Microsoft.bcl.build.tasks.dll: Is Used By Another Process

Introduction In the .NET ecosystem, particularly when working with legacy applications or cross-platform targeting (e.g., .NET Framework, Xamarin, or older UWP projects), developers may encounter a frustrating build-time error: microsoft.bcl.build.tasks.dll is used by another process While this message superficially indicates a standard file-locking issue (Windows Error ERROR_SHARING_VIOLATION ), its specific association with this DLL points to deeper systemic causes rooted in MSBuild concurrency, NuGet restore mechanisms, and antivirus interference. This article explores the architecture of the Microsoft.Bcl.Build.Tasks library, why this lock occurs, and advanced resolution strategies. 1. What is microsoft.bcl.build.tasks.dll ? The Microsoft.Bcl.Build package (often version 1.0.x or 2.x) was historically used to support Portable Class Libraries (PCLs) and backport features like System.Runtime and System.Threading.Tasks to .NET Framework 4.0 and Silverlight.