what is the difference between list<> and dictionary<> in c#

IDictionary is for key->value maps, ICollection is for sets of similar objects.

ICollection is an interface for collections of similar objects: the controls on a form, the elements in a list, the attributes in an XML tag, and so on. As of .NET 2.0, there's a generic version, so you can refer to a collection of integers as ICollection<int>.

IDictionary is an interface for mapping one type of object or value to another. It works like a real dictionary, or a phone book: you have a "key" in mind like a person’s name, and when you look it up, you get some information that’s identified by that key, like an address or phone number. Each key can only be listed once, although two different keys are still allowed to have the same value. This is also generic in .NET 2.0, so a dictionary whose keys are strings and whose values are integers would be IDictionary<string,int>.

A dictionary is actually a collection of key/value pairs: you can use an IDictionary<int,string> as an ICollection<KeyValuePair<int,string>>, and you can access the keys and values as separate collections with the Keys and Values properties.

Both ICollection and IDictionary are unordered, meaning that although you can retrieve the elements in some order with the CopyTo method or a foreach loop, that order has no special meaning, and it might change for no apparent reason. That’s the main difference between ICollection and IList: a list lets you put items in specific positions, just like an array, and they stay there until you move them.

List<> and Dictionary<,> - pretty different data structures which used for different purposes, List is simply a set of items and Dictionary is a set of key-value pairs.

Dictionary is pretty useful when you have a set of complex objects and want to have fast access by let's say ObjectName/ObjectId, in this case you create IDictionary<string, TObject> where key would be ObjectId and Value would be an object itself.

Some differences:

• List persist order of the items, Dictionary does not
• List allow fast access by index
• List support built in QuickSort algorithm for fast data sorting
• Dictionary allows ~O(1) time complexity to access an item (value) by a key

• Dictionary<K,V> is an associative array, or map. It is a container that can be indexed by values of any type.
• List<T> is an integer indexed array. It is a container that is indexed by contiguous integers.

The essential difference therefore is in how the containers are indexed.

Don't fall into the trap of believing that Dictionary<int,T> is semantically equivalent to List<T>. The difference is that the indexing of List<T> is contiguous whereas there can be gaps in the indexing for Dictionary<int,T>.