c# - 规范模式 - 使用 lambda (C#) 创建复合规范

Question

如果我有一个定义为如下表达式的规范：

public Expression<Func<Foo, bool>> IsSuperhuman = 
  x => x.CanFly && x.HasXRayVision;

我想用逻辑“超人且是女性”定义另一个规范“IsSuperheroine”，如何在新规范中重用现有规范？

Answer 1

Here's a way to do it :

Expression<Func<Foo, bool>> IsSuperhuman = x => x.CanFly && x.HasXRayVision;

Expression<Func<Foo, bool>> IsSuperheroine = AndAlso(IsSuperhuman, x => x.IsFemale);

...

public static Expression<Func<T, TResult>> AndAlso<T, TResult>(Expression<Func<T, TResult>> expr1, Expression<Func<T, TResult>> expr2)
{
    var arg = Expression.Parameter(typeof(T), expr1.Parameters[0].Name);
    var andExpr = Expression.AndAlso(
        ReplaceParameter(expr1.Body, expr1.Parameters[0], arg),
        ReplaceParameter(expr2.Body, expr2.Parameters[0], arg));
    return Expression.Lambda<Func<T, TResult>>(andExpr, arg);
}

public static Expression ReplaceParameter(Expression expr, ParameterExpression oldParam, ParameterExpression newParam)
{
    return new ReplaceParameterVisitor(oldParam, newParam).Visit(expr);
}

internal class ReplaceParameterVisitor : ExpressionVisitor
{
    private ParameterExpression _oldParam;
    private ParameterExpression _newParam;

    public ReplaceParameterVisitor(ParameterExpression oldParam, ParameterExpression newParam)
    {
        _oldParam = oldParam;
        _newParam = newParam;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node == _oldParam)
            return _newParam;
        return node;
    }
}

It is probably not the simplest way to do it, but it works...

Answer 2

你检查过 LinqKit中的谓词构建器吗？它通过让你和和或表达式一起构建表达式。