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

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);
}

CodePudding user response：

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;
}

CodePudding user response：

You can check for null on the received parameters, if any are null make that bit of criteria true by checking for null, e.g. (id == null || p.Id== id) basically says give me all Id's that match the parameter id or if it's null then give me all parameter Ids.

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 || p.Id== id) &&
        (name == null || p.Name.ToLower().Contains(name.ToLower())) &&
        (maxPrice == null || p.Price <= maxPrice);

        return products.Where(criteria);
}

This also has the added benefit of making the parameters optional.