DefaultIfEmpty for LINQ to a DataTable?

Question

I have a LINQ query written to pull at least one row from a datatable as follows:

var generalQuery =
      from contact in contacts.AsEnumerable().DefaultIfEmpty()
      where contact.Field<String>("CONTACT_TYPE").ToUpper() == "AGENT"
      select new
          {
          AgentName = contact.Field<String>("FIRST_NAME") + " " +
              contact.Field<String>("LAST_NAME"),
          AgentPhoneNumber = contact.Field<String>("PHONE"),
          AgentEmailAddress = contact.Field<String>("EMAIL")
          };

I then try to set the properties of a new instance of a class with the values in the LINQ query output:

var genInfo = new GeneralInformationType
{
    AgentName = generalQuery.FirstOrDefault().AgentName,
    AgentPhoneNumber = generalQuery.FirstOrDefault().AgentPhoneNumber,
    AgentEmailAddress = generalQuery.FirstOrDefault().AgentEmailAddress
};

The problem I am running into is that occasionally there are no results in the query. This tries to set the value of the various strings in GeneralInformationType to null and causes exceptions. I tried various methods of DefaultIfEmpty, but can't figure out where to put it.

Any help would be appreciated.

Thanks, Rob

score 1 · Answer 1 · answered Aug 02 '11 at 16:23

There are two separate problems here. First, DefaultIfEmpty is going to effectively give you a sequence with a null DataRow if the query doesn't return anything. That will then fail not when it tries to assign strings - but when in the Where clause. You're then also using FirstOrDefault which would normally return null if there are no matching entries.

I would personally remove DefaultIfEmpty call, and put all the defaulting in the code for genInfo:

var generalQuery =
       from contact in contacts.AsEnumerable()
       where contact.Field<String>("CONTACT_TYPE").ToUpper() == "AGENT"
       select new
       {
           AgentName = contact.Field<String>("FIRST_NAME") + " " +
               contact.Field<String>("LAST_NAME"),
           AgentPhoneNumber = contact.Field<String>("PHONE"),
           AgentEmailAddress = contact.Field<String>("EMAIL")
       };

// Like using DefaultIfEmpty(...).First()
var result = generalQuery.FirstOrDefault() ?? 
        new { AgentName = "Default",
              AgentPhoneNumber = "Default",
              AgentEmailAddress = "Default" };

var genInfo = new GeneralInformationType
{
    AgentName = result.AgentName,
    AgentPhoneNumber = result.AgentPhoneNumber,
    AgentEmailAddress = result.AgentEmailAddress
};

Obviously change "Default" to whatever you want. If you have more specific requirements, please explain what you're trying to do and I'm sure we'll be able to accommodate them...

score 1 · Accepted Answer · answered Aug 02 '11 at 16:24

I would recommend removing DefaultIfEmpty() in your generalQuery and adding some logic when generalQuery is empty like so:

var generalQuery =
      from contact in contacts.AsEnumerable()
      where contact.Field<String>("CONTACT_TYPE").ToUpper() == "AGENT"
      select new
          {
          AgentName = contact.Field<String>("FIRST_NAME") + " " +
              contact.Field<String>("LAST_NAME"),
          AgentPhoneNumber = contact.Field<String>("PHONE"),
          AgentEmailAddress = contact.Field<String>("EMAIL")
          };

var genInfo = new GeneralInformationType
{
    AgentName = generalQuery.Any() ? generalQuery.First().AgentName : "Default Name",
   ....
};

Both answers seem correct. I chose this one over Jon's since it was the method I used and didn't require an interim "result" object. Thanks for all your help! — SpaceCowboy74, Aug 02 '11 at 16:32

DefaultIfEmpty for LINQ to a DataTable?

2 Answers2