In visual basic, the optional parameters have been introduced to specify the parameters as optional while defining the methods, indexers, constructors, and delegates.
Generally, while calling the method we need to pass all the method parameters. In case, if we specify a few of method parameters as an optional, then we can omit those parameters while calling the method because the optional parameters are not mandatory.
If we want to make the parameter as an optional, we need to define a parameter with Optional keyword and assign the default value to that parameter as part of its definition in methods, indexers, constructors, and delegates.
While calling the method, if no argument is sent for an optional parameter, then the default value will be used otherwise it will use the value which is sent for that parameter.
Following is the example of defining the optional parameters in a method.
Public Sub GetDetails(ByVal id As Integer, ByVal name As String, Optional ByVal location As String = "Hyderabad")
' your code
End Sub
If you observe the above method, we made the location parameter as optional by assigning the default value (Hyderabad). So, while accessing GetDetails() method, if no argument sent for location parameter, then by default it will consider “Hyderabad” value otherwise it will use the value which is sent for that parameter.
The defined GetDetails() method can be accessed as shown below.
GetDetails(1,"Suresh");
GetDetails(1,"Suresh", "Chennai");
While defining the optional parameters, we need to make sure that the optional parameters are defined at the end of the parameter list, after any required parameters like as we defined in the above method otherwise we will get the compile-time error like “optional parameters must appear after all required parameters”.
Following is the invalid way of defining the optional parameters in a method.
Public Sub GetDetails(ByVal id As Integer, ByVal Optional location As String = "Hyderabad", ByVal name As String)
' your code
End Sub
The above method will throw the compile-time error because we defined a required parameter (name) after the optional parameter (location). As per rules, the optional parameters must be defined at the end of the parameter list.
In case, if you want to define multiple optional parameters, then the method declaration must be like as shown below.
Public Sub GetDetails(ByVal id As Integer, Optional ByVal name As String = "Suresh", Optional ByVal location As String = "Hyderabad")
' your code
End Sub
The defined GetDetails() method can be accessed like as shown below.
GetDetails(1);
GetDetails(1, "Rohini");
GetDetails(1, "Trishi", "Guntur");
While calling the method, if we provide an argument for any of succession optional parameters, then we must need to provide an argument for all the preceding optional parameters and the comma-separated gaps in the argument list are not supported.
Following is the invalid way of defining the optional parameters in a method.
GetDetails(1, , "Guntur");
The above caller method will throw a compile-time error like “Argument missing” because we provided an argument for the third parameter but not for a second.
However, if you know the name of the third parameter, then you can use a named argument to send a value like as shown below.
GetDetails(1, location:="Guntur");
This is how we can skip preceding optional parameters by using named arguments in caller methods.
Following is the example of defining the optional parameters during the method declaration in visual basic.
Module Module1
Sub Main(ByVal args As String())
GetDetails(1)
GetDetails(1, "Rohini")
GetDetails(1, "Trishi", "Guntur")
GetDetails(1, location:="Guntur")
Console.ReadLine()
End Sub
Public Sub GetDetails(ByVal id As Integer, Optional ByVal name As String = "Suresh", Optional ByVal location As String = "Hyderabad")
Console.WriteLine("Id:{0}", id)
Console.WriteLine("Name:{0}", name)
Console.WriteLine("Location:{0}", location)
End Sub
End Module
If you observe the above example, the GetDetails() method has one required parameter (id) and two optional parameters (name, location).
When we execute the above program, we will get the result like as shown below.
****Only Required Parameter****
Id:1
Name:Suresh
Location:Hyderabad
****Required Parameter & One Optional Parameter****
Id:1
Name:Rohini
Location:Hyderabad
****Both Required & Optional Parameters****
Id:1
Name:Trishi
Location:Guntur
****Required & Second Optional Parameter****
Id:1
Name:Suresh
Location:Guntur
This is how we can define the optional parameters in methods, indexers, constructors, and delegates based on our requirements.
In visual basic, we can also achieve the optional parameters by using method overloading functionality. Generally, the method overloading functionality will allow us to create multiple methods with the same name but with different parameters.
To know more about overloading functionality in visual basic, check method overloading.
Following are the important points which we need to remember about optional parameters in visual basic.