Delegate question

[Larry Smith]

Hi there,

Given a delegate that points to a non-static member function, is there a
cleaner way to change the object instance it's called on other than invoking
"Delegate.CreateDelegate()" (and changing the object instance while leaving
the same member function intact). Thanks in advance.

 

[Dave Sexton]

Hi Larry,

Yes, there is.

Delegates that are created using the "delegate" keyword in C# derive from
MulticastDelegate, which derives from Delegate. MulticastDelegate allows a
single delegate to contain an invocation list that consists of multiple
delegates. The delegates in the invocation list are invoked when the owning
delegate is invoked.

Using the multicast feature of delegates you can combine the first delegate
with the new delegate and then remove the first delegate from the list:

static void Test()
{
Console.WriteLine("Test");
}

static void Test2()
{
Console.WriteLine("Test2");
}

delegate void RunTest();

static void Main()
{
// create the delegate instance
RunTest multicastDel = new RunTest(Test);

// add another delegate to the invocation list
multicastDel += new RunTest(Test2);

// If multicastDel() were to be called now, the output would be:
// Test
// Test2

// remove the original delegate
multicastDel -= new RunTest(Test);

multicastDel();
}

Output:

Test2

 

[Jon Skeet [C# MVP]]

Given a delegate that points to a non-static member function, is there a
cleaner way to change the object instance it's called on other than invoking
"Delegate.CreateDelegate()" (and changing the object instance while leaving
the same member function intact). Thanks in advance.

No - you have to create a new instance, because delegates are
immutable. Using Delegate.CreateDelegate is probably the easiest way of
doing this in a general manner.

 

[Dave Sexton]

Hi Jon,

No - you have to create a new instance, because delegates are
immutable. Using Delegate.CreateDelegate is probably the easiest way of
doing this in a general manner.

Right, I forgot that += creates a new instance. My example is pointless

Sorry for any confusion, Larry.

 

[Larry Smith]

Given a delegate that points to a non-static member function, is there a

No - you have to create a new instance, because delegates are
immutable. Using Delegate.CreateDelegate is probably the easiest way of
doing this in a general manner.

Ok. Thanks for the info (appreciated)

 

[Larry Smith]

Hi Jon,


Right, I forgot that += creates a new instance. My example is pointless


Sorry for any confusion, Larry.

No problem. Thanks anyway.