Custom attributes that generate flags or options in the Intermediate Language (IL) output

You add these attributes to your code for the compiler to emit a specified Intermediate Language (IL) modifier. These attributes instruct the compiler to include the corresponding IL modifier in the output.

Attribute	Modifier	Comments
System.Runtime.InteropServices.ComImportAttribute	import
System.Runtime.InteropServices.DllImportAttribute	pinvokeimpl	You can add options listed in the constructor.
System.Runtime.InteropServices.FieldOffsetAttribute	.field	This sets the field offset for memory layout.
MarshalAsAttribute	marshal	You can set options listed in the constructor.
System.Runtime.CompilerServices.MethodImplAttribute	flag	Constructor arguments specify specific named flags such as aggressiveinlining or forwardref. These flags also specify the native, managed, or optil modifiers for the System.Runtime.CompilerServices.MethodCodeType field.
System.NonSerializedAttribute	notserialized
System.Runtime.InteropServices.OptionalAttribute	[opt]
System.Runtime.InteropServices.PreserveSigAttribute	preservesig
System.SerializableAttribute	serializable
System.Runtime.InteropServices.StructLayoutAttribute	auto, sequential, or explicit	Layout options can be set using the parameters.
System.Runtime.CompilerServices.IndexerNameAttribute		You add this attribute to an indexer to set a different method name. By default, indexers are compiled to a property named Item. You can specify a different name using this attribute.

Some of these custom attributes are applied using other C# syntax rather than adding the attribute to your source code.

Attribute	Comments
System.Runtime.InteropServices.DefaultParameterValueAttribute	Specifies the default value for the parameter. Use the default parameter syntax.
System.Runtime.InteropServices.InAttribute	Specifies the IL [in] modifier. Use the in or ref readonly modifiers.
System.Runtime.InteropServices.OutAttribute	Specifies the IL [out] modifier. Use the out modifier.
System.Runtime.CompilerServices.SpecialNameAttribute	Specifies the IL specialname modifier. The compiler automatically adds this modifier for methods that require it.
System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute	This attribute is required for the delegate* feature. The compiler adds it to any delegate* that requires its use. However, you must add this attribute to any method declaration when that method is assigned to a function pointer.

The following attributes are generally disallowed in C# source. They're listed here to aid library authors who use reflection, and to ensure you don't create custom attributes with the same fully qualified name.

Attribute	Comments
System.Runtime.CompilerServices.CompilerFeatureRequiredAttribute	Prevents downlevel compilers from using metadata it can't safely understand.
System.Runtime.CompilerServices.DecimalConstantAttribute	Encodes const decimal fields. The runtime doesn't support decimal values as constant values.
System.Reflection.DefaultMemberAttribute	Encodes indexers with System.Runtime.CompilerServices.IndexerNameAttribute. This attribute notes the default indexer when its name is different than Item. This attribute is allowed in source.
System.Runtime.CompilerServices.DynamicAttribute	Encodes whether a type in a signature is dynamic (versus object).
System.Runtime.CompilerServices.ExtensionAttribute	This attribute notes extension methods. The compiler also places this attribute on the containing classes.
System.Runtime.CompilerServices.FixedBufferAttribute	This attribute specifies fixed struct fields.
System.Runtime.CompilerServices.IsByRefLikeAttribute	This attribute specifies a ref struct.
System.Runtime.CompilerServices.IsReadOnlyAttribute	This attribute indicates that a parameter has the in modifier. It distinguishes in parameters from readonly ref or [In] ref.
System.Runtime.CompilerServices.RequiresLocationAttribute	This attribute indicates that a parameter has the readonly ref modifier. It distinguishes readonly ref from in or [In] ref.
System.Runtime.CompilerServices.IsUnmanagedAttribute	This attribute specifies the unmanaged constraint on a type parameter.
System.Runtime.CompilerServices.NullableAttribute, System.Runtime.CompilerServices.NullableContextAttribute, System.Runtime.CompilerServices.NullablePublicOnlyAttribute	These attributes encode nullable annotations in your source code.
System.ParamArrayAttribute	This attribute encodes the params modifier on array parameters.
System.Runtime.CompilerServices.ParamCollectionAttribute	This attribute encodes the params modifier on non-array parameters.
System.Runtime.CompilerServices.RefSafetyRulesAttribute	This attribute specifies the C# version that is required in order to understand ref safety annotations in the assembly. Ref safety rules evolve as C# gets new features.
System.Runtime.CompilerServices.RequiredMemberAttribute	This attribute indicates that the required modifier was placed on a member declaration. It's the encoding of the required members language feature.
System.Runtime.CompilerServices.TupleElementNamesAttribute	This attribute encodes tuple element names used in signatures.

In addition, the compiler can generate a declaration for other attributes used internally. The compiler generates these attributes in the System.Runtime.CompilerServices namespace for its own use. Some aren't in the .NET Runtime libraries. Instead, the compiler synthesizes a definition for an internal type declaration in any assembly where the attribute is needed.