Question

我正在尝试修改动态构建Contains表达式的表达式树，该表达式最终会产生像

这样的SQL

P IN (123, 124, 125, 200, 201)

而是检查执行范围检查，最终导致SQL如

(P >= 123 AND P <= 125) OR (P >= 200 AND P <= 201)

我的解决方案基于this post。

static public Expression<Func<TElement, bool>> 
BuildContainsExpression<TElement, TValue>(
    Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values)
{
    // Removed for post: Input checks and edge cases

    var equals = 
      values.Select(value => 
       (Expression)Expression.Equal(valueSelector.Body, 
                                    Expression.Constant(value, typeof(TValue))));

    var body = equals.Aggregate<Expression>((accumulate, equal) => 
                                             Expression.Or(accumulate, equal));

    return Expression.Lambda<Func<TElement, bool>>(body, p);
}

如果我提供比较值，我可以让范围检查工作：

long testValue = 5;
List<KeyValuePair<int, int>> ranges = new List<KeyValuePair<int, int>>() 
{
    new KeyValuePair<long, long>(3, 6),
    new KeyValuePair<long, long>(10, 12),
    new KeyValuePair<long, long>(20, 20),
};

List<BinaryExpression> rangeExpressions = new List<BinaryExpression>();

foreach (var pair in ranges)
{
    var greaterThanOrEqual = 
        Expression.GreaterThanOrEqual(Expression.Constant(testValue), 
                                      Expression.Constant(pair.Key));

    var lessThanOrEqual = 
        Expression.LessThanOrEqual(Expression.Constant(testValue), 
                                   Expression.Constant(pair.Value));

    var inRange = Expression.AndAlso(greaterThanOrEqual, lessThanOrEqual);
    rangeExpressions.Add(inRange);
}

var final = 
    rangeExpressions.Aggregate<Expression>((a, b) => Expression.Or(a, b));

var result = Expression.Lambda<Func<bool>>(final).Compile()();

但是，当我将该代码放入要与Linq一起使用的方法时，我无法理解如何从传入的表达式中获取用于比较的值。该方法的签名是：

Expression<Func<TElement, bool>> 
BuildRangeExpression<TElement>(
    Expression<Func<TElement, long>> valueSelector, 
    IEnumerable<long> values)

它的使用方式如下：

Expression<MyType, bool> match = 
    BuildRangeExpression<MyType, long>(my => my.ProductCode, productCodes);

var result = db.MyTypes.Where(match);

问题

我如何评估

Expression<Func<TElement, long>> valueSelector

这样我就可以使用传递给BuildRangeExpression的值而不是我当前的硬编码值

long testValue = 5;

Answer 1

我认为博客文章中的代码正是您所需要的：您所要做的就是使用valueSelector.Body而不是Expression.Constant()，并将原始参数添加到生成的表达式中： / p>

public static Expression<Func<TElement, bool>>
    BuildRangeExpression<TElement, TValue>(
    Expression<Func<TElement, TValue>> valueSelector,
    IEnumerable<Tuple<TValue, TValue>> values)
{
    var p = valueSelector.Parameters.Single();

    var equals = values.Select(
        tuple =>
        Expression.AndAlso(
            Expression.GreaterThanOrEqual(
                valueSelector.Body, Expression.Constant(tuple.Item1)),
            Expression.LessThanOrEqual(
                valueSelector.Body, Expression.Constant(tuple.Item2))));

    var body = equals.Aggregate(Expression.OrElse);

    return Expression.Lambda<Func<TElement, bool>>(body, p);
}

Answer 2

使用Expression.Parameter。

创建参数：

var param = Expression.Parameter(typeof(TElement), "arg")

而不是Expression.Constant(testvalue)，您需要放置param。

然后，您需要这样做：

var result = Expression.Lambda<Func<TElement, bool>>(final, param).Compile()

理解表达式树和参数评估

2 个答案: