Procedure Overloading (Visual Basic)
Overloading a procedure means defining it in multiple versions, using the same name but different parameter lists. The purpose of overloading is to define several closely related versions of a procedure without having to differentiate them by name. You do this by varying the parameter list.
When you overload a procedure, the following rules apply:
Same Name. Each overloaded version must use the same procedure name.
Different Signature. Each overloaded version must differ from all other overloaded versions in at least one of the following respects:
Number of parameters
Order of the parameters
Data types of the parameters
Number of type parameters (for a generic procedure)
Return type (only for a conversion operator)
Together with the procedure name, the preceding items are collectively called the signature of the procedure. When you call an overloaded procedure, the compiler uses the signature to check that the call correctly matches the definition.
Items Not Part of Signature. You cannot overload a procedure without varying the signature. In particular, you cannot overload a procedure by varying only one or more of the following items:
Procedure modifier keywords, such as Public, Shared, and Static
Parameter or type parameter names
Type parameter constraints (for a generic procedure)
Parameter modifier keywords, such as ByRef and Optional
Whether it returns a value
The data type of the return value (except for a conversion operator)
The items in the preceding list are not part of the signature. Although you cannot use them to differentiate between overloaded versions, you can vary them among overloaded versions that are properly differentiated by their signatures.
Late-Bound Arguments. If you intend to pass a late bound object variable to an overloaded version, you must declare the appropriate parameter as Object.
Suppose you are writing a Sub procedure to post a transaction against a customer's balance, and you want to be able to refer to the customer either by name or by account number. To accommodate this, you can define two different Sub procedures, as in the following example:
An alternative is to overload a single procedure name. You can use the Overloads (Visual Basic) keyword to define a version of the procedure for each parameter list, as follows:
If you also wanted to accept a transaction amount in either Decimal or Single, you could further overload post to allow for this variation. If you did this to each of the overloads in the preceding example, you would have four Sub procedures, all with the same name but with four different signatures.
The advantage of overloading a procedure is in the flexibility of the call. To use the post procedure declared in the preceding example, the calling code can obtain the customer identification as either a String or an Integer, and then call the same procedure in either case. The following example illustrates this:
Dim customer As String Dim accountNum As Integer Dim amount As Single customer = MSVB.Interaction.InputBox("Enter customer name or number") amount = MSVB.Interaction.InputBox("Enter transaction amount") Try accountNum = CInt(customer) Call post(accountNum, amount) Catch Call post(customer, amount) End Try