了解表达式树和参数评估 [英] Understanding Expression Tree and Parameter Evaluation
问题描述
我试图修改表达式树动态地构建一个包含表达式最终导致SQL像
P IN(123, 124,125,200,201)
,而不是检查执行范围检查,最终导致SQL像
(P> = 123和P< = 125)OR(P> = 200和P< = 201)
我基础上的这个帖子
静态公共表达式来; Func键< TElement,布尔>>
BuildContainsExpression< TElement,TValue>(
表达式来; Func键< TElement,TValue>> valueSelector,IEnumerable的< TValue>的值)
{
//删除了帖子:输入检查和边缘情况
无功等于=
values.Select(价值=>
(表情)Expression.Equal(valueSelector.Body,
Expression.Constant(价值的typeof(TValue))));
变种体= equals.Aggregate<表达>((积累,等于)=>
Expression.Or(积累,等于));
返回Expression.Lambda<&Func键LT; TElement,布尔>>(身体,P);
}
我能够得到检查工作,如果我提供的价值范围比较:
长testValue = 5;
名单,LT; KeyValuePair< INT,INT>>范围=新的List< KeyValuePair< INT,INT>>()
{
新KeyValuePair<长,长>(3,6),
新KeyValuePair<长,长>(10 ,12),
新KeyValuePair&下;长,长>(20,20),
};
名单,LT; BinaryExpression> rangeExpressions =新的List< BinaryExpression>();
的foreach(在范围VAR对)
{
变种greaterThanOrEqual =
Expression.GreaterThanOrEqual(Expression.Constant(testValue),
Expression.Constant( pair.Key));
变种lessThanOrEqual =
Expression.LessThanOrEqual(Expression.Constant(testValue),
Expression.Constant(pair.Value));
VAR INRANGE = Expression.AndAlso(greaterThanOrEqual,lessThanOrEqual);
rangeExpressions.Add(INRANGE);
}
变种最后=
rangeExpressions.Aggregate&所述;表达>((A,B)=> Expression.Or(A,B));
VAR的结果= Expression.Lambda<&Func键LT;布尔>>(最终).Compile()();
不过,我无法理清如何获取值从传入的表达时比较,我丢弃代码到与LINQ的使用的方法。该方法的签名是:
表达式来; Func键< TElement,布尔>>
BuildRangeExpression< TElement>(
表达式来; Func键< TElement,长>> valueSelector,
的IEnumerable<长>的值)
和它的使用方式:
表达式来; MyType的,布尔>比赛=
BuildRangeExpression< MyType的,长>(我=> my.ProductCode,productCodes);
VAR的结果= db.MyTypes.Where(比赛);
问
如何评估
表达式来; Func键< TElement,长>> valueSelector
这样我可以用传递到 BuildRangeExpression $ c中的价值$ C>,而不是我目前硬编码值
长testValue = 5;
我觉得从博客文章的代码有什么您需要:所有你所要做的就是用 valueSelector.Body
,而不是你的 Expression.Constant()
,也原来的参数添加到生成的表达式:
公共静态表达式来; Func键< TElement,布尔>>
BuildRangeExpression< TElement,TValue>(
表达式来; Func键< TElement,TValue>> valueSelector,
的IEnumerable<元组LT; TValue,TValue>>的值)
{
变种p值= valueSelector.Parameters.Single();
无功等于= values.Select(
元组=>
Expression.AndAlso(
Expression.GreaterThanOrEqual(
valueSelector.Body,Expression.Constant (tuple.Item1)),
Expression.LessThanOrEqual(
valueSelector.Body,Expression.Constant(tuple.Item2))));
变种体= equals.Aggregate(Expression.OrElse);
返回Expression.Lambda<&Func键LT; TElement,布尔>>(身体,P);
}
I'm attempting to modify an expression tree that dynamically builds a Contains expression that ultimately results in SQL like
P IN (123, 124, 125, 200, 201)
to instead check perform range checks, which ultimately results in SQL like
(P >= 123 AND P <= 125) OR (P >= 200 AND P <= 201)
I'm basing my solution on 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);
}
I'm able to get the range checking to work if I provide the value for comparison:
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()();
However, I cannot sort out how to get the value for comparison from the passed-in expression when I drop that code into the method to be used with Linq. The signature of that method is:
Expression<Func<TElement, bool>>
BuildRangeExpression<TElement>(
Expression<Func<TElement, long>> valueSelector,
IEnumerable<long> values)
and it is used like:
Expression<MyType, bool> match =
BuildRangeExpression<MyType, long>(my => my.ProductCode, productCodes);
var result = db.MyTypes.Where(match);
QUESTION
How can I evaluate
Expression<Func<TElement, long>> valueSelector
so that I can use the value passed into BuildRangeExpression
instead of my currently hard-coded value
long testValue = 5;
I think the code from the blog post has exactly what you need: all you have to do is to use valueSelector.Body
instead of your Expression.Constant()
and also add the original parameter to the generated expression:
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);
}
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