Generic Class and MethodCreate generic classes and methods with type parameter constraints and the where keyword.
This page was last reviewed on Jan 22, 2024.
Generics. Generic classes have type parameters. Separate classes, each with a different field type, can be replaced with a single generic class.
A generic class introduces a type parameter (often specified as the letter T). This becomes part of the class definition itself. Generic methods can also be designed.
Generic class example. To start, we specify a generic type. These types have type parameters. When compiled, the type parameters refer to the type specified.
Info The letter T denotes a type that is only known based on the calling location. The program can act upon T like it is a real type.
Version 1 We use an int type parameter with the Test class. The T is substituted with an int.
Version 2 This is the same as version 1, but uses a string type parameter, and Test has a type of string.
using System; class Test<T> { T _value; public Test(T t) { // The field has the same type as the parameter. this._value = t; } public void Write() { Console.WriteLine(this._value); } } class Program { static void Main() { // Version 1: use int type parameter. Test<int> test1 = new Test<int>(5); // Call the Write method. test1.Write(); // Version 2: use string type parameter. Test<string> test2 = new Test<string>("cat"); test2.Write(); } }
5 cat
Generic method example. Generic methods have type parameters. They provide a way to parameterize the types used in a method. One implementation is used with many different types.
Here This program shows a generic method. The type parameter is customarily the type T or V or TValue or similar.
Note GetInitializedList is a generic method that constructs a special List. It uses a type parameter with name T.
Note 2 The first parameter to the GetInitializedList method is also a value of type T.
using System; using System.Collections.Generic; class Program { static List<T> GetInitializedList<T>(T value, int count) { // This generic method returns a List with ten elements initialized. // ... It uses a type parameter. // ... It uses the "open type" T. List<T> list = new List<T>(); for (int i = 0; i < count; i++) { list.Add(value); } return list; } static void Main() { // Use the generic method. // ... Specifying the type parameter is optional here. // ... Then print the results. List<bool> list1 = GetInitializedList(true, 5); List<string> list2 = GetInitializedList<string>("Perls", 3); foreach (bool value in list1) { Console.WriteLine(value); } foreach (string value in list2) { Console.WriteLine(value); } } }
True True True True True Perls Perls Perls
Generic class constraints. The C# language also provides ways for you to add more features to your generic types by reducing the range of types they can be parameterized with.
Next This program uses 3 classes that demonstrate different ways of constraining type parameters.
Info The Ruby class requires that its type parameter implement IDisposable. We can thus call the Dispose method or using statement.
Also The Python type demands that its type parameter be a struct. Ints are structs so you could use Python int as the constructed type.
Finally The Perl type requires that its type parameter V be a reference type (a class). It also requires a public parameterless constructor.
using System; using System.Data; /// <summary> /// Requires type parameter that implements interface IDisposable. /// </summary> class Ruby<T> where T : IDisposable { } /// <summary> /// Requires type parameter that is a struct. /// </summary> class Python<T> where T : struct { } /// <summary> /// Requires type parameter that is a reference type with a constructor. /// </summary> class Perl<V> where V : class, new() { } class Program { static void Main() { // DataTable implements IDisposable so it can be used with Ruby. Ruby<DataTable> ruby = new Ruby<DataTable>(); // Int is a struct (ValueType) so it can be used with Python. Python<int> python = new Python<int>(); // Program is a class with a parameterless constructor (implicit) // ... so it can be used with Perl. Perl<Program> perl = new Perl<Program>(); } }
Notes, constraints. Generic constraints are not often useful. If you are developing a complex framework (like the .NET Framework itself) they are necessary.
Tip We rarely need to make our own generic types now that the .NET Framework already contains so many useful ones.
IndexOf. A common generic method that is not part of a generic type is the Array.IndexOf method. We usually can omit the type parameter when calling it.
Summary. Usually we will use generic types and methods that are provided as part of .NET. But we can create our own, and this is beneficial in certain programs.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Jan 22, 2024 (edit).
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