AsyncLocal never leaks and is safe for CallContext-like state

Even if it's typically used in a static field, the values never leak since they are bound to a transient ExecutionContext

Sometime ago I wrote on How to migrate CallContext to .NETStandard and .NETCore, and one question mentioned that the values themselves might leak, which actually is not the case, as shown here, due to the "magic" that is AsyncLocal in combination with the transient nature of the ExecutionContext:
static AsyncLocal<object> local = new AsyncLocal<object>();
​
async Task Main()
{
    WeakReference data = null;
​
    await Task.Run(() =>
    {
        var o = new object();
        data = new WeakReference(o);
        // We assign to the static async local, to see if it leaks.
        local.Value = o;
    });
​
    // After execution is finished, we do have a live reference still.
    System.Diagnostics.Debug.Assert(data.IsAlive == true);
​
    GC.Collect();
​
    // But a GC proves nobody is holding a strong reference to it.
    System.Diagnostics.Debug.Assert(data.IsAlive == false);
}
​

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Last updated 11 months ago