Difference among .NET Mutable, Readonly, Immutable, and Freezable Collections

Mutable 

Mutable is the most common collection type in the .NET world. These are collections such as List; that allow reading, as well as adding, removing and changing items.

Read-Only

Read-Only These are collections that can't be modified from the outside. However, this notion of collections doesn't guarantee that its contents will never change. For example, a dictionary's keys and values collections can't be updated directly, but adding to the dictionary indirectly updates the keys and values collections. You can create your own read only collection by inheriting from ReadOnlyCollection.

Immutable

Immutable These are collections that, once created, are guaranteed to never be changed and thread-safe. If a complicated data structure is fully immutable, it can always be safely passed to a background worker. You don't need to worry about someone modifying it at the same time. This collection type is not provided by the Microsoft .NET Framework base class library (BCL) today. You can download from NuGet by searching for Microsoft.Immutable.Collections.

Freezable

Freezable These collections behave like mutable collections until they're frozen. Then they behave like immutable collections. Although the BCL doesn't define these collections, you can find them in Windows Presentation Foundation.