LINQ & Entity Framework : issue with concatenating NULL value - linq

Question

I have an INT fields called TradeNo that may contain NULL value. The requirement is to display on the dropdown the "contract Id" and the "trade no" in bracket and if "trade no" is null, then display N/A.
Example:
44444 (2222)
55555 ( N/A )
Here’s what I thought would work. This is my functions that returns a SelectList
public IEnumerable<SelectListItem> GetContractsBySupplierDropDown(string supplier)
{
var result = from c in context.Contracts
where c.Supplier==supplier
orderby c.ContractID
select new {
Text = c.ContractID.ToString() + " (" +
(c.TradeNo.HasValue ?
c.TradeNo.Value.ToString() :
" N/A ").ToString() +
" )",
Value = c.ContractID };
return new SelectList(result, "Text", "Value");
}
The error being return is:
LINQ to Entities does not recognize the method 'System.String ToString()'
method, and this method cannot be translated into a store expression.
From what I can tell, the error displayed means that EF is trying to convert ToString to a database function?

Answer 1

Unfortunately you cannot use ToString in a number field coming from the database.
As a work around you can cast ToList before doing it, so then the contents are in the memory.
public IEnumerable<SelectListItem> GetContractsBySupplierDropDown(string supplier)
{
var query = from c in context.Contracts
where c.Supplier==supplier
orderby c.ContractID
select new {
c.ContractID,
c.TradeNo,
};
var result = from c in query.ToList()
select new {
Text = c.ContractID.ToString() + " (" +
(c.TradeNo.HasValue ?
c.TradeNo.Value.ToString() :
" N/A ") +
" )",
Value = c.ContractID
};
return new SelectList(result, "Text", "Value");
}