LINQ GroupBy while converting from string to decimal and then back to string - linq

Question

Is it possible to convert a string value to a decimal value within a LINQ expression that performs an aggregate function like SUM or AVERAGE?
Assume the example below where I have a collection of Bank Accounts where my goal is to obtain an average of each customers bank account if they have a balance. The data comes from an XML API where all the data is read in a strings.
public class BankAccount
{
string Id{ get; set; }
string CustomerId { get; set; }
string Balance { get; set; }
}
Sample data ...
{ Id = "1", CustomerId = "Bob", Balance = "1" }
{ Id = "2", CustomerId = "Bob", Balance = "2" }
{ Id = "3", CustomerId = "Sam", Balance = "4" }
{ Id = "4", CustomerId = "Sam", Balance = "" }
{ Id = "5", CustomerId = "Alice", Balance = "" }
LINQ grouping expression. Is there a way to convert the value of Balance to a decimal so an average can be taken within the LINQ statement? I tried x => Decimal.Parse(x.Balance) but got an Input string was not in a correct format error. I only need to convert the Balance property to decimal for the Average calculation as the results would be rendered as a string in the XML.
At the same time, if an account does not have a balance listed (i.e. it's blank like Sams's first account and Alice's only account above) then I don't want the Average to take that entry included in the average, though I still want the account grouped in for display.
var groupedResults = allAccounts
.GroupBy(x => new {x.CustomerId, x.Balance})
.Select(g => new BankAccount {
CustomerId = g.Customer.Key.CustomerId,
Balance = g.Average(x => x.Balance)
}).ToList();
These are the results I am looking for:
{ CustomerId = "Bob", Balance = "1.5" }
{ CustomerId = "Sam", Balance = "4" }
{ CustomerId = "Alice", Balance = "" }

Answer 1

I think to achieve the result you are looking for you should try this:
var groupedResults = allAccounts
.GroupBy(x =>x.CustomerId)
.Select(g => new BankAccount {
CustomerId = g.Key,
Balance = g.Where(x =>!string.IsNullOrEmpty(x.Balance))
.Select(x =>(decimal?)decimal.Parse(x.Balance))
.DefaultIfEmpty(null)
.Average().ToString()
}).ToList();
First just group by CustomerId, is not necessary to include the Balance there. Then, to get the average and avoid the error parsing include the condition to make sure the Balance is not empty.
Another way to do it using query syntax:
from e in allAccounts
group e by e.CustomerId into g
let temp=g.Where(x =>!string.IsNullOrEmpty(x.Balance))
select new BankAccount(){CustomerId = g.Key,
Balance =temp.Any()?
temp.Average(x =>Decimal.Parse(x.Balance)).ToString():""
};

Answer 2

decimal d;
var groupedResults = allAccounts.GroupBy(a => a.CustomerId)
.Select(g => new BankAccount { CustomerId = g.Key, Balance = g.Average(b =>
decimal.TryParse(b.Balance, out d) ? (decimal?)d : null).ToString() }).ToList();
The .TryParse part results in (decimal?)null for strings that can't be parsed, which are then ignored by .Average. Also, the last average for Alice results in (decimal?)null and then in "".