Global assembly cache APIs are obsolete

.NET Core and .NET 5 and later versions eliminate the concept of the global assembly cache (GAC) that was present in .NET Framework. As such, all .NET Core and .NET 5+ APIs that deal with the GAC either fail or perform no operation.

To help steer developers away from these APIs, some GAC-related APIs are marked as obsolete, and generate a SYSLIB0005 warning at compile time. These APIs may be removed in a future version of .NET.

Change description

The following APIs are marked obsolete.

API Marked obsolete in...
Assembly.GlobalAssemblyCache 5.0 RC1

In .NET Framework 2.x - 4.x, the GlobalAssemblyCache property returns true if the queried assembly was loaded from the GAC, and false if it was loaded from a different location on disk. In .NET Core 2.x - 3.x, the GlobalAssemblyCache always returns false, reflecting that the GAC does not exist in .NET Core.

Assembly asm = typeof(object).Assembly;
// Prints 'True' on .NET Framework, 'False' on .NET Core.

In .NET 5 and later versions, the GlobalAssemblyCache property continues to always return false. However, the property getter is also marked as obsolete to indicate to callers that they should stop accessing the property. Libraries and apps should not use the GlobalAssemblyCache API to make determinations about run-time behavior, as it always returns false in .NET Core and .NET 5 and later versions.

Assembly asm = typeof(object).Assembly;
// Prints 'False' on .NET 5+; also produces warning SYSLIB0005 at compile time.

This is a compile-time only change. There is no run-time change from previous versions of .NET Core.

Reason for change

The global assembly cache (GAC) does not exist as a concept in .NET Core and .NET 5 and later versions.

Version introduced

.NET 5.0

  • If your application queries the GlobalAssemblyCache property, consider removing the call. If you use the GlobalAssemblyCache value to choose between an "assembly in the GAC"-flow vs. an "assembly not in the GAC"-flow at run time, reconsider whether the flow still makes sense for a .NET Core or .NET 5+ application.

  • If you must continue to use the obsolete APIs, you can suppress the SYSLIB0005 warning in code.

    Assembly asm = typeof(object).Assembly;
    #pragma warning disable SYSLIB0005 // Disable the warning.
    // Prints 'False' on .NET 5+.
    #pragma warning restore SYSLIB0005 // Re-enable the warning.

    You can also suppress the warning in your project file, which disables the warning for all source files in the project.

    <Project Sdk="Microsoft.NET.Sdk">
       <!-- NoWarn below will suppress SYSLIB0005 project-wide -->

    Suppressing SYSLIB0005 disables only the GlobalAssemblyCache obsoletion warning. It does not disable any other warnings.

Affected APIs