System.Collections.Immutable by Microsoft

<PackageReference Include="System.Collections.Immutable" Version="9.0.0-preview.2.24128.5" />

.NET API 253,704 bytes

CollectionBuilderAttribute

sealed class CollectionBuilderAttribute : Attribute

public Type BuilderType { get; }

public string MethodName { get; }

public CollectionBuilderAttribute(Type builderType, string methodName)