how to check if object already exists in a list

Question

I have a list

  List myList

and I am adding items to a list and I want to check if that object is already in the list.

Answer 1

Are you sure you need a list in this case? If you are populating the list with many items, performance will suffer with myList.Contains or myList.Any; the run-time will be quadratic. You might want to consider using a better data structure. For example,

 public class MyClass
    {
        public string Property1 { get; set; }
        public string Property2 { get; set; }

    }

    public class MyClassComparer : EqualityComparer
    {
        public override bool Equals(MyClass x, MyClass y)
        {
            if(x == null || y == null)
               return x == y;

            return x.Property1 == y.Property1 && x.Property2 == y.Property2;
        }

        public override int GetHashCode(MyClass obj)
        {
            return obj == null ? 0 : (obj.Property1.GetHashCode() ^ obj.Property2.GetHashCode());
        }
    }

You could use a HashSet in the following manner:

  var set = new HashSet(new MyClassComparer());
  foreach(var myClass in ...)
     set.Add(myClass);

Of course, if this definition of equality for MyClass is 'universal', you needn't write an IEqualityComparer implementation; you could just override GetHashCode and Equals in the class itself.