趣味程式設計：C# Specification 模式實作（二）── 組合規範與邏輯運算

CompositeSpecification 抽象基類

public abstract class CompositeSpecification<T> : ISpecification<T>
{
    public abstract bool IsSatisfiedBy(T candidate);

    public ISpecification<T> And(ISpecification<T> other)
        => new AndSpecification<T>(this, other);

    public ISpecification<T> Or(ISpecification<T> other)
        => new OrSpecification<T>(this, other);

    public ISpecification<T> Not()
        => new NotSpecification<T>(this);
}

三種邏輯組合實作

public class AndSpecification<T> : CompositeSpecification<T>
{
    private ISpecification<T> _one, _other;
    public AndSpecification(ISpecification<T> x, ISpecification<T> y) { _one = x; _other = y; }
    public override bool IsSatisfiedBy(T candidate) => _one.IsSatisfiedBy(candidate) && _other.IsSatisfiedBy(candidate);
}

public class OrSpecification<T> : CompositeSpecification<T>
{
    private ISpecification<T> _one, _other;
    public OrSpecification(ISpecification<T> x, ISpecification<T> y) { _one = x; _other = y; }
    public override bool IsSatisfiedBy(T candidate) => _one.IsSatisfiedBy(candidate) || _other.IsSatisfiedBy(candidate);
}

public class NotSpecification<T> : CompositeSpecification<T>
{
    private ISpecification<T> _wrapped;
    public NotSpecification(ISpecification<T> x) { _wrapped = x; }
    public override bool IsSatisfiedBy(T candidate) => !_wrapped.IsSatisfiedBy(candidate);
}

使用範例

var spec = new NameStartsWith("A").And(new AgeGreaterThan(18)).Not(new IsBanned());
bool ok = spec.IsSatisfiedBy(user);

優點

• 將複雜業務規則分解為小型、可測試、可組合的單元。
• 符合開放封閉原則：新增規則只需新增 ISpecification 實作，不修改現有程式碼。
• 語意清晰，程式碼可讀性高。