AppDomains

AppDomains, short for Application Domains, were a feature in the .NET Framework that provided a level of isolation for code execution. Think of them as separate, secure environments within a single process. Each AppDomain had its own independent set of loaded assemblies, its own security policy, and its own runtime environment. This isolation allowed for multiple applications to run within the same process without interfering with each other.

The primary benefit of AppDomains was improved stability and security. If one application within an AppDomain crashed or encountered an unhandled exception, it was less likely to bring down the entire process. This was because the exception was typically contained within its own AppDomain. Security was also enhanced, as each AppDomain could be granted different permissions, allowing for finer-grained control over what code could do.

AppDomains were particularly useful for scenarios like web servers (e.g., ASP.NET), plugin architectures, and situations where different applications needed to coexist in a single process with distinct security contexts. They enabled features such as unloading assemblies cleanly and remoting between different AppDomains. However, managing AppDomains added complexity to application development and often incurred a performance overhead due to the isolation mechanisms.

In later versions of the .NET Framework and .NET Core/.NET 5+, the use of AppDomains has been largely deprecated or replaced with alternative isolation mechanisms. While the concept of isolation remains important, the specific implementation of AppDomains is less commonly used in modern .NET development.