CLR Activation Debugging

Diagnose .NET Framework runtime activation issues by analyzing CLR activation logs (CLRLoad logs) produced by the shim (mscoree.dll). These logs record every decision the shim makes when selecting and loading a CLR version.

When to Use

• A process fails to load the CLR at all ("Unable to find a version of the runtime to use")
• The shim picks the wrong CLR version (e.g., v2.0 instead of v4.0)
• Unexpected .NET 3.5 Feature-on-Demand (FOD) install dialogs appear
• FOD dialogs are expected but do NOT appear
• Both CLR v2 and CLR v4 load into the same process, causing failures
• A COM object fails to activate because the shim can't resolve the runtime
• Legacy hosting APIs (CorBindToRuntime) bind to an unexpected version

When Not to Use

• Modern .NET (CoreCLR / .NET 5+), this skill covers .NET Framework only (the mscoree.dll shim)
• Assembly binding failures, use Fusion logs (fuslogvw.exe), not CLR activation logs
• Runtime crashes after the CLR has loaded, activation succeeded; the problem is elsewhere

Background

The Shim Architecture

The .NET Framework shim has two layers:

• mscoree.dll (the "shell shim"), the public-facing DLL that is the registered InprocServer32 for CLR-hosted COM objects and the entry point for _CorExeMain, legacy APIs, etc.
• mscoreei.dll, the actual shim implementation where the runtime selection logic, logging, and activation decisions live. mscoree.dll forwards into mscoreei.dll.

When reading logs, the caller-name:mscoreei.dll in FOD command lines reflects this, it's mscoreei.dll doing the work.

.NET 3.5 / v2.0.50727 Version Mapping