title: Linq check if value exists otherwise set default value
body: <p>Hey I have the following query i.e.</p>
  
  <pre><code>    public int GetDefaultCurrencyForOrganization(string organizationId)
      {
          return (from a in _dataContext.allocations
                  where a.organizationId.Equals(organizationId)
                  select a.currencyId).SingleOrDefault();
      }
  </code></pre>
  
  <p>Which may return nothing for a lot of companies, and this was I want to set a value of 4 to return. Just wondering how I could add that to the above ?</p>