-derived type, that is used to style items of ItemsControl-derived type. This annotation will enable the DataContext type resolve for XAML bindings for such properties.

Property should have the tree ancestor of the ItemsControl type or marked with the XamlItemsControlAttribute attribute.

XamlOneWayBindingModeByDefaultAttribute

XAML attribute. Indicates that DependencyProperty has OneWay binding mode by default.

This attribute must be applied to DependencyProperty's CLR accessor property if it is present, to DependencyProperty descriptor field otherwise.

XamlTwoWayBindingModeByDefaultAttribute

XAML attribute. Indicates that DependencyProperty has TwoWay binding mode by default.

This attribute must be applied to DependencyProperty's CLR accessor property if it is present, to DependencyProperty descriptor field otherwise.

RouteTemplateAttribute

Indicates that the marked parameter, field, or property is a route template.

This attribute allows IDE to recognize the use of web frameworks' route templates to enable syntax highlighting, code completion, navigation, rename and other features in string literals.

RouteParameterConstraintAttribute

Indicates that the marked type is custom route parameter constraint, which is registered in application's Startup with name ConstraintName

You can specify ProposedType if target constraint matches only route parameters of specific type, it will allow IDE to create method's parameter from usage in route template with specified type instead of default System.String and check if constraint's proposed type conflicts with matched parameter's type

UriStringAttribute

Indicates that the marked parameter, field, or property is an URI string.

This attribute enables code completion, navigation, rename and other features in URI string literals assigned to annotated parameter, field or property.

CodeTemplateAttribute

Defines the code search template using the Structural Search and Replace syntax. It allows you to find and, if necessary, replace blocks of code that match a specific pattern. Search and replace patterns consist of a textual part and placeholders. Textural part must contain only identifiers allowed in the target language and will be matched exactly (white spaces, tabulation characters, and line breaks are ignored). Placeholders allow matching variable parts of the target code blocks. A placeholder has the following format: $placeholder_name$- where placeholder_name is an arbitrary identifier. Available placeholders:

• $this$ - expression of containing type

• $thisType$ - containing type

• $member$ - current member placeholder

• $qualifier$ - this placeholder is available in the replace pattern and can be used to insert qualifier expression matched by the $member$ placeholder. (Note that if $qualifier$ placeholder is used, then $member$ placeholder will match only qualified references)

• $expression$ - expression of any type

• $args$ - any number of arguments

• $arg$ - single argument

• $arg1$ ... $arg10$ - single argument

• $stmts$ - any number of statements

• $stmt$ - single statement

• $stmt1$ ... $stmt10$ - single statement

• $name{Expression, 'Namespace.FooType'}$ - expression with 'Namespace.FooType' type

• $expression{'Namespace.FooType'}$ - expression with 'Namespace.FooType' type

• $name{Type, 'Namespace.FooType'}$ - 'Namespace.FooType' type

• $type{'Namespace.FooType'}$ - 'Namespace.FooType' type

• $statement{1,2}$ - 1 or 2 statements

For more information, see Report and update deprecated APIs.

ImplicitUseKindFlags

Specifies the details of implicitly used symbol when it is marked with MeansImplicitUseAttribute or UsedImplicitlyAttribute.

ImplicitUseTargetFlags

Specifies what is considered to be used implicitly when marked with MeansImplicitUseAttribute or UsedImplicitlyAttribute.

CollectionAccessType

Provides a value for the CollectionAccessAttribute to define how the collection method invocation affects the contents of the collection.

AssertionConditionType

Specifies assertion type. If the assertion method argument satisfies the condition, then the execution continues. Otherwise, execution is assumed to be halted.

InjectedLanguage

Language of injected code fragment inside marked by LanguageInjectionAttribute string literal.

For more information, see Use [LanguageInjectionAttribute].