Dynamic Filtering in foreach (ASP.NET & EF)

Question

I have a really simple case with a controller and a repository.

Controller:

    [HttpGet]
    public async Task<IActionResult> GetProductList(ProductQuery queryparams)
    {
        var products = await uow.ProductRepo.GetProductsWithQuery(queryparams);

        var productsToReturn = mapper.Map<IEnumerable<ProductForListDto>>(products);

        return Ok(productsToReturn);
    }

Repository:

    public async Task<AbstractPagedList<Product>>GetProductsWithQuery(ProductQuery qp)
    {
        var products = DorianContext.Products
            .Include(p => p.Category)
            .Include(p => p.PriceOffers)
            .AsQueryable();

        // if (filter.CategoryId.HasValue)
        //     products = products.Where(p => p.CategoryId == filter.CategoryId);
        // if (filter.MinPrice.HasValue)
        //     products = products.Where(p => p.Price >= filter.MinPrice);
        // if (filter.MaxPrice.HasValue)
        //     products = products.Where(p => p.Price <= filter.MaxPrice);

        return await PagedList<Product>.CreateAsync(products, qp.PageNumber, qp.PageSize);
    }

Model:

    public class ProductQuery
    {
        public int? CategoryId { get; set; }
        public decimal? MinPrice { get; set; }
        public decimal? MaxPrice { get; set; }
    }

Instead of the boring commented part, how can we structure a dynamic/generic logic to make filtering for CategoryId, MinPrice and MaxPrice. (For example in a foreach block of property list of ProductQuery)

Maybe we can use a dictionary object and a foreach like the following, but I am not really sure how to get Property Names as strings from the object (I tried to use NewtonSoft.JObject but without success)

        var filterMap = new Dictionary<string, Expression<Func<Product, bool>>>()
        {
            ["categoryId"] = (v => v.CategoryId == filter.CategoryId),
            ["collectionId"] = (v => v.ProductCollectionId == filter.CollectionId),
            ["minPrice"] = (v => v.Price >= filter.MinPrice),
            ["maxPrice"] = (v => v.Price <= filter.MaxPrice)
        };

        foreach (var key in filterMap)
        {
                products = products.Where(key.Value);
        }

I don't want to use reflection. Ideas or comments with the best practices of such a case are also appreciated.

Answer 1

What I did works yes and I can continue just like this but this will result lots of duplicated logic. And because this is a toy project, I am searching for the ways to improve it. And such a project, this is overkill I agree..

So the best way to avoid to break the DRY principe is to create a Filters property into ProductQuery class like this:

public class ProductQuery
{
    public int? CategoryId { get; set; }
    public decimal? MinPrice { get; set; }
    public decimal? MaxPrice { get; set; }

    public IEnumerable<Expression<Func<Product, bool>>> Filters
    {
        get 
        {
            var filters = new List<Expression<Func<Product, bool>>>();

            if (this.CategoryId.HasValue)
                filters.Add(p => p.CategoryId == this.CategoryId);
            if (this.MinPrice.HasValue)
                filters.Add((p => p.Price >= this.MinPrice);
            if (this.MaxPrice.HasValue)
                filters.Add(p => p.Price <= this.MaxPrice);

            return filters;
        }
    }
}

So in your code you can use it like below:

public async Task<AbstractPagedList<Product>>GetProductsWithQuery(ProductQuery qp)
{
    var products = DorianContext.Products
        .Include(p => p.Category)
        .Include(p => p.PriceOffers)
        .AsQueryable();

    foreach(var filter in qp.Filters)
    {
        products = products.Where(filter);
    }

    return await PagedList<Product>.CreateAsync(products, qp.PageNumber, qp.PageSize);
}

Answer 2

Perhaps you could use a value tuple, of test-function and expression pairs:

ProductQuery filter = ... // initialize here

var exprs = new List<(Func<ProductQuery, object>, Expression<Func<Product, bool>>)>() {
    (f => f.CategoryId, p => p.CategoryId == filter.CategoryId),
    (f => f.MinPrice, p => p.Price >= filter.MinPrice),
    (f => f.MaxPrice, p => p.Price <= filter.MaxPrice)
};

foreach (var (test, expr) in exprs) {
    if (test(filter) != null) {
        products = products.Where(expr);
    }
}

---

You could go even further, by parsing the expression tree (eg p => p.CategoryId == filter.CategoryId ) and seeing which member(s) of filter are being used (eg filter.CategoryId ). Then, you could apply the condition only if that member has a value:

ProductQuery filter = ... // initialize here

var exprs = new List<Expression<Func<Product, bool>>>() {
    p => p.CategoryId == filter.CategoryId,
    p => p.Price >= filter.MinPrice,
    p => p.Price <= filter.MaxPrice
};

foreach (var expr in exprs) {
    var pi = ((expr.Body as BinaryExpression)
        .Right as MemberExpression)
        .Member as PropertyInfo;
    if (pi.GetValue(filter) != null) {
        products = products.Where(expr);
    }
}

This way, you can avoid defining the null-checking test.

The code which parses the expressions should probably be more flexible -- what if the filter property is first in the expression? what if there are conversions involved somewhere?

---

I would also suggest encapsulating the logic of building a single filter expression as a property of ProductQuery :

public Expression<Product, bool> Filter => {
    get {
        // implementation at end of answer
    }
}

which you could then call without any loops:

products = products.Where(filter.Filter);

You could implement this yourself, but I highly recommend using the LINQKit PredicateBuilder :

public Expression<Func<Product, bool>> Filter {
    get {
        var exprs = new List<Expression<Func<Product, bool>>>() {
            p => p.CategoryId == this.CategoryId,
            p => p.Price >= this.MinPrice,
            p => p.Price <= this.MaxPrice
        }.Where(expr => {
            PropertyInfo mi = ((expr.Body as BinaryExpression)
                .Right as MemberExpression)
                .Member as PropertyInfo;
            return mi.GetValue(this) != null;
        });

        var predicate = PredicateBuilder.True<Product>();
        foreach (var expr in exprs) {
            predicate = predicate.And(expr);
        }

        return predicate;
    }
}