How .NET 9 replaces object monitor locks with dedicated System.Threading.Lock:

Before C# 13, lock(new object()) compiled to Monitor.Enter() / Monitor.Exit(), requiring runtime sync block index allocations in the object header.

In .NET 9:

private readonly Lock _gate = new();

void ThreadSafeOperation()
{
    lock (_gate) // C# 13 emits _gate.EnterScope()!
    {
        // Critical Section
    }
}

Lock.EnterScope() returns a ref struct scope guard. Eliminates sync block table lookups and executes faster on multi-core CPUs!

The Roslyn compiler automatically recognizes System.Threading.Lock and generates optimal EnterScope() calls instead of Monitor.Enter.

A great modernization of C#'s classic lock keyword.