Errors and cleanup

G# uses CLR exceptions, nullable values, defer, and using for error and lifetime management. This page focuses on the implemented language surface.

Exceptions

Use throw to raise an exception and try with catch and/or finally to handle cleanup or recovery. A try statement must have at least one catch or finally semantically.

samples/Exceptions.gs

package GSharp.Example.Exceptions

import System

var trace = ""

try {
    trace = trace + "t"
} finally {
    trace = trace + "f"
}

Console.WriteLine(trace)

var caught = "before"
try {
    var n = Int32.Parse("not a number")
} catch (e Exception) {
    caught = "caught"
}

Console.WriteLine(caught)

Catch clauses name a local and may specify a type. Prefer specific exception types at library boundaries and reserve broad catches for top-level reporting or cleanup.

Error-handling rationale is in ADR-0005.

Nullable absence is not an exception

Use nullable types and nil for expected absence. Use exceptions for failure paths that interrupt normal control flow. ?: is the null-coalescing operator, ?. is null-conditional access, and !! asserts non-null.

let display = user?.Name ?: "anonymous"

Defer

defer call() schedules a call to run when the current scope exits. The parser accepts an expression, but binding requires a call. Keep deferred calls short and side-effect focused, such as unlocking, closing, or logging.

lock.Enter()
defer lock.Exit()

Using

using introduces a variable declaration whose value must be disposable. It is a resource-scope statement rather than an expression.

using let stream = File.OpenRead(path)

Use using when a value owns an unmanaged or disposable resource. Use defer when the cleanup operation is not itself represented by an IDisposable value.

The scope rules for defer and using are covered by ADR-0030.

Finally and structured cleanup

finally always expresses exception-safe cleanup around a region. Prefer using for disposable resources, defer for small cleanup calls, and finally when cleanup depends on several statements or must coordinate with catch logic.