Linq query to Join tables if value not in first table fetch from second

I'm editing my answer as per your comment. Please find the code below.

public static void KvYQ()
            {
                List<Team> modifiedTeams = new List<Team>() {
                    new Team() {
                        Id="T1"
                        ,Driver=new Driver() {
                            DriverId="D2"
                        }
                        ,Codriver=new Driver() {
                            DriverId="C1"
                        }
                    }
                    ,new Team() {
                        Id="T2"
                        ,Driver=new Driver() {
                            DriverId="D1"
                        }
                    }
                };

                List<Team> allTeams = new List<Team>() {
                    new Team() {
                        Id="T1"
                        ,Driver=new Driver() {
                            DriverId="D1"
                        }
                        ,Codriver=new Driver() {
                            DriverId="C1"
                        }
                    }
                    ,new Team() {
                        Id="T2"
                        ,Driver=new Driver() {
                            DriverId="D2"
                        }
                        ,Codriver=new Driver() {
                            DriverId="C2"
                        }
                    }
                };    

                var driverdetails = modifiedTeams.Select(x => new Team()
                {
                    Driver = x.Driver,
                    Codriver = x.Codriver

                });

                foreach (var item in driverdetails)
                {
                    Driver d = (item.Driver == null) ? allTeams.Select(x => x.Driver).FirstOrDefault() : item.Driver;
                    Driver c = (item.Codriver == null) ? allTeams.Select(x => x.Codriver).FirstOrDefault() : item.Codriver;
                    Console.WriteLine(d.DriverId + " " + c.DriverId);
                    //{D2,C1}
                    //{D1,C1}
                }


            }

You can Write the logic in one query itself. But to make it clear, i've written it in foreach.

Hope this helps

Ok, apparently you have a method to check whether a Driver is "not there" and whether a CoDriver is "not there". Although you don't specify when a Driver is not there, from your exemple I gather that this is if the property equals null.

But let's assume a Team has two functions:

bool DriverIsThere();
bool CoDriverIsThere();

Now you have a sequence of TeamIds, and you want to every TeamId to produce the DriverIds of two Drivers

If there is a modifiedTeam with TeamId: 1 If IsDriverThere(), produce Driver.DriverId, else find the Team from the collection of AllTeams with the same Id and return Driver.DriverId 2 If IsCodDriverThere(), produce CoDriver.DriverId, else find the Teadm from the collection of AllTeams with the same 'Id' and produce 'CoDriver.DriverId`.

You didn't mention what to do if there is no modifiedTeam with the requested Id, or if there is no AllTeams with this Id. Let's assume you want to throw exception.

This code for this requirement could be written in one LINQ statement. And I'm sure others will try to answer it this way. As an alternative, I'd like to promote the Yield method, because I think in this case it would be much more readable and maintainable.

I assume your IDs are unique. To enhance lookup, I'll put your modifiedTeams and allTeams into dictionaries, to enhance lookup.

I don't want to change your classes, so let's use extension methods.

See Extension Methods Demystified

Let's first define IsThere:

// returns true if Driver "is there"
public static bool IsThere(this Driver driver)
{
     return driver != null;
    // might be for some other reason, for instance: && !driver.IsSick
}

The function that takes a Team, and produces the two DriverIdsaccording to the requirements defined above is fairly straightforward:

public static IEnumerable<string> ToDriverIds(this Team team,
    Dictionary<string, Team> allTeams)
{
    if (team.Driver.IsThere())
        // Driver is there, yield return the Id
        yield return team.Driver.DriverId;
    else
    {   // Driver not available, fetch the Driver from allTeams
        // throw exception if there is no alternative team with this team ID
        Team alternativeTeam = allTeams[team.Id];
        if (alternativeTeam.IsDriverThere())
            yield return alternativeTeam.Driver.Id;
        else
           // Driver also not in alternative team: exception
           throw new InvalidOperationException("...");

        // do the same for coDriver
    }
}

An even nicer method: use a delegate to select whether you want the Driver or the Codriver:

public static Driver ToDriver(this Team team,
    Dictionary<string, Team> allTeams,
    Func<Team, Driver> driverSelector)
{
    Driver driver = driverSelector(team);
    if (driver.IsThere())
        return driver.Id;
    else
    {   // use allTeams:
        Team allternativeTeam = allTeams[team.Id];
        driver = friverSelector(alternativeTeam);
        if (driver.IsThere())
            return driver.Id;
        else
            throw new InvalidOperationException(...);
    }
}

Usage:

public static IEnumerable<string> ToDriverIds(this Team,
   Dictionary<string, Team> allTeams)
{
    // return the driverId of the driver:
    yield return Team.ToDriverId(allTeams, team => team.Driver);
    // yield return the driverId of the CoDriver:
    yield return Team.ToDriverId(allTeams, team => team.CoDriver);
}

Now from teamId to the requested driverIds:

public static IEnumerable<string> ToDriverIds(this string teamId,
    Dictionary<string, Team> modifiedTeams
    Dictionary<string, Team> allTeams)
{
    return modifiedTeams[teamId].ToDriverids(allTeams);
    // exception if there is no team with teamId
}

After all this, the final function that takes a sequence of teamIds, the modifiedTeams, and allTeams and converts into the requested output is a short one:

public static IEnumerable<string> ToIds(this IEnumerable<string> teamIds,
   IEnumerable<Team> modifiedTeams, IEnumerable<Team> allTeams)
{
    var modifiedTeamsLookup = modifiedTeams.ToDictionary(team => team.Id);
    var allTeamsLookup = allTeams.ToDictionary(team => team.Id);

    return teamIds
        .SelectMany(teamId => teamId.ToDriverIds(modifiedTeamsLookup,
            allTeamsLookup);

}