Is there a difference between nested and non nested else/else ifs?

Question

I have a task at work to rewrite old vb code into c# and i came across this

If (termNumber = TermNumberCode.FallWinter) Then
    Dim first_term As String = " ((Course.dbo.CourseList.[Year])=@TermYear) AND ((Course.dbo.CourseList.TermNumber)=3)  "

    Dim second_term As String = " ((Course.dbo.CourseList.Year)=@TermYear+1) AND ((Course.dbo.CourseList.TermNumber)=1)  "
    query &= "  AND ( ( " & first_term & " ) OR ( " & second_term & " ) )  "

Else
    If (termNumber = TermNumberCode.Summer) Then
        query &= " AND ((Course.dbo.CourseList.[Year])=@TermYear) AND ((Course.dbo.CourseList.TermNumber)=2)  "

    ElseIf (termNumber = TermNumberCode.Fall) Then
        query &= " AND ((Course.dbo.CourseList.[Year])=@TermYear) AND ((Course.dbo.CourseList.TermNumber)=3)  "

    ElseIf (termNumber = TermNumberCode.Winter) Then
        query &= " AND ((Course.dbo.CourseList.[Year])=@TermYear) AND ((Course.dbo.CourseList.TermNumber)=1)  "

    End If
End If 

I was wondering, Is this the same as writing it like this or if there is any special significance or nesting the ifs like that?

If (termNumber = TermNumberCode.FallWinter) Then
    code...

Else if (termNumber = TermNumberCode.Summer)
    code...

Else If (termNumber = TermNumberCode.Fall) 
    code...

Else //(termNumber = TermNumberCode.Winter)
    code...

Answer 1

The form

if #1..    
else
   if #2..
   else if #3..
   else ..
end

is equivalent to

if #1..
else if #2..
else if #3..
else ..

That is, the immediate nesting inside an else does not affect the logic of the nested branches. This can be repeated indefinitely - although the non-nested form is arguably more clear here.

However, in the presented code the else and a final else if is transposed, which won't make a difference if termNumber if constrained to the set checked against. But it is a difference - what if termNumber is TermNumberCode.SpringBreak ? - and thus it must be considered separately from the transformation above..

Answer 2

假设你没有内部If/End If之外的任何代码If/End If它们实际上是等价的。

Answer 3

它们在功能上是相同的。

Answer 4

This equivalence is easier to see in the C-style languages. In those languages there is no "else if" keyword - that's just a formatted combination of "else" and "if". In other words, as long as you have "if" and "else" in a language where white space is unimportant, you always get "else if".

In VB however, there is an "ElseIf" keyword and you have all those "End If" statements, so it's not just a white space rearrangement. But you still have the underlying equivalence between "Else
If" and "ElseIf".

Answer 5

VB.NET has two styles of syntax for if and else. The first style has the first statement to be controlled immediately follow Then or Else , without an intervening colon or line end. The effect of the conditional will extend to the end of the line. Use of End If is not allowed with this style.

The second style has a line end or colon immediately follow a Then or Else . In this style, the condition will affect everything until the next Else , ElseIf , or End If . Note that line-ends are not significant in this style; one may include multiple Then , Else , ElseIf , and End If statements on a single line.

To avoid parsing confusion, certain combinations of styles are forbidden. In particular, no If , Then , ElseIf , or Else of the second style may be controlled by an If of the first style, and any Else must use the same style as its matching Then . The ElseIf keyword is designed to get around these restrictions, since it is regarded as part of a second-style If / Then construct, rather than being a first-style Else . Note that one could write Else : If ... Then using a second-style construct, but separate End If statements would be required for the Then and the Else . Additionally, note that if one is using the first if/then style and includes an If as the first statement of the Else , a space between the Else and If is required.