Updating Collection from other collection for matching items - linq

Question

I have two collections as mentioned below. I have update two properties of "trade" from the other collection "refData" if those values exists in "refData".
Model:
class Trade
{
public int Id { get; set; }
public string PayIndex { get; set; }
public string RecIndex { get; set; }
public string PayCurrency { get; set; }
public string RecCurrency { get; set; }
}
class RefData
{
public string IndexLabel { get; set; }
public string Symbol { get; set; }
}
Sample Date:
var refData = new List<RefData>
{
new RefData { IndexLabel = "A1", Symbol="ABC1"},
new RefData { IndexLabel = "A2", Symbol="ABC2"},
new RefData { IndexLabel = "B1", Symbol="BCD1"},
new RefData { IndexLabel = "B2", Symbol="BCD2"},
};
var trades = new List<Trade>
{
new Trade { Id = 1, PayIndex = "A1", RecIndex = "B1"},
new Trade { Id = 2, PayIndex = "A2", RecIndex = ""},
new Trade { Id = 3, PayIndex = "", RecIndex = "B2"},
new Trade { Id = 4, PayIndex = "A3", RecIndex = "B3"}
};
I want to update PayCurrency and RecCurrency of "trades" with Symbol property of "refData" if trade's PayIndex and RecCurrency exists in "refData".
Output:
var Output = new List<Trade>
{
new Trade { Id = 1, PayIndex = "A1", RecIndex = "B1", PayCurrency = "ABC1", RecCurrency="BCD1"},
new Trade { Id = 2, PayIndex = "A2", RecIndex = "", PayCurrency = "ABC2", RecCurrency=""},
new Trade { Id = 3, PayIndex = "", RecIndex = "B2", PayCurrency = "", RecCurrency="BCD2"},
new Trade { Id = 4, PayIndex = "A3", RecIndex = "B3", PayCurrency = "", RecCurrency=""}
};

Answer 1

For c#6 and above you can do like below
var result = trades.Select(t => new Trade() {
Id= t.Id,
PayIndex = t.PayIndex,
RecIndex = t.RecIndex,
PayCurrency = refData.SingleOrDefault(r => r.IndexLabel.ToLower().Equals(t.PayIndex.ToLower()))?.Symbol ?? "",
RecCurrency = refData.SingleOrDefault(r => r.IndexLabel.ToLower().Equals(t.RecIndex.ToLower()))?.Symbol ?? ""
}).ToList();
For Older versions
var result = trades.Select(t => new Trade() {
Id= t.Id,
PayIndex = t.PayIndex,
RecIndex = t.RecIndex,
PayCurrency = refData.SingleOrDefault(r => r.IndexLabel.ToLower().Equals(t.PayIndex.ToLower())) != null ? refData.SingleOrDefault(r => r.IndexLabel.ToLower().Equals(t.PayIndex.ToLower())).Symbol : "",
RecCurrency = refData.SingleOrDefault(r => r.IndexLabel.ToLower().Equals(t.RecIndex.ToLower())) != null ? refData.SingleOrDefault(r => r.IndexLabel.ToLower().Equals(t.RecIndex.ToLower())).Symbol : ""
}).ToList();
Here is working example
Update using #SAJ answer
var output = (from r in trades
join p in refData on r.PayIndex equals p.IndexLabel
into g1
from s in g1.DefaultIfEmpty()
join t in refData on r.RecIndex equals t.IndexLabel into g2
from a in g2.DefaultIfEmpty()
select Trade { Id=r.Id,PayIndex=r.PayIndex,RecIndex=r.RecIndex, RecCurrency = a != null ? a.Symbol : "", PayCurrency = s != null ? s.Symbol : ""}).ToList();

Answer 2

You can try this
var output = (from r in trades
join p in refData on r.PayIndex equals p.IndexLabel
into g1
from s in g1.DefaultIfEmpty()
join t in refData on r.RecIndex equals t.IndexLabel into g2
from a in g2.DefaultIfEmpty()
select new { r, RecSymbol = a?.Symbol, PaySymbol = s?.Symbol }).ToList();
output.ForEach(o =>
{
o.r.PayCurrency = o.PaySymbol;
o.r.RecCurrency = o.RecSymbol;
});