Why accidental closure captures cause silent memory leaks in long-running services:

When a local function or lambda references a variable in the outer scope, the compiler allocates a hidden display class on the heap, keeping all outer variables alive in memory!

The Fix: static modifier:

public void ProcessOrder(Order order)
{
    int multiplier = 2;
    
    // 'static' keyword guarantees this local function CANNOT capture outer scope variables!
    static int CalculateTax(int amount)
    {
        return amount * 10 / 100;
    }
}

If you accidentally try to reference multiplier inside CalculateTax, the compiler produces a compile-time error!

Adding static to local functions and lambdas (static (x) => ...) is a zero-cost way to enforce zero allocations.

We enabled the Roslyn analyzer rule that flags all non-static local functions that don't need closure captures.