How To Use OLE_TRISTATE Property Data Type

MORE INFORMATION

The OLE_TRISTATE is an enumerated data type with three possible values: When you create a property of this type, Visual Basic provides a drop-down list with the above three choices in the property window. Visual Basic will not verify that the values assigned to the OLE_TRISTATE property are valid; therefore, a program or user can assign a value beyond 2 or less than zero to the property. This behavior should be considered in code.

The steps below describe how to create a simple ActiveX control with a property of type OLE_TRISTATE.

Create ActiveX Control

The following steps describe how to create an ActiveX control with a property named YesNoMaybe of type OLE_TRISTATE.

1. In Visual Basic, create a new ActiveX Control project. UserControl1 will be created by default.
2. From the Project menu, choose Project1 Properties. In the Project Name field, type "OleTriState" without the quotes. Click OK.
3. Create a Checkbox control on UserControl1 and set its Caption property to "TriState Value."
4. Add four CommandButtons to UserControl1. Set the CommandButton properties as follows;

      Command1
         (Name):  cmdUnChecked
         Caption: No
   
      Command2
         (Name):  cmdChecked
         Caption: Yes
   
      Command3
         (Name):  cmdGray
         Caption: Maybe
   
      Command4
         (Name):  cmdInvalid
         Caption: Invalid
   						

5. Add the following code to Usercontrol1:

         Option Explicit
         Private CurrentValue As OLE_TRISTATE
   
         Public Property Get YesNoMaybe() As OLE_TRISTATE
            'Retrieve the current value of the YesNoMaybe property
   
            YesNoMaybe = CurrentValue
   
         End Property
   
         Public Property Let YesNoMaybe(ByVal NewValue As OLE_TRISTATE)
            'Because Visual Basic does not check for valid values assigned to
            'an enumerated type, such as OLE_TRISTATE, you must first check
            'for values outside the range of 0-2.
   
            If NewValue > 2 or NewValue < 0 Then
               Msgbox "Invalid Value: Valid Values are 0, 1, or 2."
               Exit Property
            End If
   
            CurrentValue = NewValue
            PropertyChanged "YesNoMaybe"
         End Property
   
         Private Sub cmdUnChecked_Click()
            Me.YesNoMaybe = UnChecked
            Check1.Value = Me.YesNoMaybe
         End Sub
   
         Private Sub cmdChecked_Click()
            Me.YesNoMaybe = Checked
            Check1.Value = Me.YesNoMaybe
         End Sub
   
         Private Sub cmdGray_Click()
            Me.YesNoMaybe = Gray
            Check1.Value = Me.YesNoMaybe
         End Sub
   
         Private Sub cmdInvalid_Click()
            Me.YesNoMaybe = 3
            Check1.Value = Me.YesNoMaybe
         End Sub
   						

6. Save the Project and compile the OleTriState.Ocx.

Creating the Test Container

The test container is designed to test the OleTriState control created in the steps above.

1. Create a new Standard EXE project in Visual Basic. Form1 is created by default.
2. From the Project menu, choose Components. Select the OleTriState control created in the above steps.
3. Add an OleTriState control to Form1.
4. Run the Project.

When you run the Project you can click on each of the CommandButtons and watch the corresponding behavior in the TriState Value check box. If you select the Invalid CommandButton, a message box will appear and the value will not be changed.