Check for null in select when using LINQ to Objects left outer join

Make sure to check for null in the select of a LINQ to objects query when using a left outer join.

The following example illustrates how to get a method value out of class that is the target of the outer join. The code can be run in LINQPad as a C# program.

class Foo
{
    public int FooKey { get; set; }
    public string FooVal { get; set; }
}

class Bar
{
	public int BarKey { get; set; }
    public string BarVal { get; set; }
}

void Main()
{
	List<Foo> aListOFoo = new List<Foo> {
        new Foo { FooKey = 1, FooVal = "FooVal1" },
        new Foo { FooKey = 2, FooVal = "FooVal2" },
        new Foo { FooKey = 3, FooVal = "FooVal3" } };
    
    List<Bar> aListOBar = new List<Bar> {
        new Bar { BarKey = 1, BarVal = "BarVal1" },
        new Bar { BarKey = 2, BarVal = "BarVal2" } };
        
    var q =
        from f in aListOFoo
            join b in aListOBar on f.FooKey equals b.BarKey into fbJoin
        from bLeftJoin in fbJoin.DefaultIfEmpty()
        select new { 
            f.FooVal, 
            BarVal = bLeftJoin != null ? bLeftJoin.BarVal : null };
    
    q.Dump();
}

The output for the above code in LINQPad:

If the select used bLeftJoin.BarVal without the ternary check for null, then the query would return Object reference not set to an instance of an object.

I'm not a huge fan of LINQ outer join syntax, but LINQ brings me much happiness nonetheless :-)