Combining expressions to use with the Entity Framework

You often have to create a screen where the information is filtered on a given condition and the user can set some more options through checkboxes. For instance, display all products of a certain category and those results should be able to be filtered on their active status.

We could send one query to the database to retrieve the products and then filter them in memory, but let’s try to use the database for all of this since they’re good at this. You’ll bump into several issues when trying to solve this and like most problems in software development someone else most likely encountered the same issue and has already found a solution. In this case take a look at this, albeit old, article which illustrates the problem and gives a solution. It will enable you to write code like this:

public class ProductFinder
{
  public  ICollection<Product> FindProductsBySubCategory(int subCategoryId, bool makeFlagMarked)
  {
    using(var context = new AdventureWorksEntities())
    {
      Expression<Func<Product, bool>> whereClause = x => x.ProductSubcategory.ProductSubcategoryID == subCategoryId; 
      if(makeFlagMarked)
      {
        whereClause = whereClause.And(x => x.MakeFlag);
      }
      return context.Products.Where(whereClause).ToList();
    }
  }
}