Trying to cancel an order (c#)

I am using the following c# code to cancel an order.

// Get a list of all the orders.
var orders = await restClient.ListOrdersAsync();

        // Print the quantity of shares for each position.
        foreach (var order in orders)
            Console.WriteLine($"{order.OrderId} shares of {order.Symbol}.");
            lastOrderID = order.OrderId.ToString();

        var order = await restClient.DeleteOrderAsync(lastOrderID);

I get an error message saying orderID cannot be converted from string to System.Guid

What is the format for orderID. I thought it was string.

Looking for any help.