Restart does not rebuild when a launch config specifies a custom preLaunchTask (gap in the #496 fix)

Summary

Since the fix for #496 (C# Dev Kit v1.2.2), pressing Restart in a debug session rebuilds the project on restart — but only when the launch configuration has no preLaunchTask. If a dotnet launch configuration specifies its own preLaunchTask, Restart no longer rebuilds: it re-spins the previously-built binary and source-code changes are silently ignored. Stopping and starting the session (Stop → Start) rebuilds correctly.

The implicit build that the #496 fix runs on restart appears to be carried by the default/implicit pre-launch step; a user-supplied preLaunchTask replaces it, so the build is dropped from the restart path.

Environment

• C# Dev Kit: v3.20.199 (win32-x64)
• C# extension: v2.140.9
• VS Code: 1.129.1
• .NET SDK: 10.0.302
• OS: Windows 11

Repro

launch.json:

{
  "type": "dotnet",
  "request": "launch",
  "name": "Launch API",
  "projectPath": "${workspaceFolder}/src/Web.Api/Web.Api.csproj",
  "preLaunchTask": "noop"   // any task that does NOT build, e.g. an `echo`
}

1. F5 to start; confirm it runs.
2. Edit code so observable output changes (e.g. a returned string).
3. Press Restart (⟳ / Ctrl+Shift+F5).

Actual: the old binary is relaunched; the code change is not applied.
Expected: the project is rebuilt on restart (as it is when no preLaunchTask is present, and as Stop → Start does).

Confirmation that the preLaunchTask is the trigger: removing the preLaunchTask line (or using the auto-generated C#: <project> dynamic configuration, which has none) makes Restart rebuild correctly.

Analysis

VS Code core's restartSession runs runTasks() and then issues a DAP restart to the adapter when it advertises supportsRestartRequest:

if (session.capabilities.supportsRestartRequest) {
    const taskResult = await runTasks();
    if (taskResult === TaskRunResult.Success) {
        await doRestart(async () => {
            await session.restart();
            return true;
        });
    }
    return;
}

So on restart the only build opportunity is whatever runs inside runTasks(). With no preLaunchTask, the Dev Kit's implicit build runs there (the #496 fix); a user preLaunchTask replaces it, so nothing builds and the adapter re-spins the stale binary. This is consistent with #1822 ("restarts … resubmit the existing launch.json … there should be a way for us to reload … if we know it's a restart").

Suggested fix

On restart, run the implicit build even when a user preLaunchTask is specified — e.g. compose the implicit build with the user's tasks, or trigger the build inside the restart handler — so that adding a custom preLaunchTask doesn't silently disable the rebuild. At minimum, document that a custom preLaunchTask on a dotnet configuration must itself perform the build.

Related

• #496 — fixed the no-preLaunchTask case (v1.2.2)
• #615 — closed as duplicate of #496
• #1822 — restart resubmits the existing config

Start with VS Code's src/vs/workbench/contrib/debug/browser/debugService.ts and trace runTasks() for restartSession, then inspect the Dev Kit handling of implicit builds and custom preLaunchTask values. Reproduce with the supplied launch.json and a changed observable output. Done means Restart rebuilds before relaunching even when a non-building preLaunchTask is configured, without regressing Stop → Start or configurations without preLaunchTask.