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This is a bug in the Mono Join method implementation for IEnumerables when used with Nullables. Joins are happening for rows where both columns have "null", this is a mistake. This is still present in 3.2.5.
F# repro below (there will be a C# repro too). .NET gives the right results.
GroupJoin should also be checked.
type Customer(name:string, cost:float, quantity:Nullable<int>) =
member x.Name = name
member x.Cost = cost
member x.Quantity = quantity
let c1 = Customer( name="Don", cost=6.2, quantity=Nullable())
let c2 = Customer( name="Peter", cost=4.2, quantity=Nullable(10))
let db = [c1;c2]
db.Join(db,(fun i -> i.Quantity),(fun i -> i.Quantity),(fun i j -> (i.Cost, j.Cost))).ToArray()
// expect [(4.2, 4.2)]
// get [(6.2, 6.2); (4.2, 4.2)]
Fixed in master