为什么调用函数的返回值为空？

新手上路，请各位大侠给与帮助，多谢！！！我用的是vs2005中的vb。
（1）我先写的一个类，这里面有个函数，函数里的那个case语句不能赋值，最后导致return返回空值，不知道为什么？？
Public Class Class_A
    Private _stringA As String
    Private _stringTemp As String
    Private _string1 As String = "1111"

    Private _string2 As String = "2222"
    Private _string3 As String = "3333"

    Property stringA() As String
        Get
            Return _stringA
        End Get
        Set(ByVal value As String)
            _stringA = value
        End Set
    End Property

    Property stringTemp() As String
        Get
            Return _stringTemp
        End Get
        Set(ByVal value As String)
            _stringTemp = value
        End Set
    End Property

    Property string1() As String
        Get
            Return _string1
        End Get
        Set(ByVal value As String)
            _string1 = value
        End Set
    End Property

    '构造函数，参数为
    Sub New(ByVal A As String)
        _stringA = A

    End Sub

    '重载的构造函数
    Sub New()
    End Sub

    Public Function Function_GetString(ByVal InputA As String) As String

        Select Case InputA
            Case InputA Like "aaa"
                _stringTemp = _string2
            Case InputA Like "bbb"
                _stringTemp = _string3

        End Select

        Return stringTemp()
    End Function

End Class

（2）我做的窗体里的Load过程

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        Dim stringClass As Class_A = New Class_A("aaa")

        Dim string1 As String = stringClass.string1

        Dim string23 As String = stringClass.Function_GetString("aaa")

        Label1.Text = "返回属性值"
        Label2.Text = "返回函数值"

        '有返回值，能显示
        TextBox1.Text = string1

        '？？？？为什么下面不能返回值，不能显示
        TextBox2.Text = string23

    End Sub
End Class

--------------------编程问答-------------------- 改用 if 语句试试。

补充：VB ,  基础类