Relaxed Delegate Conversion
Relaxed delegate conversion, introduced in Visual Basic 2008, enables you to assign subs and functions to delegates or handlers even when their signatures are not identical. Therefore, binding to delegates becomes consistent with the binding already allowed for method invocations.
In place of exact signature match, relaxed conversion requires that the following conditions be met when Option Strict is set to On:
A widening conversion must exist from the data type of each delegate parameter to the data type of the corresponding parameter of the assigned function or Sub. In the following example, the delegate Del1 has one parameter, an Integer. Parameter m in the assigned lambda expressions must have a data type for which there is a widening conversion from Integer, such as Long or Double.
Narrowing conversions are permitted only when Option Strict is set to Off.
A widening conversion must exist in the opposite direction from the return type of the assigned function or Sub to the return type of the delegate. In the following examples, the body of each assigned lambda expression must evaluate to a data type that widens to Integer because the return type of del1 is Integer.
If Option Strict is set to Off, the widening restriction is removed in both directions.
Relaxed delegates also allow you to completely omit parameter specifications in the assigned method:
' The assigned lambda expression specifies no parameters, even though ' Del2 has two parameters. Because the assigned function in this ' example is a lambda expression, Option Strict can be on or off. ' Compare the declaration of d16, where a standard function is assigned. Dim d11 As Del2 = Function() 3 ' The parameters are still there, however, as defined in the delegate. Console.WriteLine(d11(5, "five")) ' Not valid. ' Console.WriteLine(d11()) ' Console.WriteLine(d11(5))
Note that you cannot specify some parameters and omit others.
The ability to omit parameters is helpful in a situation such as defining an event handler, where several complex parameters are involved. The arguments to some event handlers are not used. Instead, the handler directly accesses the state of the control on which the event is registered, and ignores the arguments. Relaxed delegates allow you to omit the arguments in such declarations when no ambiguities result. In the following example, the fully specified method OnClick can be rewritten as RelaxedOnClick.
Lambda expressions are used in the previous examples to make the type relationships easy to see. However, the same relaxations are permitted for delegate assignments that use AddressOf, Handles, or AddHandler.
In the following example, functions f1, f2, f3, and f4 can all be assigned to Del1.
' Assignments to function delegate Del1. ' Valid AddressOf assignments with Option Strict on or off: ' Integer parameters of delegate and function match. Dim d13 As Del1 = AddressOf f1 ' Integer delegate parameter widens to Long. Dim d14 As Del1 = AddressOf f2 ' Short return in f3 widens to Integer. Dim d15 As Del1 = AddressOf f3
The following example is valid only when Option Strict is set to Off.
Relaxed delegate conversion enables you to assign a function to a Sub delegate, effectively ignoring the return value of the function. However, you cannot assign a Sub to a function delegate. In the following example, the address of function doubler is assigned to Sub delegate Del3.
' Definition of Sub delegate Del3. Delegate Sub Del3(ByVal arg1 As Integer) ' Definition of function doubler, which both displays and returns the ' value of its integer parameter. Function doubler(ByVal p As Integer) As Integer Dim times2 = 2 * p Console.WriteLine("Value of p: " & p) Console.WriteLine("Double p: " & times2) Return times2 End Function