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The reason you can't do it is fairly simple. Action<T> is generic - fair enough, and you want it to be Action<object> - ok? Simplistically you would think this would work because no matter what type T was, it would ultimately be convertable to object, as everything derives from object.
Suppose though, that type T was a string, and you wanted to use Action<int> - all of a sudden, there's a conversion that is not implicitly possible - in other words, you would have to perform an explicit cast. The cast into object would have to be a special case in the compiler, so they chose not to do this - instead, like for like typing is the only way to achieve this.
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Thanks for the reply Pete, just not sure I fully understand your explanation.
What you are saying that (assuming this was allowed) this wouldn't be a problem:
Action<object> act = null;
act = new Action<int>();
but the problem would come in if I tried:
act("a");
If that's what you are saying then I'm a bit disappointed in MS because type conversion from object has always posed this challenge (I recon this is why generics was brought in) and yes its fair to expect me to pass the right value in or get a runtime error...
____________________________________________________________
Be brave little warrior, be VERY brave
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That's exactly what I'm saying. The thing is though, generics were brought in for things like type safety and to prevent aid the developer move away from boxing/unboxing. By having object as the generic type, you've just gone back to having a boxing/unboxing issue - and you're no better off than just passing Action across rather than the generic Action.
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I understand that I should avoid boxing & un-boxing (and I do want to, and I do it everywhere), in this case it's not an option. Bit of a complex scenario I'll have to explain (and the code is part of a 5 000 file solution), but I'm quite irritated that this is disallowed, I thought polymorphism / covariance / contravariance is exactly about this...
____________________________________________________________
Be brave little warrior, be VERY brave
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It is, for objects - aka reference-types, not for value-types. C# is really strict on the difference.
The posted alternative with the interface does look like it (could) solve your problem. Alternatively, you could decide only to pass objects, and to wrap your ints in a MyInt class.
I are Troll 
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It's a design issue and it makes sense when you think about it. If you want to use a collection of type object, then use ArrayList. The generic List is a strongly typed version of ArrayList, so if strong typing is not required, use ArrayList.
"You get that on the big jobs."
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Why not just to the following, from what I could see would give youthe exact same result, with the benefit of actually compiling!
public interface IBase<T>
{
Func<T> OnComplete { get; set; }
}
public class Base : IBase<object>
{
public Base(Func<object> onComplete)
{
OnComplete = onComplete;
}
public Func<object> OnComplete { get; set; }
}
public class GenericBase<T> : IBase<T>
{
public GenericBase(Func<T> onComplete)
{
OnComplete = onComplete;
}
public Func<T> OnComplete { get; set; }
}
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Generic inheritance is a bit complicated. Here's the reason that you can't cast 'down' the generic type parameter inheritance tree.
Imagine this test class:
class BadMistake {
void CompleteHandler(Dictionary<string,int> complexParameter){
}
GenericBase<Dictionary<string,int>> instance;
public void CauseDeath(){
instance = new GenericBase<Dictionary<string,int>>(CompleteHandler);
((Base)instance).OnComplete(16);
}
}
Now, CompleteHandler is expecting to be passed a Dictionary, and you can't compile something which would cause that not to be the case. But using the base class, you can pass an int (or anything else) to it.
Method parameter compatibility for generics works backwards. This example will compile if the type parameter in the base is a subclass of T, for example try:
public class Base
{
public Func<T> OnComplete {get;set;} where T:Panel
public Base(Func<T> onComplete) where T:Panel
{
OnComplete = onComplete;
}
}
public class GenericBase<T> : Base
{
public GenericBase(Func<T> onComplete) where T:UserControl : base(onComplete)
{
OnComplete = onComplete;
}
}
(if I got the syntax on the constructor there right).
Also, there isn't really a reason to make a non generic base type and inherit from it in this way. You can just use a GenericBase<object> if you want one that can deal with anything.
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Because it just can't. If asked questions such as this I like to mutter something about Covariance and Contravariance and wonder off. Usually works in the office environment anyway.
Regards,
Rob Philpott.
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Rob Philpott wrote: Covariance and Contravariance
I thought those were added in .net 4.
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Hi,
I'm growing grey early here... I severly need HELP.
When using Visual Studio 2010 (express?)...
...if you click File => "New Project", where do you specify the directory on disk?
I've been creating all sorts of project in random user directories and IT'S DRIVING ME CRAZY! Why can't I specify the directory where VS2010 creates the solution?!? Why can't I start a project in D:\projects?
This is one of the things which drives me away from .net, even tho it's the platform of choice here...
A good programmer is someone who always looks both ways before crossing a one-way street. (Doug Linder)
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Helfdane wrote: I'm growing grey early here... I severly need HELP.
Really?
Visual Studio (all versions) behaves pretty much the same as most other Windows applications (e.g. MS Word): you can create a new something in memory, and it will ask you for a path if and when that becomes unavoidable, e.g. when telling the app to save, to close, to quit, to open up another thing that causes the current one to close first, etc. Then a dialog will pop up and you will be in charge of file locations.
I tend to save early on, to get that out of the way; but there is no need to do so.
Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.
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You're not correct regarding Visual Studio 2010 Express (10.0.30319.1 RTMel). There is only a project template (installer installed only the C# template) which you can choose and the only boxes where you can type something is the solution name and the search box.
A good programmer is someone who always looks both ways before crossing a one-way street. (Doug Linder)
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I suggest you read my reply again. I did not say it was anywhere near the new-project dialog; I said the filepath is specified when you SAVE the project, not when you CREATE the project.
Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.
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I think I better go grab a coffee first before replying first thing this morning 
A good programmer is someone who always looks both ways before crossing a one-way street. (Doug Linder)
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Helfdane wrote: When using Visual Studio 2010 (express?)...
Check the about-box
Helfdane wrote: ..if you click File => "New Project", where do you specify the directory on disk?
Using the "location" textbox.
Helfdane wrote: Why can't I start a project in D:\projects?
Dunno, but you could ask your sysadmin. I can imagine how those user-specific folders are part of a regular backup-scheme. Visual Studio usually places your projects under the Documents-folder of your current user, in a subfolder with the name of the IDE, in a subfolder called "projects".
..but you should also be able to use the D:\projects location. You might even force it by moving the entire user-specific folders to the root of your D-drve, having the "My Documents" folder point there.
Helfdane wrote: This is one of the things which drives me away from .net, even tho it's the platform of choice here...
You could switch IDE[^] of course.
I are Troll
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In the New Project window, click on the Browse button and navigate to the folder you wish to save your project.
"You get that on the big jobs."
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You mean you want it to go to the same place all the time and create the solution directories there?
Menu: Tools|Options|Projects and Solutions|General - Projects Location
In VS2010 Ultimate, anyway.
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GenJerDan wrote: In VS2010 Ultimate, anyway.
Express too.
Henry Minute
Do not read medical books! You could die of a misprint. - Mark Twain
Girl: (staring) "Why do you need an icy cucumber?"
“I want to report a fraud. The government is lying to us all.”
I wouldn't let CG touch my Abacus!
When you're wrestling a gorilla, you don't stop when you're tired, you stop when the gorilla is.
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Thanx!
At first I missed the option "show all options". There it was when I enabled it.
A good programmer is someone who always looks both ways before crossing a one-way street. (Doug Linder)
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Why does this event fire so often? I am using a binding source navigator for navigating a dataset, and it is giving me a world of grief. Whenever I click the next button I want to change the picture on the screen to reflect the new record. It looks like this:
DataRowView myRow = (DataRowView)this.dtObsDataBindingSource.Current;
try
{
this.pbPhoto.SizeMode = PictureBoxSizeMode.StretchImage;
this.pbPhoto.Image = Image.FromFile(myRow["txtPicPath"].ToString() + myRow["txtPhotoName"].ToString());
this.tbl_ref_categoryTableAdapter.Fill(this.ds_review.tbl_ref_category);
this.tbl_ref_defectTableAdapter.Fill(this.ds_review.tbl_ref_defect);
this.cboCategory.SelectedValue = myRow["intCategoryId"];
if (this.cboCategory.SelectedIndex != -1)
{
this.tbl_ref_defectTableAdapter.FillByCategoryId(this.ds_review.tbl_ref_defect, (int)this.cboCategory.SelectedValue);
}
this.cboDefect.SelectedValue = myRow["intDefectId"];
this.cboLocation.SelectedValue = myRow["intLocationId"];
}
catch (NullReferenceException)
{
this.pbPhoto.SizeMode = PictureBoxSizeMode.CenterImage;
this.pbPhoto.Image = Image.FromFile(System.Windows.Forms.Application.StartupPath.ToString() + "\\noPic.jpg");
}
Long story short, the fact that this event fires three times causes me some grief, what is causing it to fire so many times? Is there a better event choice to handle this?
Cheers, --EA
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Did you read the docs? Its *SUPPOSED* to get called a lot:
I know this isn't "CurrentItemChanged", but it says in the docs that "CurrentItemChanged" is equivalent to "CurrentChanged" and will get raised the same number of times:
The CurrentChanged event is raised whenever the Current property changes for any of the following reasons:
The current position of the List changes.
The DataSource or DataMember properties change.
The membership of the underlying List changes, which causes Position to refer to a different item. Examples include adding or deleting an item before the current item, deleting or moving the current item itself, or moving an item to the current position.
The underlying list is refreshed by a new sorting or filtering operation.
When this event is triggered, the Current property will already contain its new value.
CurrentChanged is the default event for the BindingSource class.
You simply need to filter out the conditions you don't care about. Or, if you don't have enough info to do that [EDIT: in the args I mean] (which is entirely possible), use a different event.
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No I did not read the documentation, it frequently muddies the waters unnecessarily and fails to explain what I need to understand. In this instance you are right that the eventargs do not provide enough information to help, but I was able to go off of your idea and add the following:
if (myRow != (DataRowView)this.tbl_cdf_observationBindingSource.Current)
Which allows me to avoid processing everything if the datarow has not actually changed.
Cheers, --EA
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Try using PositionChanged[^] instead, if all you require is to get the next/prev record.
The standard NavBar Next and Prev buttons just do Position +=(-=) 1;
Henry Minute
Do not read medical books! You could die of a misprint. - Mark Twain
Girl: (staring) "Why do you need an icy cucumber?"
“I want to report a fraud. The government is lying to us all.”
I wouldn't let CG touch my Abacus!
When you're wrestling a gorilla, you don't stop when you're tired, you stop when the gorilla is.
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