C# Generic Class, Generic Method ExamplesCreate and use generic classes and methods. See type parameter constraints and the where keyword.
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.
C# program that describes generic class
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.
Note The syntax form for the declaration uses the <T> characters after the method name but before the formal parameter list.
Example This program shows a generic method. The type parameter is customarily the type T or V or TValue or similar.
GetInitializedList This is a generic method that constructs a special List. It uses a type parameter with name T.
Info The first parameter to the GetInitializedList method is also a value of type T.
C# program that declares generic method
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.
Ruby The Ruby class requires that its type parameter implement IDisposable. We can thus call the Dispose method or using statement.
Python This type demands that its type parameter be a struct. Ints are structs so you could use Python<int> as the constructed type.
Perl This type requires that its type parameter V be a reference type (a class). It also requires a public parameterless constructor.
C# program that uses generic type constraints
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>(); } }
Generic methods. Let us examine the C# language specification. It describes in precise detail all of the results for computing type inference matches.
Also It describes the algorithm used by the language compiler. But these details are not useful for most programs.
Tip Do not rely on language specification details in your program. Most developers do not study language specifications at length.
Generic methods, disambiguation. Much complexity in generic methods involves disambiguation. This is the process by which the exact method you want to invoke is searched for and selected.
Note You can often omit the type parameter, but not where the method call cannot be determined from the context.
Notes, constraints. Generic constrains 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.
Terms. In the C# specification, you will see lots of terms that apply to generic types. These terms include open types, closed types, bound types and unbound types.
Constructed type This is another term for a generic type—it refers to a type with a type parameter.
Open type This is also a term for a generic type. It is a type that has type parameters.
Closed type As opposed to an open type, a closed type is one that does not have type parameters.
Unbound type An unbound type is declared by a type declaration, but is not actually a type.
Bound type A bound type is one of two types—a regular type or a constructed type. It is a type of types.
MultiMap. This is a Dictionary (Map) that has many values at a single key. MultiMap allows fast lookup of Lists. The implementation here is not ideal, but uses the generic class syntax.
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.
Array.IndexOf, LastIndexOf
A 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.
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