Question

I have a requirement where I need to filter out the null values from a list, only if the null value appears after the first non-null value and before the last non-null value.

Product one = new Product { Name="A" Priority="1" Value=null };
Product two = new Product { Name="A" Priority="2" Value=null };
Product three = new Product { Name="A" Priority="3" Value="10" };
Product four = new Product { Name="A" Priority="4" Value=null };
Product five = new Product { Name="A" Priority="5" Value="20" };
Product six = new Product { Name="A" Priority="6" Value=null };

In the example, I need to first sort the list of products based on their priority and then check the first non-null value (ie., priority 3) and last non-null value (ie, priority 5), then get the list of all products with null values within priority 3 & 5. So, in our example only Product 4 with Priority 4 is the record I am looking for.
I got to the part of actually grouping them by products and sorting them by priority but stuck on how to proceed after that

from p in Products
group p by p.Product into grp
select new
{
   Product = grp.Key
   Values = grp.OrderBy(x => x.Priority)
}

Can someone point me as how to proceed? I am thinking I may to use the indexes to identify all the non-null and iterate through to get min and max Priority values and later query for all records with blank null values within the min/max priority.

Answer 1

How about this (you can use .Select instead of .SelectMany to get separate groups for each product. .SelectMany combines all the valid result records into a single list):

Products
    .GroupBy(p => p.Name)
    .SelectMany (grp =>
        grp.OrderBy(p => p.Priority)           // sort by priority
            .SkipWhile(p => p.Value == null)   // skip null entries at beginning
            .Reverse()                         // reverse
            .SkipWhile(p => p.Value == null)   // skip null entries at end
            .Reverse()                         // reverse back to normal
            .Where(p => p.Value == null)       // then find null entries
    );

Demo: http://ideone.com/2dU9L