Type constraints (classes and interfaces)

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Example

Type constraints are able to force a type parameter to implement a certain interface or class.

interface IType;
interface IAnotherType;

// T must be a subtype of IType
interface IGeneric<T>
    where T : IType
{
}

// T must be a subtype of IType
class Generic<T>
    where T : IType
{
}

class NonGeneric
{
    // T must be a subtype of IType
    public void DoSomething<T>(T arg)
        where T : IType
    {
    }
}

// Valid definitions and expressions:
class Type : IType { }
class Sub : IGeneric<Type> { }
class Sub : Generic<Type> { }
new NonGeneric().DoSomething(new Type());

// Invalid definitions and expressions:
class AnotherType : IAnotherType { }
class Sub : IGeneric<AnotherType> { }
class Sub : Generic<AnotherType> { }
new NonGeneric().DoSomething(new AnotherType());

Syntax for multiple constraints:

class Generic<T, T1>
    where T : IType 
    where T1 : Base, new()
{
}

Type constraints works in the same way as inheritance, in that it is possible to specify multiple interfaces as constraints on the generic type, but only one class:

class A { /* ... */ }
class B { /* ... */ }

interface I1 { }
interface I2 { }

class Generic<T>
    where T : A, I1, I2
{
}

class Generic2<T>
    where T : A, B //Compilation error
{
}

Another rule is that the class must be added as the first constraint and then the interfaces:

class Generic<T>
    where T : A, I1
{
}

class Generic2<T>
    where T : I1, A //Compilation error
{
}

All declared constraints must be satisfied simultaneously for a particular generic instantiation to work. There is no way to specify two or more alternative sets of constraints.

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Contributors: 6
2016-08-10
Licensed under: CC-BY-SA

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