共用方式為

Overloaded Properties and Methods (Visual Basic)

  • 發行項

Overloading is the creation of more than one procedure, instance constructor, or property in a class with the same name but different argument types.

Overloading Usage

Overloading is especially useful when your object model dictates that you employ identical names for procedures that operate on different data types. For example, a class that can display several different data types could have Display procedures that look like this:

Overloads Sub Display(ByVal theChar As Char)
    ' Add code that displays Char data. 
End Sub 
Overloads Sub Display(ByVal theInteger As Integer)
    ' Add code that displays Integer data. 
End Sub 
Overloads Sub Display(ByVal theDouble As Double)
    ' Add code that displays Double data. 
End Sub

Without overloading, you would need to create distinct names for each procedure, even though they do the same thing, as shown next:

Sub DisplayChar(ByVal theChar As Char)
    ' Add code that displays Char data. 
End Sub 
Sub DisplayInt(ByVal theInteger As Integer)
    ' Add code that displays Integer data. 
End Sub 
Sub DisplayDouble(ByVal theDouble As Double)
    ' Add code that displays Double data. 
End Sub

Overloading makes it easier to use properties or methods because it provides a choice of data types that can be used. For example, the overloaded Display method discussed previously can be called with any of the following lines of code:

' Call Display with a literal of type Char.
Display("9"c)
' Call Display with a literal of type Integer.
Display(9)
' Call Display with a literal of type Double.
Display(9.9R)

At run time, Visual Basic calls the correct procedure based on the data types of the parameters you specify.

Overloading Rules

You create an overloaded member for a class by adding two or more properties or methods with the same name. Except for overloaded derived members, each overloaded member must have different parameter lists, and the following items cannot be used as a differentiating feature when overloading a property or procedure:

  • Modifiers, such as ByVal or ByRef, that apply to a member, or parameters of the member.

  • Names of parameters

  • Return types of procedures

The Overloads keyword is optional when overloading, but if any overloaded member uses the Overloads keyword, then all other overloaded members with the same name must also specify this keyword.

Derived classes can overload inherited members with members that have identical parameters and parameter types, a process known as shadowing by name and signature. If the Overloads keyword is used when shadowing by name and signature, the derived class's implementation of the member will be used instead of the implementation in the base class, and all other overloads for that member will be available to instances of the derived class.

If the Overloads keyword is omitted when overloading an inherited member with a member that has identical parameters and parameter types, then the overloading is called shadowing by name. Shadowing by name replaces the inherited implementation of a member, and it makes all other overloads unavailable to instances of the derived class and its decedents.

The Overloads and Shadows modifiers cannot both be used with the same property or method.

Example

The following example creates overloaded methods that accept either a String or Decimal representation of a dollar amount and return a string containing the sales tax.

To use this example to create an overloaded method

  1. Open a new project and add a class named TaxClass.

  2. Add the following code to the TaxClass class.

    Public Class TaxClass
    Overloads Function TaxAmount(ByVal decPrice As Decimal, 
         ByVal TaxRate As Single) As String
        TaxAmount = "Price is a Decimal. Tax is $" & 
           (CStr(decPrice * TaxRate))
    End Function 

    Overloads Function TaxAmount(ByVal strPrice As String, 
          ByVal TaxRate As Single) As String
        TaxAmount = "Price is a String. Tax is $" & 
           CStr((CDec(strPrice) * TaxRate))
    End Function 
End Class

  3. Add the following procedure to your form.

    Sub ShowTax()
    ' 8% tax rate. 
    Const TaxRate As Single = 0.08
    ' $64.00 Purchase as a String. 
    Dim strPrice As String = "64.00" 
    ' $64.00 Purchase as a Decimal. 
    Dim decPrice As Decimal = 64
    Dim aclass As New TaxClass
    'Call the same method with two different kinds of data.
    MsgBox(aclass.TaxAmount(strPrice, TaxRate))
    MsgBox(aclass.TaxAmount(decPrice, TaxRate))
End Sub

  4. Add a button to your form and call the ShowTax procedure from the Button1_Click event of the button.

  5. Run the project and click the button on the form to test the overloaded ShowTax procedure.

At run time, the compiler chooses the appropriate overloaded function that matches the parameters being used. When you click the button, the overloaded method is called first with a Price parameter that is a string and the message, "Price is a String. Tax is $5.12" is displayed. TaxAmount is called with a Decimal value the second time and the message, "Price is a Decimal. Tax is $5.12" is displayed.

See Also

Reference

Sub Statement (Visual Basic)

Shadows (Visual Basic)

ByVal (Visual Basic)

ByRef (Visual Basic)

Overloads (Visual Basic)

Shadows (Visual Basic)

Concepts

Shadowing in Visual Basic

Inheritance Basics (Visual Basic)

Other Resources

Objects and Classes in Visual Basic