List of Diagnostics Produced by .NET Libraries APIs

Obsoletions

Per https://github.com/dotnet/designs/blob/master/accepted/2020/better-obsoletion/better-obsoletion.md, we now have a strategy for marking existing APIs as [Obsolete]. This takes advantage of the new diagnostic id and URL template mechanisms introduced to ObsoleteAttribute in .NET 5.

The diagnostic id values reserved for obsoletions are SYSLIB0001 through SYSLIB0999. When obsoleting an API, claim the next three-digit identifier in the SYSLIB0### sequence and add it to the list below. The URL template for all obsoletions is https://aka.ms/dotnet-warnings/{0}. The {0} placeholder is replaced by the compiler with the SYSLIB0### identifier.

The acceptance criteria for adding an obsoletion includes:

• Add the obsoletion to the table below, claiming the next diagnostic id
  • Ensure the description is meaningful within the context of this table, and without requiring the context of the calling code
• Add new constants to src\libraries\Common\src\System\Obsoletions.cs, following the existing conventions
  • A ...Message const using the same description added to the table below
  • A ...DiagId const for the SYSLIB0### id
• Annotate src files by referring to the constants defined from Obsoletions.cs
  • Specify the UrlFormat = Obsoletions.SharedUrlFormat
  • Example: [Obsolete(Obsoletions.CodeAccessSecurityMessage, DiagnosticId = Obsoletions.CodeAccessSecurityDiagId, UrlFormat = Obsoletions.SharedUrlFormat)]
  • If the Obsoletions type is not available in the project, link it into the project
    • <Compile Include="$(CommonPath)System\Obsoletions.cs" Link="Common\System\Obsoletions.cs" />
• Annotate ref files using the hard-coded strings copied from Obsoletions.cs
  • This matches our general pattern of ref files using hard-coded attribute strings
  • Example: [System.ObsoleteAttribute("The UTF-7 encoding is insecure and should not be used. Consider using UTF-8 instead.", DiagnosticId = "SYSLIB0001", UrlFormat = "https://aka.ms/dotnet-warnings/{0}")]
• If the library builds against downlevel targets earlier than .NET 5.0, then add an internal copy of ObsoleteAttribute
  • The compiler recognizes internal implementations of ObsoleteAttribute to enable the DiagnosticId and UrlFormat properties to light up downlevel
  • An MSBuild property can be added to the project's first <PropertyGroup> to achieve this easily
  • Example: <IncludeInternalObsoleteAttribute>true</IncludeInternalObsoleteAttribute>
  • This will need to be specified in both the src and ref projects
• If the library contains types that are forwarded within a generated shim
  • Errors will be received when running build libs, with obsoletion errors in src/libraries/shims/generated files
  • This is resolved by adding the obsoletion's diagnostic id to the <NoWarn> property for partial facade assemblies
  • That property is found in src/libraries/Directory.Build.targets
  • Search for the "Ignore Obsolete errors within the generated shims that type-forward types" comment and add the appropriate diagnostic id to the comment and the <NoWarn> property (other SYSLIB diagnostics already exist there)
• Apply the breaking-change label to the PR that introduces the obsoletion
  • A bot will automatically apply the needs-breaking-change-doc-created label when the breaking-change label is detected
• Follow up with the breaking change process to communicate and document the breaking change
  • In the breaking-change issue filed in dotnet/docs, specifically mention that this breaking change is an obsoletion with a SYSLIB diagnostic id
  • The documentation team will produce a PR that adds the obsoletion to the SYSLIB warnings page
  • That PR will also add a new URL specific to this diagnostic ID; e.g. SYSLIB0001
  • Connect with @gewarren or @BillWagner with any questions
• Register the SYSLIB0### URL in aka.ms
  • The vanity name will be dotnet-warnings/syslib0###
  • Ensure the link's group owner matches the group owner of dotnet-warnings/syslib0001
  • Connect with @jeffhandley, @GrabYourPitchforks, or @gewarren with any questions

An example obsoletion PR that can be referenced where each of the above criteria was met is:

• Implement new GetContextAPI overloads (#49186)

The PR that reveals the implementation of the <IncludeInternalObsoleteAttribute> property was:

• Mark DirectoryServices CAS APIs as Obsolete (#40756)

Obsoletion Diagnostics (SYSLIB0001 - SYSLIB0999)

Diagnostic ID	Description
SYSLIB0001	The UTF-7 encoding is insecure and should not be used. Consider using UTF-8 instead.
SYSLIB0002	PrincipalPermissionAttribute is not honored by the runtime and must not be used.
SYSLIB0003	Code Access Security is not supported or honored by the runtime.
SYSLIB0004	The Constrained Execution Region (CER) feature is not supported.
SYSLIB0005	The Global Assembly Cache is not supported.
SYSLIB0006	Thread.Abort is not supported and throws PlatformNotSupportedException.
SYSLIB0007	The default implementation of this cryptography algorithm is not supported.
SYSLIB0008	The CreatePdbGenerator API is not supported and throws PlatformNotSupportedException.
SYSLIB0009	AuthenticationManager is not supported. Methods will no-op or throw PlatformNotSupportedException.
SYSLIB0010	This Remoting API is not supported and throws PlatformNotSupportedException.
SYSLIB0011	BinaryFormatter serialization is obsolete and should not be used. See https://aka.ms/binaryformatter for recommended alternatives.
SYSLIB0012	Assembly.CodeBase and Assembly.EscapedCodeBase are only included for .NET Framework compatibility. Use Assembly.Location instead.
SYSLIB0013	Uri.EscapeUriString can corrupt the Uri string in some cases. Consider using Uri.EscapeDataString for query string components instead.
SYSLIB0014	WebRequest, HttpWebRequest, ServicePoint, and WebClient are obsolete. Use HttpClient instead.
SYSLIB0015	DisablePrivateReflectionAttribute has no effect in .NET 6.0+.
SYSLIB0016	Use the Graphics.GetContextInfo overloads that accept arguments for better performance and fewer allocations.
SYSLIB0017	Strong name signing is not supported and throws PlatformNotSupportedException.
SYSLIB0018	ReflectionOnly loading is not supported and throws PlatformNotSupportedException.
SYSLIB0019	RuntimeEnvironment members SystemConfigurationFile, GetRuntimeInterfaceAsIntPtr, and GetRuntimeInterfaceAsObject are not supported and throw PlatformNotSupportedException.
SYSLIB0020	JsonSerializerOptions.IgnoreNullValues is obsolete. To ignore null values when serializing, set DefaultIgnoreCondition to JsonIgnoreCondition.WhenWritingNull.
SYSLIB0021	Derived cryptographic types are obsolete. Use the Create method on the base type instead.
SYSLIB0022	The Rijndael and RijndaelManaged types are obsolete. Use Aes instead.
SYSLIB0023	RNGCryptoServiceProvider is obsolete. To generate a random number, use one of the RandomNumberGenerator static methods instead.
SYSLIB0024	Creating and unloading AppDomains is not supported and throws an exception.
SYSLIB0025	SuppressIldasmAttribute has no effect in .NET 6.0+.
SYSLIB0026	X509Certificate and X509Certificate2 are immutable. Use X509CertificateLoader to create a new certificate.
SYSLIB0027	PublicKey.Key is obsolete. Use the appropriate method to get the public key, such as GetRSAPublicKey.
SYSLIB0028	X509Certificate2.PrivateKey is obsolete. Use the appropriate method to get the private key, such as GetRSAPrivateKey, or use the CopyWithPrivateKey method to create a new instance with a private key.
SYSLIB0029	ProduceLegacyHmacValues is obsolete. Producing legacy HMAC values is not supported.
SYSLIB0030	HMACSHA1 always uses the algorithm implementation provided by the platform. Use a constructor without the useManagedSha1 parameter.
SYSLIB0031	EncodeOID is obsolete. Use the ASN.1 functionality provided in System.Formats.Asn1.
SYSLIB0032	Recovery from corrupted process state exceptions is not supported; HandleProcessCorruptedStateExceptionsAttribute is ignored.
SYSLIB0033	Rfc2898DeriveBytes.CryptDeriveKey is obsolete and is not supported. Use PasswordDeriveBytes.CryptDeriveKey instead.
SYSLIB0034	CmsSigner(CspParameters) is obsolete and is not supported. Use an alternative constructor instead.
SYSLIB0035	ComputeCounterSignature without specifying a CmsSigner is obsolete and is not supported. Use the overload that accepts a CmsSigner.
SYSLIB0036	Regex.CompileToAssembly is obsolete and not supported. Use GeneratedRegexAttribute with the regular expression source generator instead.
SYSLIB0037	AssemblyName members HashAlgorithm, ProcessorArchitecture, and VersionCompatibility are obsolete and not supported.
SYSLIB0038	SerializationFormat.Binary is obsolete and should not be used. See https://aka.ms/serializationformat-binary-obsolete for more information.
SYSLIB0039	TLS versions 1.0 and 1.1 have known vulnerabilities and are not recommended. Use a newer TLS version instead, or use SslProtocols.None to defer to OS defaults.
SYSLIB0040	EncryptionPolicy.NoEncryption and AllowEncryption significantly reduce security and should not be used in production code.
SYSLIB0041	The default hash algorithm and iteration counts in Rfc2898DeriveBytes constructors are outdated and insecure. Use a constructor that accepts the hash algorithm and the number of iterations.
SYSLIB0042	ToXmlString and FromXmlString have no implementation for ECC types, and are obsolete. Use a standard import and export format such as ExportSubjectPublicKeyInfo or ImportSubjectPublicKeyInfo for public keys and ExportPkcs8PrivateKey or ImportPkcs8PrivateKey for private keys.
SYSLIB0043	ECDiffieHellmanPublicKey.ToByteArray() and the associated constructor do not have a consistent and interoperable implementation on all platforms. Use ECDiffieHellmanPublicKey.ExportSubjectPublicKeyInfo() instead.
SYSLIB0044	AssemblyName.CodeBase and AssemblyName.EscapedCodeBase are obsolete. Using them for loading an assembly is not supported.
SYSLIB0045	Cryptographic factory methods accepting an algorithm name are obsolete. Use the parameterless Create factory method on the algorithm type instead.
SYSLIB0046	ControlledExecution.Run method may corrupt the process and should not be used in production code.
SYSLIB0047	XmlSecureResolver is obsolete. Use XmlResolver.ThrowingResolver instead when attempting to forbid XML external entity resolution.
SYSLIB0048	RSA.EncryptValue and DecryptValue are not supported and throw NotSupportedException. Use RSA.Encrypt and RSA.Decrypt instead.
SYSLIB0049	JsonSerializerOptions.AddContext is obsolete. To register a JsonSerializerContext, use either the TypeInfoResolver or TypeInfoResolverChain properties.
SYSLIB0050	Formatter-based serialization is obsolete and should not be used.
SYSLIB0051	This API supports obsolete formatter-based serialization. It should not be called or extended by application code.
SYSLIB0052	This API supports obsolete mechanisms for Regex extensibility. It is not supported.
SYSLIB0053	AesGcm should indicate the required tag size for encryption and decryption. Use a constructor that accepts the tag size.
SYSLIB0054	Thread.VolatileRead and Thread.VolatileWrite are obsolete. Use Volatile.Read or Volatile.Write respectively instead.
SYSLIB0055	The underlying hardware instruction does not perform a signed saturate narrowing operation, and it always returns an unsigned result. Use the unsigned overload instead.
SYSLIB0056	LoadFrom with a custom AssemblyHashAlgorithm is obsolete. Use overloads without an AssemblyHashAlgorithm.
SYSLIB0057	Loading certificate data through the constructor or Import is obsolete. Use X509CertificateLoader instead to load certificates.
SYSLIB0058	KeyExchangeAlgorithm, KeyExchangeStrength, CipherAlgorithm, CipherStrength, HashAlgorithm and HashStrength properties of SslStream are obsolete. Use NegotiatedCipherSuite instead.
SYSLIB0059	SystemEvents.EventsThreadShutdown callbacks are not run before the process exits. Use AppDomain.ProcessExit instead.
SYSLIB0060	The constructors on Rfc2898DeriveBytes are obsolete. Use the static Pbkdf2 method instead.
SYSLIB0061	The Queryable MinBy and MaxBy taking an IComparer<TSource> are obsolete. Use the new ones that take an IComparer<TKey>.
SYSLIB0062	XSLT Script blocks are not supported.
SYSLIB0063	This constructor has been deprecated and argument bool isConnected does not have any effect. Use NamedPipeClientStream(PipeDirection direction, bool isAsync, SafePipeHandle safePipeHandle) instead.
SYSLIB0064	RSACryptoServiceProvider.Encrypt and RSACryptoServiceProvider.Decrypt methods that take a Boolean are obsolete. Use the overload that accepts RSAEncryptionPadding instead.

Analyzer Warnings

The diagnostic id values reserved for .NET Libraries analyzer warnings are SYSLIB1001 through SYSLIB1999. When creating a new analyzer that ships as part of the Libraries (and not part of the SDK), claim the next three-digit identifier in the SYSLIB1### sequence and add it to the list below.

Analyzer Diagnostics (SYSLIB1001 - SYSLIB1999)

Diagnostic ID	Description
SYSLIB1001	Logging method names cannot start with _
SYSLIB1002	Don't include log level parameters as templates in the logging message
SYSLIB1003	InvalidLoggingMethodParameterNameTitle
SYSLIB1004	Logging class cannot be in nested types
SYSLIB1005	Could not find a required type definition
SYSLIB1006	Multiple logging methods cannot use the same event id within a class
SYSLIB1007	Logging methods must return void
SYSLIB1008	One of the arguments to a logging method must implement the Microsoft.Extensions.Logging.ILogger interface
SYSLIB1009	Logging methods must be static
SYSLIB1010	Logging methods must be partial
SYSLIB1011	Logging methods cannot use the allows ref struct constraint
SYSLIB1012	Redundant qualifier in logging message
SYSLIB1013	Don't include exception parameters as templates in the logging message
SYSLIB1014	Logging template has no corresponding method argument
SYSLIB1015	Argument is not referenced from the logging message
SYSLIB1016	Logging methods cannot have a body
SYSLIB1017	A LogLevel value must be supplied in the LoggerMessage attribute or as a parameter to the logging method
SYSLIB1018	Don't include logger parameters as templates in the logging message
SYSLIB1019	Couldn't find a field of type Microsoft.Extensions.Logging.ILogger
SYSLIB1020	Found multiple fields of type Microsoft.Extensions.Logging.ILogger
SYSLIB1021	Can't have the same template with different casing
SYSLIB1022	Logging method contains malformed format strings
SYSLIB1023	Generating more than 6 arguments is not supported
SYSLIB1024	Argument is using the unsupported out parameter modifier
SYSLIB1025	Multiple logging methods cannot use the same event name within a class
SYSLIB1026	C# language version not supported by the logging source generator.
SYSLIB1027	Primary constructor parameter of type Microsoft.Extensions.Logging.ILogger is hidden by a field
SYSLIB1028	Argument is using the unsupported params parameter modifier
SYSLIB1029	Logging method parameter is a ref struct
SYSLIB1030	JsonSourceGenerator did not generate serialization metadata for type
SYSLIB1031	JsonSourceGenerator encountered a duplicate JsonTypeInfo property name
SYSLIB1032	JsonSourceGenerator encountered a context class that is not partial
SYSLIB1033	JsonSourceGenerator encountered a type that has multiple [JsonConstructor] annotations
SYSLIB1034	JsonSourceGenerator encountered a [JsonStringEnumConverter] annotation
SYSLIB1035	JsonSourceGenerator encountered a type that has multiple [JsonExtensionData] annotations
SYSLIB1036	JsonSourceGenerator encountered an invalid [JsonExtensionData] annotation
SYSLIB1037	JsonSourceGenerator encountered a type with init-only properties for which deserialization is not supported
SYSLIB1038	JsonSourceGenerator encountered a property annotated with [JsonInclude] that has inaccessible accessors
SYSLIB1039	JsonSourceGenerator encountered a [JsonDerivedTypeAttribute] annotation with [JsonSourceGenerationMode.Serialization] enabled
SYSLIB1040	Invalid GeneratedRegex attribute
SYSLIB1041	Multiple GeneratedRegex attribute
SYSLIB1042	Invalid GeneratedRegex arguments
SYSLIB1043	GeneratedRegex method must have a valid signature
SYSLIB1044	GeneratedRegex only supports C# 11 and newer
SYSLIB1045	Use 'GeneratedRegexAttribute' to generate the regular expression implementation at compile-time
SYSLIB1046	SYSLIB1045-SYSLIB1049 reserved for System.Text.RegularExpressions.Generator.
SYSLIB1047	SYSLIB1045-SYSLIB1049 reserved for System.Text.RegularExpressions.Generator.
SYSLIB1048	SYSLIB1045-SYSLIB1049 reserved for System.Text.RegularExpressions.Generator.
SYSLIB1049	SYSLIB1045-SYSLIB1049 reserved for System.Text.RegularExpressions.Generator.
SYSLIB1050	Invalid LibraryImportAttribute usage
SYSLIB1051	Specified type is not supported by source-generated P/Invokes
SYSLIB1052	Specified configuration is not supported by source-generated P/Invokes
SYSLIB1053	Specified LibraryImportAttribute arguments cannot be forwarded to DllImportAttribute
SYSLIB1054	Use 'LibraryImportAttribute' instead of 'DllImportAttribute' to generate P/Invoke marshalling code at compile time
SYSLIB1055	Invalid CustomMarshallerAttribute usage
SYSLIB1056	Specified native type is invalid
SYSLIB1057	Marshaller type does not have the required shape
SYSLIB1058	Invalid NativeMarshallingAttribute usage
SYSLIB1059	Marshaller type does not support allocating constructor
SYSLIB1060	Specified marshaller type is invalid
SYSLIB1061	Marshaller type has incompatible method signatures
SYSLIB1062	Project must be updated with '<AllowUnsafeBlocks>true</AllowUnsafeBlocks>'
SYSLIB1063	SYSLIB1063-SYSLIB1069 reserved for Microsoft.Interop.LibraryImportGenerator.
SYSLIB1064	SYSLIB1063-SYSLIB1069 reserved for Microsoft.Interop.LibraryImportGenerator.
SYSLIB1065	SYSLIB1063-SYSLIB1069 reserved for Microsoft.Interop.LibraryImportGenerator.
SYSLIB1066	SYSLIB1063-SYSLIB1069 reserved for Microsoft.Interop.LibraryImportGenerator.
SYSLIB1067	SYSLIB1063-SYSLIB1069 reserved for Microsoft.Interop.LibraryImportGenerator.
SYSLIB1068	SYSLIB1063-SYSLIB1069 reserved for Microsoft.Interop.LibraryImportGenerator.
SYSLIB1069	SYSLIB1063-SYSLIB1069 reserved for Microsoft.Interop.LibraryImportGenerator.
SYSLIB1070	Invalid 'JSImportAttribute' usage
SYSLIB1071	Invalid 'JSExportAttribute' usage
SYSLIB1072	Specified type is not supported by source-generated JavaScript interop
SYSLIB1073	Specified configuration is not supported by source-generated JavaScript interop
SYSLIB1074	JSImportAttribute requires unsafe code
SYSLIB1075	JSExportAttribute requires unsafe code
SYSLIB1076	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1077	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1078	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1079	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1080	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1081	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1082	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1083	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1084	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1085	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1086	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1087	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1088	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1089	SYSLIB1070-SYSLIB1089 reserved for System.Runtime.InteropServices.JavaScript.JSImportGenerator.
SYSLIB1090	Invalid 'GeneratedComInterfaceAttribute' usage
SYSLIB1091	Method is declared in different partial declaration than the 'GeneratedComInterface' attribute.
SYSLIB1092	Usage of '[LibraryImport
SYSLIB1093	Analysis for COM interface generation has failed
SYSLIB1094	The base COM interface failed to generate source. Code will not be generated for this interface.
SYSLIB1095	Invalid 'GeneratedComClassAttribute' usage
SYSLIB1096	Use 'GeneratedComInterfaceAttribute' instead of 'ComImportAttribute' to generate COM marshalling code at compile time
SYSLIB1097	This type implements at least one type with the 'GeneratedComInterfaceAttribute' attribute. Add the 'GeneratedComClassAttribute' to enable passing this type to COM and exposing the COM interfaces for the types with the 'GeneratedComInterfaceAttribute' from objects of this type.
SYSLIB1098	.NET COM hosting with 'EnableComHosting' only supports built-in COM interop. It does not support source-generated COM interop with 'GeneratedComInterfaceAttribute'.
SYSLIB1099	COM Interop APIs on 'System.Runtime.InteropServices.Marshal' do not support source-generated COM and will fail at runtime
SYSLIB1100	Configuration binding generator: type is not supported.
SYSLIB1101	Configuration binding generator: property on type is not supported.
SYSLIB1102	Configuration binding generator: project's language version must be at least C# 12.
SYSLIB1103	Configuration binding generator: value types are invalid inputs to configuration 'Bind' methods.*
SYSLIB1104	Configuration binding generator: Generator cannot determine the target configuration type.*
SYSLIB1105	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1106	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1107	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1108	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1109	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1110	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1111	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1112	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1113	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1114	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1115	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1116	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1117	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1118	SYSLIB1100-SYSLIB1118 reserved for Microsoft.Extensions.Configuration.Binder.SourceGeneration.
SYSLIB1201	Options validation generator: Can't use 'ValidateObjectMembersAttribute' or ValidateEnumeratedItemsAttribute on fields or properties with open generic types.
SYSLIB1202	Options validation generator: A member type has no fields or properties to validate.
SYSLIB1203	Options validation generator: A type has no fields or properties to validate.
SYSLIB1204	Options validation generator: A type annotated with OptionsValidatorAttribute doesn't implement the necessary interface.
SYSLIB1205	Options validation generator: A type already includes an implementation of the 'Validate' method.
SYSLIB1206	Options validation generator: Can't validate private fields or properties.
SYSLIB1207	Options validation generator: Member type is not enumerable.
SYSLIB1208	Options validation generator: Validators used for transitive or enumerable validation must have a constructor with no parameters.
SYSLIB1209	Options validation generator: OptionsValidatorAttribute can't be applied to a static class.
SYSLIB1210	Options validation generator: Null validator type specified for the ValidateObjectMembersAttribute or 'ValidateEnumeratedItemsAttribute' attributes.
SYSLIB1211	Options validation generator: Unsupported circular references in model types.
SYSLIB1212	Options validation generator: Member potentially missing transitive validation.
SYSLIB1213	Options validation generator: Member potentially missing enumerable validation.
SYSLIB1214	Options validation generator: Can't validate constants, static fields or properties.
SYSLIB1215	Options validation generator: Validation attribute on the member is inaccessible from the validator type.
SYSLIB1216	C# language version not supported by the options validation source generator.
SYSLIB1217	The validation attribute is only applicable to properties of type string, array, or ICollection; it cannot be used with other types.
SYSLIB1218	SYSLIB1201-SYSLIB1219 reserved for Microsoft.Extensions.Options.SourceGeneration.
SYSLIB1219	SYSLIB1201-SYSLIB1219 reserved for Microsoft.Extensions.Options.SourceGeneration.
SYSLIB1220	JsonSourceGenerator encountered a [JsonConverterAttribute] with an invalid type argument.
SYSLIB1221	JsonSourceGenerator does not support this C# language version.
SYSLIB1222	Constructor annotated with JsonConstructorAttribute is inaccessible.
SYSLIB1223	Attributes deriving from JsonConverterAttribute are not supported by the source generator.
SYSLIB1224	Types annotated with JsonSerializableAttribute must be classes deriving from JsonSerializerContext.
SYSLIB1225	Type includes ref like property, field or constructor parameter.
SYSLIB1226	'JsonIgnoreCondition.Always' is not valid on type-level 'JsonIgnoreAttribute' annotations.
SYSLIB1227	SYSLIB1220-SYSLIB1229 reserved for System.Text.Json.SourceGeneration.
SYSLIB1228	SYSLIB1220-SYSLIB1229 reserved for System.Text.Json.SourceGeneration.
SYSLIB1229	SYSLIB1220-SYSLIB1229 reserved for System.Text.Json.SourceGeneration.
SYSLIB1230	Deriving from a GeneratedComInterface-attributed interface defined in another assembly is not supported.
SYSLIB1231	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.
SYSLIB1232	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.
SYSLIB1233	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.
SYSLIB1234	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.
SYSLIB1235	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.
SYSLIB1236	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.
SYSLIB1237	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.
SYSLIB1238	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.
SYSLIB1239	SYSLIB1230-SYSLIB1239 reserved for Microsoft.Interop.ComInterfaceGenerator.

Diagnostic Suppressions (SYSLIBSUPPRESS****)

Suppression ID	Suppressed Diagnostic ID	Description
SYSLIBSUPPRESS0001	CA1822	Do not offer to make methods static when the methods need to be instance methods for a custom marshaller shape.
SYSLIBSUPPRESS0002	IL2026	ConfigurationBindingGenerator: suppress RequiresUnreferencedCode diagnostic for binding call that has been intercepted by a generated static variant.
SYSLIBSUPPRESS0003	IL3050	ConfigurationBindingGenerator: suppress RequiresDynamicCode diagnostic for binding call that has been intercepted by a generated static variant.

Experimental APIs

APIs can be marked as [Experimental] if their shape or functionality is included in a release but not yet officially supported. Experimental APIs offer the opportunity to collect customer feedback on these APIs in a major release, usually refining the APIs and removing the [Experimental] attribute in the next release. The [Experimental] attribute differs from [RequiresPreviewFeatures], wherein:

• [RequiresPreviewFeatures] APIs require a corresponding preview feature in another product area such as the compiler or SDK
  • Using these APIs requires enabling preview features for the project and all its consumers
• [Experimental] APIs are self-contained within the libraries and do not require preview features in other parts of the product
  • These APIs can be used by suppressing specific diagnostics without enabling preview features for the project

See Preview APIs - .NET | Microsoft Learn for more information.

Experimental Diagnostics (SYSLIB5001 - SYSLIB5999)

The diagnostic id values reserved for experimental APIs are SYSLIB5001 through SYSLIB5999. When marking an API as [Experimental], claim the next three-digit identifier in the SYSLIB5### sequence and add it to the list below. The URL template for all experimental APIs is https://aka.ms/dotnet-warnings/{0}. The {0} placeholder is replaced by the compiler with the SYSLIB5### identifier.

Diagnostic id values for experimental APIs must not be recycled, as that could silently opt customers into new experimental APIs where they had previously suppressed the ID for a previous usage of the value.

Diagnostic ID	Introduced	Removed	Description
SYSLIB5001	.NET 9	.NET 10	Tensor<T> and related APIs in System.Numerics.Tensors are experimental
SYSLIB5002	.NET 9	TBD	SystemColors alternate colors are experimental
SYSLIB5003	.NET 9	TBD	System.Runtime.Intrinsics.Arm.Sve is experimental
SYSLIB5004	.NET 9	TBD	X86Base.DivRem is experimental since performance is not as optimized as T.DivRem
SYSLIB5005	.NET 9	.NET 10	System.Formats.Nrbf is experimental
SYSLIB5006	.NET 10	TBD	Types for Post-Quantum Cryptography (PQC) are experimental.