expression does not produce a value when calling a

问题:

i want to call this public sub hungrys(gutom as string) from my zooanimal class to my form1.vb. for reference, please refer to my code. i always get an error "expression does not produce a value" in my Textbox10.text = za.hungrys(gutom as string)

Public Class ZooAnimal

Public Sub New()
hungry = isHungry()

Public Function isHungry() As Boolean
    If age > 0 Then
        hungry = True
    End If
    If age <= 0 Then
        hungry = False
    End If
    Return hungry
End Function

 Public Sub hungrys(ByRef gutom As String)
    If hungry = True Then
        gutom = "The zoo animal is hungry"
    End If
    If hungry = False Then
        gutom = "The zoo animal is not hungry "
    End If
End Sub

Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Dim za As New ZooAnimal
Dim gutom As String = ""

TextBox10.Text = za.hungrys(gutom)

回答1:

If you are trying to get a value out of a Sub with a ByRef parameter rather than the return value of a Function then this:

TextBox10.Text = za.hungrys(gutom)

would need to be this:

za.hungrys(gutom)
TextBox10.Text = gutom

The first line calls the Sub and assigns a new value to the variable and the second line displays the variable's value in the TextBox.

Unless it's as a learning exercise though, there's no good reason to use a ByRef parameter there. You'd normally write that method like this:

Public Function hungrys() As String
    Dim gutom As String

    If hungry Then
        gutom = "The zoo animal is hungry"
    Else
        gutom = "The zoo animal is not hungry "
    End If

    Return gutom
End Sub

and then call it like this:

Dim gutom As String = za.hungrys()

TextBox10.Text = gutom

or just:

TextBox10.Text = za.hungrys()

Notice that that method uses an If...Else rather than two separate If blocks.

By the way, that is a terrible, terrible name for a method. Firstly, it should start with an upper-case letter. Secondly, "hungrys" doesn't tell you what the method does. If I read that with no context, I'd have little idea what it's purpose was.

回答2:

Change your Sub to Function.
Sub are procedures that do not Return a value.
Function do return values.
It's just that simple.

Also, declare the data type of your parameter gutom As Boolean because it stores a value that could be True or False.

Do not use redundant If.. Use If-Else instead.

Public Function hungrys(ByVal gutom As Boolean) As String
  If hungry = True Then
      gutom = "The zoo animal is hungry"
  Else
      gutom = "The zoo animal is not hungry "
  End If
  Return gutom
End Function

Call it By

TextBox10.Text = za.hungrys(True)