如何在.NET中创建表达式构建器

Question

我已经编写了一些代码来允许在我们的网站上过滤产品,而且我的代码味道非常糟糕.用户可以选择1-*这些过滤器,这意味着我需要特定于该WHERE子句.

我想我正在寻找一种构建lambda表达式的方法,所以对于每个过滤器我都可以"修改"我的WHERE子句 - 但我不确定如何在.NET中执行此操作,并且必须有一种方法.

处于当前状态的代码(实际上是硬编码的,而不是动态的,添加更多过滤器选项会很麻烦).

public static class AgeGroups
{
    public static Dictionary<string, int> Items = new Dictionary<string, int>(){
        { "Modern (Less than 10 years old)", 1 },
        { "Retro (10 - 20 years old)", 2 },
        { "Vintage(20 - 70 years old)", 3 },
        { "Antique(70+ years old)", 4 }
    };

    public static IQueryable<ProductDTO> FilterAgeByGroup(IQueryable<ProductDTO> query, List<string> filters)
    {
        var values = new List<int>();
        var currentYear = DateTime.UtcNow.Year;
        foreach (var key in filters)
        {
            var matchingValue = Items.TryGetValue(key, out int value);

            if (matchingValue)
            {
                values.Add(value);
            }
        }

        if (Utility.EqualsIgnoringOrder(values, new List<int> { 1 }))
        {
            query = query.Where(x => x.YearManufactured >= currentYear - 10);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 2 }))
        {
            query = query.Where(x => x.YearManufactured <= currentYear - 10 && x.YearManufactured >= currentYear - 20);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 3 }))
        {
            query = query.Where(x => x.YearManufactured <= currentYear - 20 && x.YearManufactured >= currentYear - 70);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 4 }))
        {
            query = query.Where(x => x.YearManufactured <= currentYear - 70);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 1, 2}))
        {
            query = query.Where(x => x.YearManufactured >= currentYear - 20);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 1, 3 }))
        {
            query = query.Where(x => x.YearManufactured >= currentYear - 10 || (x.YearManufactured <= currentYear - 20 && x.YearManufactured >= currentYear - 70));
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 1, 4 }))
        {
            query = query.Where(x => x.YearManufactured >= currentYear - 10 ||  x.YearManufactured <= currentYear - 70);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 2, 3 }))
        {
            query = query.Where(x => x.YearManufactured <= currentYear - 10 && x.YearManufactured >= currentYear - 70);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 2, 4 }))
        {
            query = query.Where(x => (x.YearManufactured <= currentYear - 10 && x.YearManufactured >= currentYear - 20) 
                                     || x.YearManufactured <= currentYear - 70);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 1, 2, 3 }))
        {
            query = query.Where(x => x.YearManufactured >= currentYear - 70);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 1, 2, 4 }))
        {
            query = query.Where(x => x.YearManufactured >= currentYear - 20 || x.YearManufactured <= currentYear - 70);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 2, 3, 4}))
        {
            query = query.Where(x => x.YearManufactured <= currentYear - 10);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 1, 3, 4}))
        {
            query = query.Where(x => x.YearManufactured >= currentYear - 10 || x.YearManufactured <= 20);
        }
        else if (Utility.EqualsIgnoringOrder(values, new List<int> { 1, 2, 3, 4 }))
        {
            // all
        }
        return query;
    }
}

Answer 1

我自己最近也遇到了这个问题。通过另一个问题的帮助，我发现了http://www.albahari.com/nutshell/predicatebuilder.aspx。基本上，您想要构建一个谓词并将其传递到查询的 where 子句中。

public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> where1, 
     Expression<Func<T, bool>> where2)
{
    InvocationExpression invocationExpression = Expression.Invoke(where2, 
         where1.Parameters.Cast<Expression>());
    return Expression.Lambda<Func<T, bool>>(Expression.OrElse(where1.Body, 
         invocationExpression), where1.Parameters);
}

public static IQueryable<ProductDTO> FilterAgeByGroup(IQueryable<ProductDTO> query,  
   List<string> filters, int currentYear)
{
    var values = new HashSet<int>();
    //Default value
    Expression<Func<ProductDTO, bool>> predicate = (ProductDTO) => false;

    foreach (var key in filters)
    {
        var matchingValue = Items.TryGetValue(key, out int value);

        if (matchingValue)
        {
            values.Add(value);
        }
    }

    if (values.Count == 0)
        return query;

    if (values.Contains(1))
    {
        predicate = predicate.Or(x => x.YearManufactured >= currentYear - 10);
    }

    if (values.Contains(2))
    {
        predicate = predicate.Or(x => x.YearManufactured <= currentYear - 10 && 
            x.YearManufactured >= currentYear - 20);
    }

    if (values.Contains(3))
    {
        predicate = predicate.Or(x => x.YearManufactured <= currentYear - 20 && 
            x.YearManufactured >= currentYear - 70);
    }

    if (values.Contains(4))
    {
        predicate = predicate.Or(x => x.YearManufactured <= currentYear - 70);
    }

    return query.Where(predicate);
}