Why won't this work as an IIF function, but wi

问题:

The following works:

If 1=1
    rdoYes.checked = True
Else
    rdoNo.checked = True
End If

However, the following doesn't work:

IIF(1=1, rdoYes.checked = True, rdoNo.checked = True)

Why is this?

Thanks!

回答1:

It does "work"; it just doesn't do what you want.

IIf in VB.NET is a function (don't use it, ever, by the way), which takes these parameters:

1. A Boolean condition to check
2. An Object to return if the condition is True
3. A different Object to return if the condition is False

In your usage, your condition is 1 = 1; then your two other parameters are rdoYes.Checked = True and rdoNo.Checked = True, both Boolean expressions from the VB compiler's perspective (so, really, they're equivalent to the simpler rdoYes.Checked and rdoNo.Checked).

Remember that in VB.NET, the = sign is only an assignment if it is on its own line. This is how the compiler distinguishes between statements such as x = 5 and If x = 5 Then.

This is not directly related to your question, but you should also be aware that IIf is deprecated and you should almost always favor If instead:

' Let us just suppose it made sense to write this: '
' Notice the If instead of IIf. '
Dim result = If(1 = 1, rdoYes.Checked, rdoNo.Checked)

回答2:

The IIF() function will return something based on what you enter for the first parameter. Since VB.Net doesn't differ between = as in assignment and = as in comparison (== in many other languages), the second statement is ambiguous.

---

You can do this with using late binding (delegates in VB.Net):

(Function(c) InlineAssignHelper(c.Checked, true)).Invoke(IIf(1 = 1, chkYes, chkNo))

  Private Function InlineAssignHelper(Of T)(ByRef target As T, ByVal value As T) As T
    target = value
    Return value
  End Function

回答3:

Because IIf takes expressions and returns a result of one of them, and rdoYes.checked = True is not an expression and cannot be returned.

回答4:

iif doesn't do what you think it does -- the important part is the return from it, so you might be able to do:

iif(1=1, rdoYes, rdoNo).checked = True

(I'm not sure that's valid VB ... it's been more than a decade since I've had to code in it)