Introduction
Delegates are .NET's type safe equivalent to function pointers.
Delegates go further than this though. Instead of a single
delegate having the ability to point to, and invoke, a single
function, delegates in .NET give you the ability to have a single
delegate point to a list of methods, each which will be called in
turn.
Creating a single cast delegate
A single cast delegate is one which points to a single method. To
create a delegate you must first declare a delegate type that
has the same signature as the methods you wish it to invoke. For
instance, if we wished to have a delegate call a function that took
a String*
as a parameter and returned an int
, we might declare it
as
__delegate int MyDelegate(String *str);
Single cast delegates are implicitely derived from Delegate
.
To use a delegate to invoke your methods you must create an instance
of the delegate and pass in an object and the method of that object
you wish to call.
For instance, suppose we had a managed class
__gc class MyClass
{
public:
int MethodA(String *str)
{
Console::WriteLine(S"MyClass::MethodA - The value of str is: {0}", str);
return str->Length;
}
}
we could declare an object of type MyClass and a delegate to call the
objects methods like
MyClass *pMC = new MyClass();
MyDelegate *pDelegate = new MyDelegate(pMC, &MyClass::MethodA);
To invoke the object's method using the delegate is as simple as calling
pDelegate->Invoke("Invoking MethodA");
This would output:
MyClass::MethodA - The value of str is: Invoking MethodA
Creating a multi cast delegate
Multi cast delegates allow you to chain together methods so that each one
will be called in turn when the delegate is invoked. To create a multicast
method use the static Delegate::Combine
method to combine delegates
into a multicast delegate.
When creating multicast delegates in beta 1, you had to declare your delegates
you wished to combine as multicast delegates using the
__delegate(multicast)
directive. This creates a delegate
derived from MulticastDelegate
. In beta 2 you do not use the
(multicast)
specifier, and you can use Delegate::Combine
your single cast delegates into a multicast delegate.
Below is some sample code showing 2 multicast delegates being combined (beta 2)
__delegate int MyDelegate(String *str);
__gc class MyClass
{
public:
int MethodA(String *str)
{
Console::WriteLine(S"MyClass::MethodA - The value of str is: {0}", str);
return str->Length;
}
int MethodB(String* str)
{
Console::WriteLine(S"MyClass::MethodB - The value of str is: {0}", str);
return str->Length * 2;
}
};
...
MyClass *pMC = new MyClass();
MyDelegate *pDelegate1 = new MyDelegate(pMC, &MyClass::MethodA);
MyDelegate *pDelegate2 = new MyDelegate(pMC, &MyClass::MethodB);
MyDelegate *pMultiDelegate =
static_cast<MyDelegate *>(Delegate::Combine(pDelegate, pDelegate2));
When you invoke the multicast delegate pMultiDelegate it will first call
MyClass::MethodA
, then MyClass::MethodB
. For instance,
calling
pMultiDelegate->Invoke("Invoking Multicast delegate");
would result in
MyClass::MethodA - The value of str is: Invoking multicast delegate
MyClass::MethodB - The value of str is: Invoking multicast delegate
Type Safety
Delegates are inherently type safe. You cannot compile calls to a delegate
using the wrong parameters, or assign the return value of a delegate to a
type that cannot be implicitly cast from the return type of the method the
delegate is calling. Because an object and method are passed to the delegates
constructor, the compiler has all the information it needs to ensure that
errors caused by mismatched parameters and return types do not occur.
Chris Maunder is the co-founder of
CodeProject, DeveloperMedia and ContentLab, and has been a prominent figure in the software development community for nearly 30 years. Hailing from Australia, Chris has a background in Mathematics, Astrophysics, Environmental Engineering and Defence Research. His programming endeavours span everything from FORTRAN on Super Computers, C++/MFC on Windows, through to to high-load .NET web applications and Python AI applications on everything from macOS to a Raspberry Pi. Chris is a full-stack developer who is as comfortable with SQL as he is with CSS.
In the late 1990s, he and his business partner David Cunningham recognized the need for a platform that would facilitate knowledge-sharing among developers, leading to the establishment of CodeProject.com in 1999. Chris's expertise in programming and his passion for fostering a collaborative environment have played a pivotal role in the success of CodeProject.com. Over the years, the website has grown into a vibrant community where programmers worldwide can connect, exchange ideas, and find solutions to coding challenges. Chris is a prolific contributor to the developer community through his articles and tutorials, and his latest passion project,
CodeProject.AI.
In addition to his work with CodeProject.com, Chris co-founded ContentLab and DeveloperMedia, two projects focussed on helping companies make their Software Projects a success. While at CodeProject, Chris' roles included Architecture and coding, Product Development, Content Creation, Community Growth, Client Satisfaction and Systems Automation, and many, many sales meetings. All while keeping his sense of humour.