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Generics in C# and the CLR differ from similar constructs in other languages Although other languages provide similar functionality, C# is significantly more type-safe Generics in C# is a language feature and a platform feature the underlying 20 runtime contains deep support for generics in its engine C++ templates differ significantly from C# generics, because C# takes advantage of the CIL C# generics are compiled into the CIL, causing specialization to occur at execution time for each value type only when it is used, and only once for reference types A distinct feature not supported by C++ templates is explicit constraints C++ templates allow you to compile a method call that may or may not belong to the type parameter As a result, if the member does not exist in the type parameter, an error occurs, likely with a cryptic error message and referring to an unexpected location in the source code However, the advantage of the C++ implementation is that operators (+, -, and so on) may be called on the type C# does not support the calling of operators on the type parameter because operators are static they can t be identified by interfaces or base class constraints The problem with the error is that it occurs only when using the template, not when defining it Because C# generics can declare constraints, the compiler can prevent such errors when defining the generic, thereby identifying invalid assumptions sooner Furthermore, when declaring a variable of a generic type, the error will point to the declaration of the variable, not to the location in the generic implementation where the member is used It is interesting to note that Microsoft s CLI support in C++ includes both generics and C++ templates because of the distinct characteristics of each
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To specify an interface for the constraint you declare an interface constraint This constraint even circumvents the need to cast in order to call an explicit interface member implementation
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11: Generics
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Base Class Constraints Sometimes you might want to limit the constructed type to a particular class derivation You do this using a base class constraint, as shown in Listing 1122
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Listing 1122: Declaring a Base Class Constraint
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public class EntityDictionary<TKey, TValue> : SystemCollectionsGenericDictionary<TKey, TValue> where TValue : EntityBase { }
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In contrast to SystemCollectionsGenericDictionary<TKey, TValue> on its own, EntityDictionary<TKey, TValue> requires that all TValue types derive from the EntityBase class By requiring the derivation, it is possible to always perform a cast operation within the generic implementation, because the constraint will ensure that all type parameters derive from the base and, therefore, that all TValue type parameters used with EntityDictionary can be implicitly converted to the base The syntax for the base class constraint is the same as that for the interface constraint, except that base class constraints must appear first when multiple constraints are specified However, unlike interface constraints, multiple base class constraints are not allowed since it is not possible to derive from multiple classes Similarly, base class constraints cannot be specified for sealed classes or specific structs For example, C# does not allow a constraint for a type parameter to be derived from string or SystemNullable<T>
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struct/class Constraints Another valuable generic constraint is the ability to restrict type parameters to a value type or a reference type The compiler does not allow specifying SystemValueType as the base class in a constraint Instead, C# provides special syntax that works for reference types as well Instead of specifying a class from which T must derive, you simply use the keyword struct or class, as shown in Listing 1123
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Constraints
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Listing 1123: Specifying the Type Parameter as a Value Type
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public struct Nullable<T> : IFormattable, IComparable, IComparable<Nullable<T>>, INullable where T : struct { // }
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Because a base class constraint requires a particular base class, using struct or class with a base class constraint would be pointless, and in fact could allow for conflicting constraints Therefore, you cannot use struct and class constraints with a base class constraint There is one special characteristic for the struct constraint It limits possible type parameters as being only value types while at the same time preventing type parameters that are SystemNullable<T> type parameters Why Without this last restriction, it would be possible to define the nonsense type Nullable<Nullable<T>>, which is nonsense because Nullable<T> on its own allows a value type variable that supports nulls, so a nullable-nullable type becomes meaningless Since the nullable operator ( ) is a C# shortcut for declaring a nullable value type, the Nullable<T> restriction provided by the struct constraint also prevents code such as the following:
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