C# List of objects, how do I get the sum of a property

using System.Linq;

...

double total = myList.Sum(item => item.Amount);

And if you need to do it on items that match a specific condition...

double total = myList.Where(item => item.Name == "Eggs").Sum(item => item.Amount);

Another alternative:

myPlanetsList.Select(i => i.Moons).Sum();