Understanding Expression Tree and Parameter Evaluation

c# expression-trees

Question

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;

Accepted Answer

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);
}

Popular Answer

Use Expression.Parameter.

Create a parameter:

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

Instead of Expression.Constant(testvalue), you will need to put param.

Then, you need to do:

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


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Licensed under: CC-BY-SA with attribution
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Is this KB legal? Yes, learn why