Represents a strongly typed list of objects that can be accessed by index. Provides methods to search, sort, and manipulate lists.
Assembly: mscorlib (in mscorlib.dll)
The class uses both an equality comparer and an ordering comparer.
Methods such as Contains, IndexOf, LastIndexOf, and Remove use an equality comparer for the list elements. The default equality comparer for type T is determined as follows. If type T implements the IEquatable(T) generic interface, then the equality comparer is the Equals(T) method of that interface; otherwise, the default equality comparer is Object.Equals(Object).
Methods such as BinarySearch and Sort use an ordering comparer for the list elements. The default comparer for type T is determined as follows. If type T implements the IComparable(T) generic interface, then the default comparer is the CompareTo(T) method of that interface; otherwise, if type T implements the nongeneric IComparable interface, then the default comparer is the CompareTo(Object) method of that interface. If type T implements neither interface, then there is no default comparer, and a comparer or comparison delegate must be provided explicitly.
The is not guaranteed to be sorted. You must sort the before performing operations (such as BinarySearch) that require the to be sorted.
Elements in this collection can be accessed using an integer index. Indexes in this collection are zero-based.
accepts a null reference (Nothing in Visual Basic) as a valid value for reference types and allows duplicate elements.
In deciding whether to use the or ArrayList class, both of which have similar functionality, remember that the class performs better in most cases and is type safe. If a reference type is used for type T of the class, the behavior of the two classes is identical. However, if a value type is used for type T, you need to consider implementation and boxing issues.
If a value type is used for type T, the compiler generates an implementation of the class specifically for that value type. That means a list element of a object does not have to be boxed before the element can be used, and after about 500 list elements are created the memory saved not boxing list elements is greater than the memory used to generate the class implementation.
Make certain the value type used for type T implements the IEquatable(T) generic interface. If not, methods such as Contains must call the Object.Equals(Object) method, which boxes the affected list element. If the value type implements the IComparable interface and you own the source code, also implement the IComparable(T) generic interface to prevent the BinarySearch and Sort methods from boxing list elements. If you do not own the source code, pass an IComparer(T) object to the BinarySearch and Sort methods
It is to your advantage to use the type-specific implementation of the class instead of using the ArrayList class or writing a strongly typed wrapper collection yourself. The reason is your implementation must do what the .NET Framework does for you already, and the common language runtime can share Microsoft intermediate language code and metadata, which your implementation cannot.
The following code example demonstrates several properties and methods of the generic class of type string. (For an example of a of complex types, see the Contains method.)
The default constructor is used to create a list of strings with the default capacity. The Capacity property is displayed and then the Add method is used to add several items. The items are listed, and the Capacity property is displayed again, along with the Count property, to show that the capacity has been increased as needed.
The Contains method is used to test for the presence of an item in the list, the Insert method is used to insert a new item in the middle of the list, and the contents of the list are displayed again.
The default Item property (the indexer in C#) is used to retrieve an item, the Remove method is used to remove the first instance of the duplicate item added earlier, and the contents are displayed again. The Remove method always removes the first instance it encounters.
The TrimExcess method is used to reduce the capacity to match the count, and the Capacity and Count properties are displayed. If the unused capacity had been less than 10 percent of total capacity, the list would not have been resized.
Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
A can support multiple readers concurrently, as long as the collection is not modified. Enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with one or more write accesses, the only way to ensure thread safety is to lock the collection during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.
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The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.