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Is there any trick to avoid double checks when constructing Where criteria conditionally?

July 11, 2022

I am wondering whether I can simplify my code below, especially the following 2 statements:

// Statement A
Expression<Func<Product, bool>>? criteria = p =>
    (id == null ? true : p.Id == id) &&
    (name == null ? true : p.Name.ToLower().Contains(name.ToLower())) &&
    (maxPrice == null ? true : p.Price <= maxPrice);


// Statement B
if (id is null && name is null && maxPrice is null)
    criteria = null;

I am not happy with the statement B because of two reasons:

I have to check again each parameter that has been checked in the statement A.
It is prone to error (forgetting to check all parameters)

Attempt

The following does not compile because

An expression tree cannot contain an assignment operator.

bool sign = false;
Criteria = p =>
    (id == null ? (sign = true) : p.Id == id) &&
    (name == null ? (sign = true) : p.Name.ToLower().Contains(name.ToLower()))
;

if (sign == false)
    Criteria = null;

Question

Is there any way to make it much simpler and less prone to errors?
Any suggestions are always welcome.

static IQueryable<Product> Filter(this IQueryable<Product> products,
    int? id = null,
    string? name = null,
    decimal? maxPrice = null)
{
    Expression<Func<Product, bool>>? criteria = p =>
        (id == null ? true : p.Id == id) &&
        (name == null ? true : p.Name.ToLower().Contains(name.ToLower())) &&
        (maxPrice == null ? true : p.Price <= maxPrice);


    if (id is null && name is null && maxPrice is null)
        criteria = null;

    if (criteria is null)
        return products;
    else
        return products.Where(criteria);
}

>Solution :

Just follow the pretty standard pattern of dynamically adding Where based on presence of filtering conditions:

static IQueryable<Product> Filter(this IQueryable<Product> products,
    int? id = null,
    string? name = null,
    decimal? maxPrice = null)
{
    if (id is not null)
    {
        products = products.Where(p => p.Id == id);
    }

    if (name is not null)
    {
        products = products.Where(p => p.Name.ToLower().Contains(name.ToLower()));
    }

    if (maxPrice is not null)
    {
        products = products.Where(p => p.Price <= maxPrice);
    }

    return products;
}