The `Microsoft.Bcl.Build` NuGet package is a pivotal tool used in .NET development, especially when dealing with the complexities arising from targeting multiple .NET platforms, specifically while working with portable class libraries (PCLs). This package assists developers in overcoming versioning issues and compatibility challenges. Here's a comprehensive exploration into what the `Microsoft.Bcl.Build` package does and its utility in the .NET ecosystem.

Background

In .NET, targeting different platforms can present challenges due to differences in available APIs and runtime behavior. This becomes particularly evident when you're developing libraries intended to be shared across various platforms. The `Microsoft.Bcl.Build` package simplifies maintenance and enhances compatibility for such scenarios.

Functionality

The primary role of the `Microsoft.Bcl.Build` package is to facilitate portability and compatibility for libraries by ensuring that all the necessary binding redirects are correctly configured during the build process. Here's a breakdown of its core functionality:

1. Binding Redirects: When you create a library that targets multiple platforms or runtime versions, different versions of dependencies may be required on different platforms. The `Microsoft.Bcl.Build` package automatically handles binding redirects in your application's configuration file. This ensures that the correct versions of assemblies are loaded at runtime.
2. Platform Extensions: It provides additional API surface to .NET Standard and Portable Class Libraries (PCLs). This includes back-porting newer APIs to earlier .NET versions, making it easier for libraries targeting PCL profiles to use newer .NET features without having to abandon support for older runtimes.
3. Build-Time Improvements: The package also improves the build process by ensuring that any necessary NuGet dependencies specific to a particular target framework are included and appropriately configured.

Technical Explanation and Examples

When targeting multiple frameworks, you often encounter scenarios where newer methods or properties are unavailable on older platform versions. Here, `Microsoft.Bcl.Build` steps in to fill this gap by redirecting API calls to versions compatible with the target platform.

Example Scenario

Suppose your library needs to utilize newer asynchronous APIs, such as `Task.Delay`, which might not be available in earlier .NET Framework versions. Normally, this could render your library incompatible with older platforms unless:

• You implement conditional compilation and exclude features.
• Introduce custom implementations, increasing maintenance overhead.

With `Microsoft.Bcl.Build`, you can leverage these newer features, as it automatically resolves compatibility issues by embedding the necessary redirects and dependencies to provide these APIs wherever possible.

Implementation Steps

1. Installation: Install the package via NuGet Package Manager:

• Compatibility: The package is particularly useful in older .NET environments. With .NET Core and .NET 5+ unifying the framework, the reliance on such packages might decrease for new projects.
• Alternatives: For modern .NET projects, targeting `netstandard` or the unified `.NET 5+` is recommended, as these provide built-in support for multiple platforms without the need for such packages.