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I can now write code to create a linked list in which each node is a different data type . The code could look something like this:
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private static void DifferentDataLinkedList() { Node head = new TypedNode<Char>('.'); head = new TypedNode<DateTime>(DateTime.Now, head); head = new TypedNode<String>("Today is ", head); Console.WriteLine(head.ToString()); }
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Sometimes generic syntax confuses developers . After all, there can be a lot of less-than (<) and greater-than (>) signs sprinkled throughout your source code, and this hurts readability . To improve syntax, some developers define a new non-generic class type that is derived from a generic type and that specifies all of the type arguments . For example, to simplify code like this:
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List<DateTime> dtl= new List<DateTime>();
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Some developers might first define a class like this:
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internal sealed class DateTimeList : List<DateTime> { // No need to put any code in here! }
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Now, the code that creates a list can be rewritten more simply (without less-than and greater-than signs) like this:
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DateTimeList dtl = new DateTimeList();
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While this seems like a convenience, especially if you use the new type for parameters, local variables, and fields, you should never define a new class explicitly for the purpose of making your source code easier to read . The reason is because you lose type identity and equivalence, as you can see in the following code:
Boolean sameType = (typeof(List<DateTime>) == typeof(DateTimeList));
When the code above runs, sameType will be initialized to false because you are comparing two different type objects . This also means that a method prototyped as accepting a DateTimeList will not be able to have a List<DateTime> passed to it . However, a method prototyped as accepting a List<DateTime> can have a DateTimeList passed to it since DateTimeList is derived from List<DateTime> . Programmers may become easily confused by all of this .
Part II Designing Types
Fortunately, C# does offer a way to use simplified syntax to refer to a generic closed type while not affecting type equivalence at all; you can use the good old using directive at the top of your source code file . Here is an example:
using DateTimeList = System.Collections.Generic.List<System.DateTime>;
Here, the using directive is really just defining a symbol called DateTimeList . As the code compiles, the compiler substitutes all occurrences of DateTimeList with System.Collections.Generic.List<System.DateTime> . This just allows developers to use a simplified syntax without affecting the actual meaning of the code, and therefore, type identity and equivalence are maintained . So now, when the following line executes, sameType will be initialized to true .
Boolean sameType = (typeof(List<DateTime>) == typeof(DateTimeList));
As another convenience, you can use C# s implicitly typed local variable feature, where the compiler infers the type of a method s local variable from the type of the expression you are assigning to it:
using System; using System.Collections.Generic; ... internal sealed class SomeType { private static void SomeMethod () { // Compiler infers that DateTimeList is of type // System.Collections.Generic.List<System.DateTime> var dtl = List<DateTime>(); ... } }
Code Explosion
When a method that uses generic type parameters is JIT-compiled, the CLR takes the method s IL, substitutes the specified type arguments, and then creates native code that is specific to that method operating on the specified data types . This is exactly what you want and is one of the main features of generics . However, there is a downside to this: the CLR keeps generating native code for every method/type combination . This is referred to as code explosion . This can end up increasing the application s working set substantially, thereby hurting performance . Fortunately, the CLR has some optimizations built into it to reduce code explosion . First, if a method is called for a particular type argument, and later, the method is called again using the same type argument, the CLR will compile the code for this method/type combination just once . So if one assembly uses List<DateTime>, and a completely different assembly
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