---
title: "assertIsolated(_:file:line:)"
framework: swift
role: symbol
role_heading: Instance Method
path: "swift/actor/assertisolated(_:file:line:)"
---

# assertIsolated(_:file:line:)

Stops program execution if the current task is not executing on this actor’s serial executor.

## Declaration

```swift
@backDeployed(before: macOS 14.0, iOS 17.0, watchOS 10.0, tvOS 17.0)
nonisolated func assertIsolated(_ message: @autoclosure () -> String = String(), file: StaticString = #fileID, line: UInt = #line)
```

## Parameters

- `message`: The message to print if the assertion fails.
- `file`: The file name to print if the assertion fails. The default is where this method was called.
- `line`: The line number to print if the assertion fails The default is where this method was called.

## Discussion

Discussion This function’s effect varies depending on the build flag used: In playgrounds and -Onone builds (the default for Xcode’s Debug configuration), stops program execution in a debuggable state after printing message. In -O builds (the default for Xcode’s Release configuration), the isolation check is not performed and there are no effects. note: This check is performed against the actor’s serial executor, meaning that / if another actor uses the same serial executor–by using that actor’s serial executor as its own unownedExecutor–this check will succeed , as from a concurrency safety perspective, the serial executor guarantees mutual exclusion of those two actors.