Overloading Methods in Visual Basic .NET
Paul D. Sheriff
Summary: This article discusses how Microsoft Visual Basic .NET figures out which method to call during compile based on the parameter types that you pass. This technique is called overloading a method. (8 printed pages)
- Learn why to overload a method
- Create overloaded methods
- Create your own constructors for a class
The following should be true for you to get the most out of this document:
- You understand OOP techniques
- You know how to build classes in Microsoft .NET
Have you ever wanted to create a group of functions that each did essentially the same thing, with only their parameters changed? You can accomplish this by using optional parameters, or by changing the name of the function. Both techniques work, but they are not very elegant. In an OOP language such as Microsoft® Visual Basic® .NET, you are allowed to create methods in a class that have the same name but different argument lists. Visual Basic .NET can figure out which method to call during compile based on the parameter types that you pass. This technique is called overloading a method. The benefit of using the same name is that the user interface is kept consistent; only the inputs are different. The functionality within the method changes for each new overloaded method.
The Line class you created in the document "Creating Classes" uses a GetWord method to return the first word in its line of text. In this document, you will create two more versions of this GetWord method. In the first version, you will pass in a number representing the position of the word you wish to retrieve from the line. In the second version, you will pass in a search string and the method will return the first word in the line of text that contains that string.
The signature of a method consists of its name, parameters, and return type—in other words, the arguments that differentiate one "flavor" of the method from another. The signatures for these three methods look like the following lines of code:
Function GetWord() As String Function GetWord(ByVal Position As Integer) As String Function GetWord(ByVal Search As String) As String
The name of the method (GetWord) stays the same in each of the above methods, but the argument list is different for each one. The compiler can resolve each of these names based on these different "signatures." Notice that the return value is the same for each method. Keeping the return value the same is not a requirement, but you normally will not change the return value on overloaded methods. Changing only the return type on a method does not overload a method. If you try to do this, Visual Basic .NET gives you an error message telling you that you cannot overload a method by changing the return type. You must change at least one argument for a method to have a different signature and be overloaded.
Reasons to Overload a Method
Overloading a method allows you to keep your interface consistent, and allows you to logically call the same method regardless of the types of data you are passing in. You will find that using the same name will help you remember what a procedure does, as opposed to having to come up with new names or a naming convention, to help you keep things straight.
Build Two Overloaded Methods
Let's create two new GetWord methods that overload the original GetWord method in the Line class. The first new GetWord method will accept an integer parameter and return the word at the position that you pass to the method. The second GetWord method will accept a string parameter, and will search for that string within the original line of text. If the search string is found, the word at that position will be returned from this method.
Figure 1. Screen used to test the overloading of methods
- Create a new Windows Application project using Visual Studio .NET. Or, if you read the document called "Creating Classes," you can use that same project, and follow the steps below to change the Windows Form to work with this project.
- Create the form shown in Figure 1 by adding the appropriate controls and setting the properties of those controls, as outlined in Table 1.
Table 1. Controls used to build the form to test the GetWord overload method in the Line class
Control Type Property Value Label Name Label1 Text Line of Text TextBox Name txtLine Text The rain in Spain stays mainly in the plain CommandButton Name btnDisplay Text Display Length GroupBox Name fraWord Text Find Occurrence of Word Label Name Label2 Text Enter Word TextBox Name txtWordNum Text 1 CommandButton Name btnGetWord Text Get Word TextBox Name txtWord Text ReadOnly True GroupBox Name fraWord2 Text Find Word Label Name Label3 Text Enter Word to Find TextBox Name txtSearch Text Spain CommandButton Name btnGetWord2 Text Get Word TextBox Name txtWord2 Text ReadOnly True
- From the Visual Studio .NET menu, add a new class by clicking Project and then clicking Add Class.
- When prompted, set the Name of this class to Line.vb and click OK.
- Add the following code to make this Line class:
Public Class Line Private mstrLine As String Property Line() As String Get Return mstrLine End Get Set mstrLine = Value End Set End Property ReadOnly Property Length() As Integer Get Return mstrLine.Length End Get End Property Public Function GetWord() As String Dim astrWords() As String astrWords = mstrLine.Split(" ".ToCharArray()) Return astrWords(0) End Function End Class
- Change the GetWord method and add the keyword Overloads, as shown in the code below.
Public Overloads Function GetWord() As String
The Overloads keyword is required whenever you create several versions of the same method. You will be creating additional versions of the GetWord method next.
- Create a new GetWord method that will accept an Integer value and still return a string. The declaration is in the code listing below.
Public Overloads Function GetWord(_ ByVal Position As Integer) As String Dim astrWords() As String astrWords = mstrLine.Split(" ".ToCharArray()) If Position > astrWords.Length Then Return "" Else Return astrWords(Position - 1) End If End Function
The GetWord method converts the string to an array of individual words using the Split method of the String class. Because the mstrLine variable is defined as a string object, it has a Split method. This Split method takes a Char array of delimiters. Instead of declaring a Char array variable and populating it with a single space, you can take advantage of a technique known as boxing.
Boxing is the process of converting a literal, like a single space, to the implied object (in this case, a string object) for that literal. Once this boxing has occurred, you can apply the ToCharArray method of this newly created string object to return an array of one character. This single element array is passed to the Split method to use as the delimiter that breaks apart the words in the string.
Now that you have an array of words, you can return the word that corresponds to the number you passed into this method. Of course, you should always check to make sure that the number you passed is not greater than the number of words in the array.
Try It Out
- Open the frmLineTest.vb form.
- Double-click the first Get Word button on the form and add the following code to the Click event.
Protected Sub btnGetWord_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles btnGetWord.Click Dim oLine As Line = New Line() oLine.Line = txtLine.Text txtWord.Text = _ oLine.GetWord(txtWordNum.Text.ToInt32()) End Sub
Because this version of the GetWord method expects an Integer data type, you need to use the ToInt32 method to convert the value in the Text property to an Integer.
- To run the project, place a number in the first text box, and click the Get Word button. Depending on the number you typed in, a word should now appear in the read-only text box next to this first button
- Add another new version of the GetWord method that will accept a search string as an argument. This version of GetWord needs to search for that string within the line of text, and return the word where that string is found.
Public Overloads Function GetWord(_ ByVal Search As String) As String Dim astrWords() As String Dim intLoop As Integer astrWords = mstrLine.Split(" ".ToCharArray()) For intLoop = 0 To astrWords.Length - 1 If astrWords(intLoop).IndexOf(Search) > -1 Then Exit For End If Next If intLoop < astrWords.Length Then Return astrWords(intLoop) End If End Function
In this method, after you create the array of words, you loop through the array and check to see if the search string you passed is contained in any word in the array. To accomplish this, you use the IndexOf method of the string to see whether the string you are searching for is part of the string. If the IndexOf method finds the search string, it will return a number greater than –1. Once you have found the string, the For loop exits, and the word at that position is returned from this method.
Try It Out
- Open the frmLineTest.vb form.
- Double-click the second Get Word button and add the following code to the Click event procedure for this button.
Protected Sub btnGetWord2_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles btnGetWord2.Click Dim oLine As Line = New Line() oLine.Line = txtLine.Text txtWord2.Text = _ oLine.GetWord(txtSearch.Text) End Sub
In the btnGetWord2_Click event procedure, you pass the search string to the GetWord method. The Text property returns a String data type, so the GetWord method that accepts the String data type is the method that is called.
- To run the project, place a few characters into the second text box on this form, and then click the second Get Word button. Depending on the characters you typed in, you should see a word appear in the text box next to the button, or a blank string.
When you instantiate a new object, Visual Basic .NET calls an implicit New method. If you don't write the New method yourself, Visual Basic .NET builds one for you. If you wish to initialize any variables within an object at the time you create the new object, you can write an overloaded New method. You can then pass one or more values to this New method. You will probably find that overloading the New method is your most common use of overloading.
- Open the Line.vb class module and add the code shown below.
Public Sub New() ' Do Nothing End Sub Public Sub New(ByVal Line As String) mstrLine = Line End Sub
The first New method you created handles the creation of a new Line object when you do not pass any parameters to it. You need to use the Overloads keyword because you will also be creating another New method with a parameter.
The other New method accepts a string argument. This string argument will be passed when the object is created and assigned to the private variable that holds the line of text.
Try It Out
- Open the frmLineTest.vb form.
- Double-click on either of the GetWord command buttons.
- Change the code to match the code below.
Protected Sub btnGetWord_Click(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles btnGetWord.Click Dim oLine As Line = New Line(txtLine.Text) txtWord.Text = _ oLine.GetWord(txtWordNum.Text.ToInt32()) End Sub
- Press F5 to try out this sample.
Notice that the declaration of the oLine object now passes the value from the txtLine text box. This is different from Visual Basic 6.0 where a Dim statement was not executable code. The Dim statement is now executable.
The ability to overload methods is completely new to Visual Basic .NET and has no equivalent in previous versions of Visual Basic.
Overloading methods is a great way to keep the intent of a method clear while allowing different parameters to affect the value you receive back from the method. Remember that you must change the parameters, and not just the return type to overload a method. When deciding which methods to overload, the intent of the methods should stay consistent. Just because you can overload a method does not necessarily mean you should. Always think about the intent of the method and whether the various methods you create do essentially the same thing.
About the Author
Paul D. Sheriff is the owner of PDSA, Inc., a custom software development and consulting company in Southern California. Paul is the MSDN Regional Director for Southern California, is the author of a book on Visual Basic 6 called Paul Sheriff Teaches Visual Basic, and has produced over 72 videos on Visual Basic, SQL Server, .NET and Web Development for Keystone Learning Systems. Paul has co-authored a book entitled ASP.NET Jumpstart. Visit the PDSA, Inc. Web site (www.pdsa.com) for more information.
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Copyright © 2001 Informant Communications Group and Microsoft Corporation
Technical Editing: PDSA, Inc.