Using Linq to merge to List - linq

Question

I have the code like this.
public class Employee
{
public int ID { get; set; }
public string Name { get; set; }
public string NickName { get; set; }
}
var table1 = new List<Employee>() {
new Employee(){ ID=1, Name="A"},
new Employee(){ ID=2, Name="B"},
new Employee(){ ID=3, Name="C"},
new Employee(){ ID=5, Name="E"},
};
var table2 = new List<Employee>() {
new Employee(){ ID=1, NickName="NickA"},
new Employee(){ ID=2, NickName="NickB"},
new Employee(){ ID=3, NickName="NickC"},
new Employee(){ ID=4, NickName="NickD"}
};
I want to merge two lists above to something like this:
{ID=1, Name="A", NickName="NickA"}
{ID=2, Name="B", NickName="NickB"}
{ID=3, Name="C", NickName="NickC"}
{ID=4, Name=null, NickName="NickD"}
{ID=5, Name="E", NickName=null}
How can I do it using LinQ. Please help me out!

Answer 1

You can group by ID, and pick the "first of" value for each field, like this:
var res = table1.Concat(table2).GroupBy(e => e.ID)
.Select(g => new Employee {
ID = g.Key
, Name = g.Select(e => e.Name).FirstOrDefault(s => s != null)
, NickName = g.Select(e => NickName).FirstOrDefault(s => s != null)
});
Note that if you take this approach it does not matter from what table the Name or the NickName comes: you could mix assignments in your lists, and let LINQ group them back for you.

Answer 2

Something like:
var results = table1.Select(t1 => {
var t2 = table2.First(x => x.ID == t1.ID);
return new Employee {
ID = t1.ID,
Name = t1.Name,
NickName = t2.NickName
};
}).ToList();
If table2 is sufficiently large (tens of thousands of entries or more) consider (ie. performance measure) building a Dictionary or sort the array to provide quicker lookup by ID (in the latter case using the BinarySearch method with the custom IComparer just comparing ids). The cost of setting up this intermediate structure is non-trivial, and modern computers can search a few thousand simple rows very quickly.