␡
- Commenting Your Code
- Guarding Your Code
- Installing the Code Contracts Library
- Using Code Contracts
- Contracts on Interfaces and Abstract Methods
- Suppressing Warnings
- Building Contract Reference Assemblies
- Summary
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Contracts on Interfaces and Abstract Methods
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Contracts on Interfaces and Abstract Methods
Interfaces are not allowed to contain method bodies. This means that their contracts must be written using a separate contract class. The contract class and its associated interface are linked using a pair of attributes. Listing 11 shows how to write contracts for an interface.
Listing 11—Interface Contracts
[ContractClass(typeof(IFooContract))]
interface IFoo
{
int Count { get; }
void Put(int value);
}
[ContractClassFor(typeof(IFoo))]
abstract class IFooContract : IFoo
{
int IFoo.Count
{
get
{
Contract.Ensures(0 <= Contract.Result<int>());
return default(int);
}
}
void IFoo.Put(int value)
{
Contract.Requires(0 <= value);
}
}
Writing contracts for an abstract method in an abstract class is accomplished in the same way as interfaces, as shown in Listing 12.
Listing 12—Contracts on Abstract Methods
[ContractClass(typeof(FooContract))]
abstract class Foo
{
public abstract int Count { get; }
public abstract void Put(int value);
}
[ContractClassFor(typeof(Foo))]
abstract class FooContract : IFoo
{
public override int Count
{
get
{
Contract.Ensures(0 <= Contract.Result<int>());
return default(int);
}
}
public override void Put(int value)
{
Contract.Requires(0 <= value);
}
}
In both cases, you needed to specify a dummy return value for the get accessor of the Count property. You actually need to do this for any property or method that has a return type other than void. The easiest way to do this is to always use default(T) for any type T.