how to check if object already exists in a list

Question

It depends on the needs of the specific situation. For example, the dictionary approach would be quite good assuming:

1. The list is relatively stable (not a lot of inserts/deletions, which dictionaries are not optimized for)
2. The list is quite large (otherwise the overhead of the dictionary is pointless).

If the above are not true for your situation, just use the method Any():

Item wonderIfItsPresent = ...
bool containsItem = myList.Any(item => item.UniqueProperty == wonderIfItsPresent.UniqueProperty);

This will enumerate through the list until it finds a match, or until it reaches the end.

Simply use Contains method. Note that it works based on the equality function Equals

bool alreadyExist = list.Contains(item);

If it's maintainable to use those 2 properties, you could:

bool alreadyExists = myList.Any(x=> x.Foo=="ooo" && x.Bar == "bat");

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<MyClass>
    {
        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<MyClass>(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.

Another point to mention is that you should ensure that your equality function is as you expect. You should override the equals method to set up what properties of your object have to match for two instances to be considered equal.

Then you can just do mylist.contains(item)