c#-4.0 - 仕様パターンの実装方法

Question

私のプロジェクトでは; 以下に示す特定のパターンクラスを含めました。これを実装する方法がわかりません。これらのコードは、以前の開発者によって含まれています。

public interface ISpecification<T>
{
    Expression<Func<T, bool>> SpecExpression { get; }
    bool IsSatisfiedBy(T obj);
}

public static class IExtensions
{
    public static ISpecification<T> And<T>(
        this ISpecification<T> left,
        ISpecification<T> right)
    {
        return new And<T>(left, right);
    }

    public static ISpecification<T> Or<T>(
        this ISpecification<T> left,
        ISpecification<T> right)
    {
        return new Or<T>(left, right);
    }

    public static ISpecification<T> Negate<T>(this ISpecification<T> inner)
    {
        return new Negated<T>(inner);
    }
}

public abstract class SpecificationBase<T> : ISpecification<T>
{
    private Func<T, bool> _compiledExpression;

    private Func<T, bool> CompiledExpression
    {
        get { return _compiledExpression ?? (_compiledExpression = SpecExpression.Compile()); }
    }

    public abstract Expression<Func<T, bool>> SpecExpression { get; }

    public bool IsSatisfiedBy(T obj)
    {
        return CompiledExpression(obj);
    }
}

public class And<T> : SpecificationBase<T>
{
    ISpecification<T> left;
    ISpecification<T> right;

    public And(
        ISpecification<T> left,
        ISpecification<T> right)
    {
        this.left = left;
        this.right = right;
    }

    // AndSpecification
    public override Expression<Func<T, bool>> SpecExpression
    {
        get
        {
            var objParam = Expression.Parameter(typeof(T), "obj");

            var newExpr = Expression.Lambda<Func<T, bool>>(
                Expression.AndAlso(
                    Expression.Invoke(left.SpecExpression, objParam),
                    Expression.Invoke(right.SpecExpression, objParam)
                ),
                objParam
            );

            return newExpr;
        }
    }
}

public class Or<T> : SpecificationBase<T>
{
    ISpecification<T> left;
    ISpecification<T> right;

    public Or(
        ISpecification<T> left,
        ISpecification<T> right)
    {
        this.left = left;
        this.right = right;
    }

    // OrSpecification
    public override Expression<Func<T, bool>> SpecExpression
    {
        get
        {
            var objParam = Expression.Parameter(typeof(T), "obj");

            var newExpr = Expression.Lambda<Func<T, bool>>(
                Expression.OrElse(
                    Expression.Invoke(left.SpecExpression, objParam),
                    Expression.Invoke(right.SpecExpression, objParam)
                ),
                objParam
            );

            return newExpr;
        }
    }
}

 public class Negated<T> : SpecificationBase<T>
{
    private readonly ISpecification<T> _inner;

    public Negated(ISpecification<T> inner)
    {
        _inner = inner;
    }

    // NegatedSpecification
    public override Expression<Func<T, bool>> SpecExpression
    {
        get
        {
            var objParam = Expression.Parameter(typeof(T), "obj");

            var newExpr = Expression.Lambda<Func<T, bool>>(
                Expression.Not(
                    Expression.Invoke(this._inner.SpecExpression, objParam)
                ),
                objParam
            );

            return newExpr;
        }
    }
}

簡単な例で上記の仕様を実装する方法は? この仕様の用途は何ですか?