LINQ和可选参数

Question

I'm designing a web service to be consumed by an MVC app (pretty simple stuff), but I need a single method in the web service that takes up to four optional parameters (ie catId, brandId, lowestPrice and highestPrice).

How do I write the Linq query so that it essentially executes

databaseObject.Products.Where(p=> (p.Category == ANY if catId==null, else catId))

I hope that makes sense.

Answer 1

The parameters of the method could accept null values and the Where restriction could be evaluated for each not null parameter:

IQueryable<Product> q = databaseObject.Products;

if (catId != null)
{
    q = q.Where(p => p.Category == catId);
}
if (brandId != null)
{
    q = q.Where(p => p.Brand == brandId);
}
// etc. the other parameters

var result = q.ToList();

Answer 2

If this is Linq To SQL:

databaseObject.Products.Where(p=> (catId == null || p.Category == catId) );

Linq To SQL, efficiently writes the SQL sent to backend without a where clause if CatId is null. You can have multiple of these constructs, and only those with a nonNull value is included in the where construct.

Answer 3

databaseObject.Products.Where(p=> ((catId==null) ||  (p.Category == catId)))

对于其他3个可选参数，您可以将它们输入，在一个linq语句中进行整个搜索。

Answer 4

Something along the lines of the following should do the trick:

IEnumerable<Product> GetProducts(int? categoryId)
{
    var result = databaseObject.Products.Where(product => ProductCategoryPredicate(product, categoryId)
}

/// <summary>
/// Returns true if the passed in categoryId is NULL
/// OR if it's not null, if the passed in categoryId matches that
/// of the passed in product
///
/// This method will get called once for each product returned from 
/// databaseObject.Products</summary>
bool ProductCategoryPredicate(Product product, int? categoryId)
{
    if (categoryId.HasValue)
    {
        return product.Category == categoryId.Value;
    }
    else
    {
        return true;
    }
}

This could/can be simplified into a single line LINQ statement (see below) but I've written it long-hand above for clarity:

IEnumerable<Product> GetProducts(int? categoryId)
{
    var result = databaseObject.Products.Where(product => !categoryId.HasValue || product.Category == categoryId.Value);
}