.net - ORing LINQ 查询，使用表达式树构建

Question

所以，这相当复杂。

我在一个集合中有一组规则，一个规则包含这三个属性。

Field, Op, and Data (all strings)

因此规则可能类似于“State”、“eq”、“CA”

我的一般规则是所有规则都通过 AND 组合在一起。但是，需要注意的是，如果它们具有相同的 Field 值，则所有这些都进行 OR 运算。这允许我们说“State”、“eq”、“CA”或“State”、“eq”、“TX”和“FirstName”、“eq”、“John”。

问题是我当前应用规则的方式不起作用，因为它只是使用每个规则不断构建 linq 表达式，使其变得越来越明确。

var result = rules.Aggregate(_repository.All, (current, rule) => current.ExtendQuery(rule))

ExtendQuery 是我编写的扩展方法，它使用 ExpressionTrees 来生成新查询，该查询将当前规则应用于传入的查询。 (有效地将它们全部组合在一起)

现在对我来说修改 .Aggregate 行以按字段对规则进行分组，然后为每个字段生成唯一的查询并不困难，但是我如何获取它将它们“或”在一起而不是“AND”？

然后对于每个查询，我如何将它们“AND”在一起？联盟？

ExtendQuery 看起来像这样

    public static IQueryable<T> ExtendQuery<T>(this IQueryable<T> query, QueryableRequestMessage.WhereClause.Rule rule) where T : class
    {
        var parameter = Expression.Parameter(typeof(T), "x");
        Expression property = Expression.Property(parameter, rule.Field);
        var type = property.Type;

        ConstantExpression constant;
        if (type.IsEnum)
        {
            var enumeration = Enum.Parse(type, rule.Data);
            var intValue = (int)enumeration;
            constant = Expression.Constant(intValue);
            type = typeof(int);
            //Add "Id" by convention, this is all because enum support is lacking at this point in Entity Framework
            property = Expression.Property(parameter, rule.Field + "Id");
        }
        else if(type == typeof(DateTime))
        {
            constant = Expression.Constant(DateTime.ParseExact(rule.Data, "dd/MM/yyyy", CultureInfo.CurrentCulture));
        }
        else if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
        {
            //This will convert rule.Data to the baseType, not a nullable type (because that won't work)
            var converter = TypeDescriptor.GetConverter(type);
            var value = converter.ConvertFrom(rule.Data);
            constant = Expression.Constant(value);

            //We change the type of property to get converted to it's base type
            //This is because Expression.GreaterThanOrEqual can't compare a decimal with a Nullable<decimal>
            var baseType = type.GetTypeOfNullable();
            property = Expression.Convert(property, baseType);
        }
        else
        {
            constant = Expression.Constant(Convert.ChangeType(rule.Data, type));
        }

        switch (rule.Op)
        {
            case "eq": //Equals
            case "ne": //NotEquals
                {
                    var condition = rule.Op.Equals("eq")
                                        ? Expression.Equal(property, constant)
                                        : Expression.NotEqual(property, constant);
                    var lambda = Expression.Lambda(condition, parameter);
                    var call = Expression.Call(typeof(Queryable), "Where", new[] { query.ElementType }, query.Expression, lambda);
                    query = query.Provider.CreateQuery<T>(call);
                    break;
                }
            case "lt": //Less Than
                    query = type == typeof (String) 
                        ? QueryExpressionString(query, Expression.LessThan, type, property, constant, parameter)
                        : QueryExpression(query, Expression.LessThan, property, constant, parameter); break;
            case "le": //Less Than or Equal To
                query = type == typeof (String)
                        ? QueryExpressionString(query, Expression.LessThanOrEqual, type, property, constant, parameter)
                        : QueryExpression(query, Expression.LessThanOrEqual, property, constant, parameter); break;
            case "gt": //Greater Than
                query = type == typeof (String)
                        ? QueryExpressionString(query, Expression.GreaterThan, type, property, constant, parameter)
                        : QueryExpression(query, Expression.GreaterThan, property, constant, parameter); break;
            case "ge": //Greater Than or Equal To 
                query = type == typeof (String)
                        ? QueryExpressionString(query, Expression.GreaterThanOrEqual, type, property, constant, parameter)
                        : QueryExpression(query, Expression.GreaterThanOrEqual, property, constant, parameter); break;
            case "bw": //Begins With
            case "bn": //Does Not Begin With
                query = QueryMethod(query, rule, type, "StartsWith", property, constant, "bw", parameter); break;
            case "ew": //Ends With
            case "en": //Does Not End With
                query = QueryMethod(query, rule, type, "EndsWith", property, constant, "cn", parameter); break;
            case "cn": //Contains
            case "nc": //Does Not Contain
                query = QueryMethod(query, rule, type, "Contains", property, constant, "cn", parameter); break;
            case "nu": //TODO: Null
            case "nn": //TODO: Not Null
                break;
        }

        return query;
    }

    private static IQueryable<T> QueryExpression<T>(
        IQueryable<T> query,
        Func<Expression, Expression, BinaryExpression> expression,
        Expression property,
        Expression value,
        ParameterExpression parameter
    ) where T : class
    {
        var condition = expression(property, value);
        var lambda = Expression.Lambda(condition, parameter);
        var call = Expression.Call(typeof(Queryable), "Where", new[] { query.ElementType }, query.Expression, lambda);
        query = query.Provider.CreateQuery<T>(call);
        return query;
    }

    private static IQueryable<T> QueryExpressionString<T>(
        IQueryable<T> query,
        Func<Expression, Expression, BinaryExpression> expression,
        Type type, 
        Expression property, 
        Expression value, 
        ParameterExpression parameter)
    {
        var containsmethod = type.GetMethod("CompareTo", new[] { type });
        var callContains = Expression.Call(property, containsmethod, value);
        var call = expression(callContains, Expression.Constant(0, typeof(int)));
        return query.Where(Expression.Lambda<Func<T, bool>>(call, parameter));
    }

    private static IQueryable<T> QueryMethod<T>(
        IQueryable<T> query,
        QueryableRequestMessage.WhereClause.Rule rule,
        Type type,
        string methodName,
        Expression property,
        Expression value,
        string op,
        ParameterExpression parameter
    ) where T : class
    {
        var containsmethod = type.GetMethod(methodName, new[] { type });
        var call = Expression.Call(property, containsmethod, value);
        var expression = rule.Op.Equals(op)
                             ? Expression.Lambda<Func<T, bool>>(call, parameter)
                             : Expression.Lambda<Func<T, bool>>(Expression.IsFalse(call), parameter);
        query = query.Where(expression);
        return query;
    }

Answer 1

所以实际上很容易。

现在，您的代码为每个规则生成一个 where ，您需要的是一个条件稍微复杂的 where ，因此需要对代码进行一些修改:

private static Expression GetComparisonExpression(this Rule rule, ParameterExpression parameter)
    {
        Expression property = Expression.Property(parameter, rule.Field);
        ConstantExpression constant = Expression.Constant(4);

        /* the code that generates constant and does some other stuff */

        switch (rule.Op)
        {
            case "eq": //Equals
            case "ne": //NotEquals
                {
                    var condition = rule.Op.Equals("eq")
                                        ? Expression.Equal(property, constant)
                                        : Expression.NotEqual(property, constant);
                    return condition;
                }

            default:
                throw new NotImplementedException();
        }
    }

这是原始代码所需内容的片段。此方法不会包装查询，而只是根据给定参数与规则中的内容生成比较表达式。

现在从生成查询的语句开始:

 var result = rules.Generate(_repository.All);

生成方法按属性名称 Field 对规则进行分组，并为每个组生成 and (这只是 && 运算符)条件表达式:

(group1Comparision) && (group2Comparison) && so on


public static IQueryable<T> Generate<T>(this  IEnumerable<Rule> rules, IQueryable<T> query) where T : class
{
    if (rules.Count() == 0)
        return query;

    var groups = rules.GroupBy(x => x.Field).ToArray();

    var parameter = Expression.Parameter(typeof(T));
    var comparison = groups.First().GetComparisonForGroup(parameter);

    foreach (var group in groups.Skip(1))
    {
        var otherComparions = group.GetComparisonForGroup(parameter);
        comparison = Expression.AndAlso(comparison, otherComparions);
    }

    var lambda = Expression.Lambda(comparison, parameter);
    var call = Expression.Call(typeof(Queryable), "Where", new[] { query.ElementType }, query.Expression, lambda);
    return query.Provider.CreateQuery<T>(call);

}

请注意，按属性名称分组会使规则的原始顺序变得无关紧要。

最后一件事是为组创建比较，因此 || 运算符:

public static Expression GetComparisonForGroup(this IEnumerable<Rule> group, ParameterExpression parameter)
{
        var comparison = group.Select((rule) => rule.GetComparisonExpression(parameter)).ToArray();

        return comparison.Skip(1).Aggregate(comparison.First(),
            (left, right) => Expression.OrElse(left, right));
}

因此，对于给定的规则列表，不需要外部库:

var rules = new Rule[]
            {
                new Rule{ Field = "A", Data = "4", Op="ne"},
                new Rule{ Field = "B", Data = "4", Op="eq"},
                new Rule{ Field = "A", Data = "4", Op="eq"},
                new Rule{ Field = "C", Data = "4", Op="ne"},
                new Rule{ Field = "A", Data = "4", Op="eq"},
                new Rule{ Field = "C", Data = "4", Op="eq"},
            };

我生成了这样的条件，该条件被引入到对查询的单个 Where 调用中:

($var1.A != 4 || $var1.A == 4 || $var1.A == 4) && $var1.B == 4 && ($var1.C != 4 || $var1.C == 4)