Question

Following on from a previous question i asked, I’m now trying to figure out how to build dynamic expressions for both AND & OR queries.

Given the following string array:

string[] ranges = new string[]{"0-100", "100-200", "500-1000"};

I would like to dynamically express this in a linq expression – Something along the lines of:

var v = from p in products
        where 
        (p.Amount >= 0 && p.Amount <= 100) ||
        (p.Amount >= 101 && p.Amount <= 200) ||
        (p.Amount >= 500 && p.Amount <= 1000)
        select p;

How do i dynamically build the linq expression in this loop?

string[] ranges = new string[]{"0-100", "100-200", "500-1000"};

var query = products.AsQueryable();

foreach (var item in ranges)
{
    int min = int.Parse(item.Split('-').First());
    int max = int.Parse(item.Split('-').Last());                
    //Linq expression?
}

Answer 1

Use predicate builder:

string[] ranges = new string[]{"0-100", "100-200", "500-1000"};
var predicate = PredicateBuilder.False<Product>();

foreach (var item in ranges)
{
    int min = int.Parse(item.Split('-').First());
    int max = int.Parse(item.Split('-').Last());                
    predicate = predicate.Or(p => p.Amount >= min && p.Amount <= max);
}

Notice how we start with the boolean state of false, and or together predicates in the loop. Conversely, you can start with a state of true and and together the predicates.

Finally, not sure if this is possible with query comprehension syntax, but your ultimate query can then look like:

var v = products.Where(predicate);