Clr Activation Debugging

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

.NET 2.0, 3.0, and 3.5 all share the same CLR runtime version: v2.0.50727. The "3.0" and "3.5" releases were library additions on top of CLR v2.0. For activation purposes, they are all "v2.0.50727." When the shim resolves to v2.0.50727 or FOD offers to install "NetFx3", it's installing the CLR v2.0 runtime (plus the 3.0/3.5 libraries). Similarly, CLR v4.0 (v4.0.30319) covers all .NET Framework versions from 4.0 through 4.8.x.

.NET 3.5 Availability on Recent Windows

On recent Windows versions (Windows 11 Insider Preview Build 27965 and future platform releases), .NET Framework 3.5 is no longer available as a Windows optional component (Feature-on-Demand). It must be installed from a standalone MSI. This means the FOD dialog (fondue.exe /enable-feature:NetFx3) will not succeed on these systems even if it fires. On Windows 10 and Windows 11 through 25H2, FOD remains available. .NET Framework 3.5 reaches end of support on January 9, 2029.

Shim HRESULT Codes

When the shim fails, it returns specific HRESULTs in the 0x8013xxxx range. These are the errors you'll see from callers (not in the activation logs themselves, which log human-readable messages):

If a user reports one of these HRESULTs (especially 0x80131700), CLR activation logs are the right diagnostic tool.

Prerequisites

CLR activation logging must be enabled to produce log files. If the user doesn't have logs yet, instruct them to enable logging:

Via environment variable (recommended — scoped to current session):

set COMPLUS_CLRLoadLogDir=C:\CLRLoadLogs

Via registry (machine-wide — affects all .NET Framework processes):

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework
  CLRLoadLogDir = "C:\CLRLoadLogs" (REG_SZ)

On 64-bit systems, also set under Wow6432Node if 32-bit processes are involved.

⚠️ The log directory must already exist. The shim will not create it. If it doesn't exist, no logs will be written and there will be no error or indication of failure.

Logs are written as {ProcessName}.CLRLoad{NN}.log (NN = 00–99, one per process instance). Logs cannot be read until the process exits — the file is held open.

After capturing, remove the env var or registry key to stop logging.

Inputs

Workflow

Step 1: Load Reference Material

Resolve bundled paths from the directory that contains this SKILL.md, not from the user's workspace. Load the reference files in this order — they contain the detailed log format, decision flow, and CLSID registry documentation:

1. references/log-format.md — Log line format, fields, and all known log message types
2. references/activation-flow.md — The shim's decision tree for runtime selection
3. references/com-activation.md — COM (DllGetClassObject) activation specifics, CLSID registry layout

If a direct read fails, list this skill's references/ directory once and retry only when the listing shows the expected file. Do not use workspace file or text search to locate the skill installation.

If one or more expected reference files remain unavailable, use the loaded references where possible and the inline knowledge below for the missing coverage. Include Reference coverage: reduced; unavailable: <paths>; used inline guidance for missing references. in the final diagnosis, with <paths> replaced by the missing relative paths.

Step 2: Survey the Log Files

Get the big picture before diving into any single log:

1. List all log files and group by process name — this shows which executables triggered CLR activation
2. For each process, scan for outcome lines:
   • Decided on runtime: vX.Y.Z — successful resolution
   • ERROR: — failed resolution
   • Launching feature-on-demand — FOD dialog was shown
   • Could have launched feature-on-demand — FOD would have fired but was suppressed
   • V2.0 Capping is preventing consideration — v4+ was skipped due to capping

grep -l "ERROR:\|Launching feature-on-demand\|Could have launched" *.log
grep -c "Launching feature-on-demand" *.log

3. Build a summary table:

Step 3: Analyze Problematic Logs

For each log file with an unexpected outcome, trace the full activation flow. Read the log top-to-bottom and identify:

⚠️ Nested log entries: The shim's own internal calls can trigger additional log entries within an activation sequence that is already being logged. For example, a DllGetClassObject call may internally call ComputeVersionString, which calls FindLatestVersion, each generating log lines. When the FOD check runs ("Checking if feature-on-demand installation would help"), it re-runs the entire version computation — producing a second ComputeVersionString block within the same activation. Don't mistake these nested/re-entrant entries for separate activation attempts.

3a. Entry Point

The first FunctionCall: or MethodCall: line tells you how activation was triggered:

3b. Input Parameters

Immediately after the entry point, the log dumps the version computation inputs:

• IsLegacyBind: Is this a legacy (pre-v4) activation path? If 1, the shim uses the single-runtime "legacy" view of the world. Legacy APIs (CorBindToRuntimeEx, DllGetClassObject for legacy COM, LoadLibraryShim, etc.) set this.
• IsCapped: If 1, the shim's roll-forward semantics are capped at Whidbey (v2.0.50727) — it will NOT consider v4.0+ when enumerating installed runtimes. This is the mechanism that makes v4 installation non-impactful: legacy codepaths continue to behave as if v4 doesn't exist. On a v4-only machine with no .NET 3.5, a capped enumeration sees no runtimes at all. Capping does NOT prevent loading v4+ if a specific v4 version string is explicitly provided (e.g., via CorBindToRuntimeEx("v4.0.30319", ...) or via config with useLegacyV2RuntimeActivationPolicy).
• SkuCheckFlags: Controls SKU (edition) compatibility checking.
• ShouldEmulateExeLaunch: Whether to pretend this is an EXE launch for policy purposes.
• LegacyBindRequired: Whether a legacy bind is strictly required.

3c. Config File Processing

Look for config file parsing results:

• Parsing config file: {path} — the shim is looking for a .config file
• Config File (Open). Result:00000000 — config file found and opened successfully
• Config File (Open). Result:80070002 — config file not found (HRESULT for ERROR_FILE_NOT_FOUND)
• Found config file: {path} — config was successfully read
• UseLegacyV2RuntimeActivationPolicy is set to {0|1} — whether <startup useLegacyV2RuntimeActivationPolicy="true"> is present. When 1, all runtimes are treated as candidates for legacy codepaths — meaning legacy shim APIs can enumerate and choose v4+. This can be used with multiple <supportedRuntime> entries, with other config options, or even with no <supportedRuntime> entries at all (in which case legacy APIs can simply enumerate v4). Side effect: turns off in-proc SxS with pre-v4 runtimes — locks them out of the process.
• Config file includes SupportedRuntime entry. Version: vX.Y.Z, SKU: {sku} — each <supportedRuntime> found in config

Key insight: If a process has no config file AND is doing a capped legacy bind, the shim has nothing to direct it to v4.0. It will enumerate installed runtimes (capped to ≤v2.0), find nothing if 3.5 isn't installed, and fail. This is by design — v4 is intentionally invisible to these codepaths to keep v4 installation non-impactful.

3d. Version Resolution

• Installed Runtime: vX.Y.Z. VERSION_ARCHITECTURE: N — what's installed on the machine
• {exe} was built with version: vX.Y.Z — version from the binary's PE header (managed assemblies only; native EXEs won't have this)
• Using supportedRuntime: vX.Y.Z — the shim picked a version from the config's <supportedRuntime> list
• FindLatestVersion is returning the following version: vX.Y.Z ... V2.0 Capped: {0|1} — result of policy-based latest-version search
• Default version of the runtime on the machine: vX.Y.Z or (null) — what the shim settled on; (null) means nothing was found
• Decided on runtime: vX.Y.Z — final decision — this is the version that will be loaded