了解表达式树和参数评估(Understanding Expression Tree and Para

我试图修改表达式树用于动态创建一个包含表达式最终导致SQL像

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

以代替检查执行范围检查，这最终导致SQL等

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

我在我的基础上解决这个职位 。

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);
}

我能够得到检查工作，如果我提供比较值的范围：

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()();

然而，我无法理清如何获取值从比较传入的表情时，我滴那个代码插入方法使用LINQ使用。 该方法的签名是：

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

它用于这样的：

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

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

题

如何评估

Expression<Func<TElement, long>> valueSelector

所以，我可以使用传入值BuildRangeExpression ，而不是我目前硬编码的值

long testValue = 5;

Answer 1:

我想从博客文章的代码有您需要的东西：你所要做的就是用valueSelector.Body ，而不是你的Expression.Constant()也是原始参数添加到生成的表达式：

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 。

创建一个参数：

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

取而代之的Expression.Constant(testvalue) ，你将需要把param 。

然后，你需要做的：

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

文章来源: Understanding Expression Tree and Parameter Evaluation