C # where clause
|
where clause specifies the type of constraints, which can be used as a variable parameter of the type defined in the generic declaration.
1. interface constraints. For example, a generic class declaration MyGenericClass , this type parameter T can be achieved IComparable <T> Interface:
public class MyGenericClass<T> where T:IComparable { }
2. The base class constraint: Indicates a type must be specified class as a base class (or just the class itself) , it can be used as the generic type parameter type. Such constraints once used, must appear before all other constraints of the type parameter.
class MyClassy<T, U>
where T : class where U : struct { } 3.where clause may further comprise constructor constraint. Can create an instance of type parameters using the new operator; however, this must be constrained type parameter constructor constraint new () is. new () constraint allows the compiler to know: any type of parameters provide must have no parameters accessible (or default) constructor. E.g:
public class MyGenericClass <T> where T: IComparable, new
()
new () constraint appear in the final where clause.
{ // The following line is not possible without new() constraint: T item = new T(); } 4. For a plurality of types of parameters, each parameter type uses a where clause, for example:
interface
MyI { }
class Dictionary<TKey,TVal> where TKey: IComparable, IEnumerable where TVal: MyI { public void Add(TKey key, TVal val) { } } The parameter type may also be attached to the constraint of a generic method, for example:
public bool MyMethod<T>
(T t) where T : IMyInterface { }
Note that for both the commission and methods for describing the type parameter constraint syntax is the same:
delegate T MyDelegate<T>() where T : new()
|
Link from : http://hi.baidu.com/%D0%A6%D3%F0%BA%A8%CC%EC/blog/item/9701871c28d2fb13413417fd.html
转载于:https://www.cnblogs.com/zhangchenliang/archive/2012/01/09/2316985.html