SoftCircuits.SparseCollections (nuget): The SparseCollections library provides the SparseArray<T> and SparseMatrix<T> collection classes. The array class allow statements such as "array[1000000] = 5" or "array[-1000000] = 6" without having to create a large array. The matrix class does the same thing using a two-dimensional-array metaphor.  Now targets either .NET Standard 2.0 or .NET 5.0, and includes support for nullable reference types.
Keywords: sparse-array|sparse-matrix|sparse|array|matrix|collections|.net|sparce-array|sparse-data|sparse-matrices