EdSF - yes, that is the answer that i was looking for :) this was one of my versions, why this is happening.

It's sad - because i'm now stuck with slower function (or more code)...

when you posted your answer before edit, i haven't really understood what you've meant.

so i began to write console test application :)

Module Module1

	Function f1() As String
		Console.WriteLine("f1")
		Return "return from f1"
	End Function

	Function f2() As String
		Console.WriteLine("f2")
		Return "return from f2"
	End Function

	 Sub Main()
		Dim str = ""
		Console.WriteLine(If(str.Length > 0, f1, f2))
		Console.WriteLine(IIf(str.Length > 0, f1, f2))

		Console.WriteLine("-------")

		Dim dec As Decimal?
		dec = If(str.Length > 0, f1, Nothing)
		If dec Is Nothing Then Console.WriteLine("Nothing") Else Console.WriteLine(dec)

		Console.WriteLine("-------")

		dec = IIf(str.Length > 0, f1, Nothing)
		If dec Is Nothing Then Console.WriteLine("Nothing") Else Console.WriteLine(dec)
	 End Sub

End Module

anyway - thank you for clarification, i totally have thought, that IF and IIF have only one difference - speed.

P.S. it's interesting though, why IF returns nothing in this case: IF(str.length>0, nothing, nothing) and does not convert nothing to decimal 0D.