Error in LINQ Left JOIN

Question

i have written below query in LINQ to perform left join but its throwing error:

var qry = from c in dc.category_feature_Name_trans_SelectAll_Active()
          join p in dc.product_category_feature_trans_SelectAll()
          on c.cft_id equals p.cft_id into cp
          from p in cp.DefaultIfEmpty()                      
          select new
          {
              c.cft_id,
              c.feature_id,
              c.feature_name,
              p.product_id ,
              p.value 
          };

Error:

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the 
current web request. Please review the stack trace for more information about
the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 57:                       on c.cft_id equals p.cft_id into cp
Line 58:                       from p in cp.DefaultIfEmpty()                      
error Line 59:                       select new
Line 60:                       {
Line 61:                           c.cft_id,

Please help me.

Answer 1

cp.DefaultIfEmpty() returns a sequence which will have a single null value in if cp was empty.

That means you have to account for the fact that the p in

from p in cp.DefaultIfEmpty()

may be null. Now, you haven't really said what you want to happen in that case. You might want something like this:

var qry = from c in dc.category_feature_Name_trans_SelectAll_Active()
          join p in dc.product_category_feature_trans_SelectAll()
          on c.cft_id equals p.cft_id into cp
          from p in cp.DefaultIfEmpty()                      
          select new
          {
              c.cft_id,
              c.feature_id,
              c.feature_name,
              product_id = p == null ? null : p.product_id,
              value = p == null ? null : p.value 
          };

... or you may want some different handling. We don't know the types of p.product_id or p.value, which doesn't help. (For example, you'll need a bit more work with the above code if product_id is a value type.)