Linq中的StrongTypingException

Question

Anybody can tell why the ToList() throws an exception?

var duplicates =  
    from typeMappings in _liveTable.Where(r =>
        (r.ProviderId == providerId) && (r.ExchangeId == exchangeId))
    join dataDictionary in _liveDataSet.DataDictionary.Where(r => 
        (r.DataDictionaryTypeId == dataDictionaryTypeId)) 
    on typeMappings.DataDictionaryId equals dataDictionary.DataDictionaryId
    select typeMappings.ConfigId;

if (duplicates.ToList().Count > 0) 
{ ... }

The Exception message is:
'duplicates.ToList()' threw an exception of type 'System.Data.StrongTypingException' System.Collections.Generic.List {System.Data.StrongTypingException}

Thanks

Answer 1

From MSDN:

StrongTypingException

The exception that is thrown by a strongly typed DataSet when the user accesses a DBNull value.

So the problem occurs because one of the properties you access in the query is null. Check which properties of your DataTable are allowed to be null, and check it with a call to IsNull before you try to get the value.

Answer 2

Try to use that to eleminate Null value

var duplicates =  
from typeMappings in _liveTable.Where(r =>
    (r.ProviderId == providerId) && (r.ExchangeId == exchangeId))
join dataDictionary in _liveDataSet.DataDictionary.Where(r => 
    (r.DataDictionaryTypeId == dataDictionaryTypeId)) 
on typeMappings.DataDictionaryId equals dataDictionary.DataDictionaryId
select new
       { ConfigId = typeMappings.ConfigId = null ? "anyValueyouwhant" : typeMappings.ConfigId};

juste to test without null value