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The C# Language
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Here is an example that demonstrates the reference type constraint:
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// Demonstrate a reference constraint using System; class MyClass { // } // Use a reference constraint class Test<T> where T : class { T obj; public Test() { // The following statement is legal only // because T is guaranteed to be a reference // type, which can be assigned the value null obj = null; } // } class ClassConstraintDemo { static void Main() { // The following is OK because MyClass is a class Test<MyClass> x = new Test<MyClass>(); // The next line is in error because int is a value type Test<int> y = new Test<int>(); } }
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First, notice how Test is declared:
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class Test<T> where T : class {
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The class constraint requires that any type argument for T be a reference type In this program, this is necessary because of what occurs inside the Test constructor:
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public Test() { // The following statement is legal only // because T is guaranteed to be a reference // type, which can be assigned the value null obj = null; }
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Here, obj (which is of type T) is assigned the value null This assignment is valid only for reference types As a general rule, you cannot assign null to a value type (The exception to this rule is the nullable type, which is a special structure type that encapsulates a value type and allows the value null See 20 for details) Therefore, without the constraint, the
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18:
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Generics
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assignment would not have been valid and the compile would have failed This is one case in which the difference between value types and reference types might be important to a generic routine The value type constraint is the complement of the reference type constraint It simply ensures that any type argument is a value type, including a struct or an enum (In this context, a nullable type is not considered a value type) Here is an example:
// Demonstrate a value type constraint using System; struct MyStruct { // } class MyClass { // } class Test<T> where T : struct { T obj; public Test(T x) { obj = x; } // } class ValueConstraintDemo { static void Main() { // Both of these declarations are legal Test<MyStruct> x = new Test<MyStruct>(new MyStruct()); Test<int> y = new Test<int>(10); // But, the following declaration is illegal! Test<MyClass> z = new Test<MyClass>(new MyClass()); } }
PART I
In this program, Test is declared as shown here:
class Test<T> where T : struct {
Because T of Test now has the struct constraint, T can be bound only to value type arguments This means that Test<MyStruct> and Test<int> are valid, but Test<MyClass> is not To prove this, try removing the comment symbols from the start of the last line in the program and recompiling An error will be reported
Part I:
The C# Language
Using a Constraint to Establish a Relationship Between Two Type Parameters
There is a variation of the base class constraint that allows you to establish a relationship between two type parameters For example, consider the following generic class declaration:
class Gen<T, V> where V : T {
In this declaration, the where clause tells the compiler that the type argument bound to V must be identical to or inherit from the type argument bound to T If this relationship is not present when an object of type Gen is declared, then a compile-time error will result A constraint that uses a type parameter, such as that just shown, is called a naked type constraint The following example illustrates this constraint:
// Create relationship between two type parameters using System; class A { // } class B : A { // } // Here, V must be or inherit from T class Gen<T, V> where V : T { // } class NakedConstraintDemo { static void Main() { // This declaration is OK because B inherits A Gen<A, B> x = new Gen<A, B>(); // This declaration is in error because // A does not inherit B Gen<B, A> y = new Gen<B, A>(); } }
First, notice that class B inherits class A Next, examine the two Gen declarations in Main( ) As the comments explain, the first declaration
Gen<A, B> x = new Gen<A, B>();
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