|
I am getting the classic "Object reference not set to an instance of an object." error inside a System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg) call.
Before you tell me the canned response about null references, here is the trace:
System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.ComponentManager.System.Windows.Forms.UnsafeNativeMethods+IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at eCandidus.Master.Main() in C:\Enterprise Candidus\eCandidus\eCandidus\Master.cs:line 104
The error happens randomly while scrolling a dropdown list combo box, and happens in different combo boxes in different forms. Note that is being caught in a last resort try/catch at the very outmost call posible.
Anyone run into this one before?
|
|
|
|
|
Hi,
my question is.. im using winforms..where one form needs a connection to a SQL Server 7.0 database..can anyone tell me what are the classes involved is it :
System.Data; and System.Data.SqlClient; ??
and also how do i code the connection part...some example would shed some light...i've searched online and got many examples but they're very confusing.
forgive me if im asking for too much...im still new to C#..
plus where can i get the .Net SDK documentation.
Arvinder Gill
ArvinderGill
|
|
|
|
|
You can get the .NET Framework SDK both online and for downloading from http://msdn.microsoft.com/netframework[^].
There are MANY examples - both simple and advanced - dealing with ADO.NET in there. Rather than typing out a lot of code and explaining it as many other articles have done and what the .NET Framework SDK covers in the ADO.NET sections, I'll just let you read it.
Microsoft MVP, Visual C#
My Articles
|
|
|
|
|
I'm creating a tool that queries game servers on the internet.
Though I seem to be having a weird problem when it comes to threading.
I managed to get round the problem of not being able to create threads and pass parameters to those methods.
There is a public variable called queryThread. For example it is: 64. The following code loops 64 times creating threads called queryServerRange. Each with the name of its thread number. (I use this number later on for the thread to figure out which servers it will query).
int i = 0;
while (i < queryThread)
{
i++;
Thread queryRangeThread = new Thread( new ThreadStart( queryServerRange ) );
queryRangeThread.Name = i.ToString();
queryRangeThread.Start();
Random R = new Random();
Thread.Sleep(R.Next(200));
}
Now the following code then first of all figures out which thread number it is,
how many servers there are to be queried in total (from the hashtable servers.Count).
In knowing its thread number, I do some maths to figure out the range this thread will handle.
i.e. server number 0 to 128.
I did it in this way also in an attempt to minimize deadlock/shared variable problems.
However my problem is this: intermittently the whole program will freeze in what I can only see as deadlock,
as yet I can't seem to find a solution.
There are occasions when the problem will not occur, but I can only see this
as being a fluke and down to servers responding in time, etc.
An earlier problem I had was in adding items to the listview below (some items were not appearing),
but I solved this problem with a monitor (Monitor.Enter(servers);).
I looked at doing the same whilst assigning myServer and calling .Refresh() however this slowed the program
right down and made it no faster than querying the servers one by one.
Another strange thing I've noticed, is that the lockups only seem to occur inside the Visual Studio IDE,
when I run the program completely outside of the IDE it doesn't appear to lock up ever.
Does anyone have any smart ideas of how I can stop my code locking up?
Thanks,
Peter.
Here's the code...
private void queryServerRange()
{
int remainder;
int number = Math.DivRem(int.Parse(servers.Count.ToString()), queryThread, out remainder);
int threadNumber = int.Parse(Thread.CurrentThread.Name);
int startPoint;
if (threadNumber == 1)
startPoint = number * (threadNumber - 1);
else
startPoint = number * (threadNumber - 1) + 1;
int endPoint;
if (threadNumber.Equals(queryThread) && !remainder.Equals(0))
endPoint = (number * threadNumber) + remainder;
else
endPoint = number * threadNumber;
//MessageBox.Show("start: " + startPoint.ToString() + " end: " + endPoint.ToString());
try
{
while (startPoint < endPoint)
{
if (status.pressedCancel)
{
Thread.CurrentThread.Abort();
}
Random R = new Random();
Thread.Sleep(R.Next(50));
int listViewIndex = -1;
object obj1 = servers[(long)startPoint];
GameServer myServer = new GameServer();
myServer = (GameServer) obj1;
// Set the servers' time until it gives up trying to contact it.
myServer.Timeout = timeout;
myServer.Refresh();
Monitor.Enter(servers);
// If the server is not in listViewServers and the server has the same filter,
// then add this server into the listView
if (listViewIndex == -1 && (filter.Equals(myServer.Filter.ToString()) || filter.Equals("") ))
{
if (! myServer.HostName.Equals("Timed Out"))
{
//Create a listItem so that the server can be added in one go to the listViewServer control
System.Windows.Forms.ListViewItem theServer = new System.Windows.Forms.ListViewItem(myServer.HostName.ToString());
//Add the servers' host name.
System.Windows.Forms.ListViewItem.ListViewSubItem item1 = new System.Windows.Forms.ListViewItem.ListViewSubItem (theServer, myServer.Ip.ToString() + ":" + myServer.Port.ToString());
//Add the servers' ip address and port number.
System.Windows.Forms.ListViewItem.ListViewSubItem item2 = new System.Windows.Forms.ListViewItem.ListViewSubItem (theServer, myServer.MapName.ToString());
//Add the number of players and total number of players allowed.
System.Windows.Forms.ListViewItem.ListViewSubItem item3 = new System.Windows.Forms.ListViewItem.ListViewSubItem (theServer, myServer.NumPlayers.ToString() +"/" + myServer.MaxPlayers.ToString());
//Add the servers' ping time.
System.Windows.Forms.ListViewItem.ListViewSubItem item4 = new System.Windows.Forms.ListViewItem.ListViewSubItem (theServer, myServer.Ping.ToString()+ "ms");
//Add if the server is a favourite.
System.Windows.Forms.ListViewItem.ListViewSubItem item5 = new System.Windows.Forms.ListViewItem.ListViewSubItem (theServer, "");
//Add the SubItems to the item.
theServer.SubItems.Add(item1);
theServer.SubItems.Add(item2);
theServer.SubItems.Add(item3);
theServer.SubItems.Add(item4);
theServer.SubItems.Add(item5);
theServer.Tag = myServer.serverID.ToString();
//Add the item to the listView.
listViewServers.Items.Add(theServer);
}
}
Monitor.Exit(servers);
status.labelRefresh.Text = "Refreshing Servers (" + (status.progressBarDownloading.Value) +"/" + status.progressBarDownloading.Maximum.ToString() + ")...";
status.progressBarDownloading.Value = status.progressBarDownloading.Value + 1;
startPoint++;
}
Thread.CurrentThread.Abort();
}
catch (Exception ex)
{
MessageBox.Show(ex.ToString());
}
}
}
|
|
|
|
|
You can use the lock keyword against an instance of an object (use a static object like the Type - using typeof(YourClass) - for static objects, and an instance member for instance resources). This uses the Monitor when it compiles, but it does so in a better way than what you've done (which doesn't take exceptions into account). For example, the following block:
lock(typeof(MyClass))
{
}translates to
Monitor.Enter(typeof(MyClass));
try
{
}
finally
{
Monitor.Exit(typeof(MyClass));
}This way, if an exception occurs where the comment is, the handle is unlocked. There are many other locking mechanisms you should read about in the System.Threading namespace class documentation in the .NET Framework SDK.
Note also that any implementation of ICollection must implement a SyncRoot (a property that returns an object that can be locked) that you can use to synchronize access to a collection or list. Other classes have such support, too, and some even can generate a synchronized instance for you (see Hashtable.Synchronized, for example).
Microsoft MVP, Visual C#
My Articles
|
|
|
|
|
I'm trying to use WNetEnumResource() in C#. I've imported the functions from the system DLL's, allocated buffer memory etc but there's one little step that's got me beat. My code looks approximately like this (extraneous guff removed).
[StructLayout(LayoutKind.Sequential)]
private struct NETRESOURCE
{
public uint dwScope;
public uint dwType;
public uint dwDisplayType;
public uint dwUsage;
public string lpLocalName;
public string lpRemoteName;
public string lpComment;
public string lpProvider;
};
<p>
[DllImport("Mpr.dll", EntryPoint="WNetEnumResourceW",
SetLastError=true, CallingConvention=CallingConvention.Winapi)]
<p>
unsafe private static extern ErrorCodes WNetEnumResource(
IntPtr hEnum, ref uint lpcCount, byte *buffer, ref uint lpBufferSize);
<p>
private unsafe void EnumerateServers()
{
uint bufferSize = 16384;
byte *buffer = (byte *) Marshal.AllocHGlobal((int) bufferSize);
IntPtr handle = (IntPtr) 0;
ErrorCodes result;
uint cEntries = 1;
<p>
result = WNetOpenEnum(ResourceScope.RESOURCE_GLOBALNET, ResourceType.RESOURCETYPE_ANY, 0, 0, out handle);
<p>
if (result == ErrorCodes.NO_ERROR)
{
aData.Add("WNetOpenEnum succeeded");
<p>
do
{
result = WNetEnumResource(handle, ref cEntries, buffer, ref bufferSize);
<p>
if (result == ErrorCodes.NO_ERROR)
{
NETRESOURCE *p = (NETRESOURCE *) buffer;
}
else if (result != ErrorCodes.ERROR_NO_MORE_ITEMS)
break;
} while (result != ErrorCodes.ERROR_NO_MORE_ITEMS);
}
else
aData.Add("WNetOpenEnum failed");
<p>
Marshal.FreeHGlobal((IntPtr) buffer);
}
As you can see, I've declared the 3rd parameter (the buffer) in the call to WNetEnumResource() function as a byte pointer and used the Marshal.AllocHGlobal to allocate a buffer. Single stepping through the code shows that the initial WNetOpenEnum() succeeds as does the WNetEnumResource() call.
What I can't seem to do (no matter what I try) is to cast the buffer to a pointer to a NETRESOURCE structure. The compiler informs me that I can't take the address or the size of a managed type.
I've probably missed something blindingly obvious  but any hints or help would be appreciated.
Rob Manderson
Paul Watson wrote:What sense would you most dislike loosing?
Ian Darling replied.
Telepathy
Then I'd no longer be able to find out everyones dirty little secrets The Lounge, December 4 2003
|
|
|
|
|
And, of course, after posting this I got to thinking and realised I'm bringing a c++ approach to the problem. A bit of spelunking around in System.Runtime.InteropServices revealed the Marshal.PtrToStructure() function which, after changing the NETRESOURCE into a class rather than a struct solves the problem nicely.
Is there a better approach?
Rob Manderson
Paul Watson wrote:What sense would you most dislike loosing?
Ian Darling replied.
Telepathy
Then I'd no longer be able to find out everyones dirty little secrets The Lounge, December 4 2003
|
|
|
|
|
You don't need to use an unsafe context. Instead of using byte* just use ref byte; ref and out should typically be used for pointers to value types, while they shouldn't be used for reference types - since they're already references - exception when a pointer to a pointer is needed.
Instead of declaring your param as an IntPtr for your struct or enum, you can typically use the ref keyword as well. Take, for example, the typical declaration of SendMessage:
[DllImport("user32.dll")]
private static extern IntPtr SendMessage(IntPtr hWnd, int handle,
IntPtr wParam, IntPtr lParam);This means that if you want to pass a pointer to a struct (say, for a window message to a common control) that you have to use the method Marshal.StructureToPtr and then pass it. Instead, a much easier - and perfectly ligitimate - alternative would be like so:
[DllImport("user32.dll")]
IntPtr SendMessage(IntPtr hWnd, int msg,
ref MyStruct wParam, IntPtr lParam);All this is done without requiring an unsafe context, which is typically only useful for when you have to perform address references or allocate memory for native calls without using CoTaskMemAlloc or GlobalAlloc (which are available from the Marshal as managed calls).
Keep these things in mind, remember that enums are value types too, and you should find a better, easier alternative.
Microsoft MVP, Visual C#
My Articles
|
|
|
|
|
You're absolutely right. I iterated my way to a working solution using the unsafe keyword and then, following your advice here, removed it and it still compiles and works.
I kinda feel much like I did 15 or so years ago when I was learning how many *'s to bung in front of a pointer to dereference it  Heh I'm still in the early stages of learning C#
My final solution looks like this:
[DllImport("Mpr.dll", EntryPoint="WNetEnumResourceA",
CallingConvention=CallingConvention.Winapi)]
<p>
private static extern ErrorCodes WNetEnumResource(
IntPtr hEnum, ref uint lpcCount, IntPtr buffer,
ref uint lpBufferSize);
I guess I have a little way to go in unlearning the pointer way. Nonetheless, I still had problems with trying to handle the IntPtr buffer parameter. I could see that I was getting back the correct values but could see no way to convert the buffer into a struct (in the sense of being able to use struct members to refer to the various fields).
I changed the struct to a class (thanks to Inside C# by Tom Archer) and wrote this line...
Marshal.PtrToStructure(buffer, p);
where p is a NETRESOURCE instance and now I can reference each value using p. syntax. You can see that I was thinking in c++ terms and trying to cast a pointer to a different type.
Hmmm this works but I can't see how to call the WNetEnumResource function in such a way as to return more than 1 instance of the NETRESOURCE structure. The Marshal.PrtToStructure method gives me no way to step down the buffer to the second instance, or the third, of the NETRESOURCE structure within the buffer given that I cannot take a sizeof or the address of a managed object. I can see why the compiler forbids me this
On the other hand, I can't see how to do it in c++ either This is a port of some code I wrote in c++ and, single stepping through the c++ code, I see that each time (whatever value I set as the count of entries to be returned) it returns 1 entry. This makes sense actually - the MSDN entry for the WNetEnumResource API says that the buffer must be large enough to contain not only the NETRESOURCE structure itself but also the strings it points to (which is why I allocated a very large buffer). I'm guessing that the API fills in the structure values and copies the strings immediately afterward - and if it does that then one can't use standard c++ pointer arithmetic to step down the buffer.
Rob Manderson
Paul Watson wrote:What sense would you most dislike loosing?
Ian Darling replied.
Telepathy
Then I'd no longer be able to find out everyones dirty little secrets The Lounge, December 4 2003
|
|
|
|
|
You actually should stick with a struct to keep it allocated on the stack as opposed to taking the risk that an instance of it will be kept in the heap. Who knows what problems this could cause with the unmanaged code.
There are plenty of ways to convert structs to pointers and pointers to structs (after all, a buffer begins with a pointer and is a certain size). If you review all the methods of the Marshal class, there are several methods to get you exactly what you need, such as Marshal.UnsafeAddrOfPinnedArrayElement, which wraps the unsafe code in a trusted assembly (a base class library assembly) and returns you an IntPtr. If the method takes an address of a struct, you should be able to pass an array of that struct (without a ref since an array is a reference type already). When you get it back, you can either treat the array as-is or use the aforementioned method.
Microsoft MVP, Visual C#
My Articles
|
|
|
|
|
I have a Panel with AutoScroll set to true and when using the mouse to move the scroll bar the Panel position is updated only when the mouse button is released. How can I make the panel scroll while holding down the mouse button and moving the scroll bar?
|
|
|
|
|
Works fine for me in both .NET 1.0 and 1.1. Are you sure you're using the System.Windows.Forms.Panel from the .NET base class libraries?
Microsoft MVP, Visual C#
My Articles
|
|
|
|
|
Yes, I am using System.Windows.Forms.Panel but I have not set up any events to specifically react on the scrollbar moving, is that necesssary? The funny thing is that it works fine when using the mouse wheel to scroll just not when clicking on the scroll bar, holding down left mouse button and moving the mouse. Any other ideas to what can cause this?
|
|
|
|
|
There are no events for a such control exposed by classes derived from ScrollableControl. In fact, the only way to know when something is being scrolled is to override WndProc and handy notification window messages the "old fashion way" (if you've done any Win32 programming, I'm sure you'll understand).
What version of .NET are you using? Scrolling should work just fine in any event. I've tried in 1.1 just now (our application at work is still 1.0 and it works correctly) and it works for keyboard and mouse events both (i.e., it continuously scrolls). Any other conditions you think are worth mentioning (like performing owner draw or something)?
Microsoft MVP, Visual C#
My Articles
|
|
|
|
|
I have a COM method that requires a GUID to the interface used by a particular Item.
I'm not sure how to get the GUID via c# (I can look it up in shell32 though), or how to convert it to a COM syntax when I have it.
I've checked the net, but there is almost nothing on this subject.
So how do i pass a GUID from C# to COM? C# handles it as a string... is this how I should proceed?
Cheers
Cata
|
|
|
|
|
See the System.Guid structure, which marshals to the GUID structure in C/C++. This even has methods for creating an instance of the Guid struct from a string.
BTW, COM is not a syntax - it's a programming concept. GUIDs can be expressed in many different ways, from a nested array of bytes to a string. Since both are typically in hex form, this is trivial to translate.
And if you're wanting to use a GUID from an attributed interface already declared, you can use Marshal.GenerateGuidForType to get the GUID (or to generate one if a Type is not attributed with the GuidAttribute).
Microsoft MVP, Visual C#
My Articles
|
|
|
|
|
I'm trying to call the following line of code:
Guid myGUID = Marshal.GenerateGuidForType(System.Type.GetType("IShellFolder"));<br />
But I always end up with a null pointer and an exception. The type ISHellFolder is defined as:
<br />
[Guid("000214E6-0000-0000-C000-000000000046"),<br />
InterfaceType(ComInterfaceType.InterfaceIsIUnknown)] <br />
interface IShellFolder<br />
{<br />
...<br />
...<br />
}<br />
Not entirely sure why it's not working.
Cata
|
|
|
|
|
The Catalyst wrote:
Guid myGUID = Marshal.GenerateGuidForType(System.Type.GetType("IShellFolder"));
But I always end up with a null pointer and an exception.
Thats becos Type.GetType() requires a fully qualified assembly name, hence returning null and causing your exception.
leppie::AllocCPArticle("Zee blog");
Seen on my Campus BBS: Linux is free...coz no-one wants to pay for it.
|
|
|
|
|
Type.GetType requires a fully-qualified class name (includes namespace) and assembly name if defined in a different assembly (and, optionally, other Type information). That's the first thing wrong with it.
I also mentioned previously in this thread to use typeof when you know the type at design-time.
Guid guid = Marshal.GenerateGuideForType(typeof(IShellFolder)); Did you even look at the documentation for Type.GetType? You really need to start reading the .NET Framework SDK documentation and pick up a book like I gave you a link for before on COM interoperability with .NET, although this particular question is just a fundamental type issue with the .NET Framework (using C# syntax).
Microsoft MVP, Visual C#
My Articles
|
|
|
|
|
I have not got books, but I do have many articles, tutorials, and the SDK which I am working through. While I understand the use of books, I find I have trouble working from them. To be honest, the only thing i've read on programming was the first 40 pages of a Java book. Beyond that i've learned everything myself, C#, HTML etc.
I was unaware of 'typeof', i'm sure i'll use it in the future. However, I still have a problem with my function SHBindToParent.
As for my problem with COM that this question derived from. I wasn't marshaling the GUID as a structure:
[MarshalAs(UnmanagedType.LPStruct)]
Once this was done, It worked fine.
As always your help is much appreciated.
Cata
|
|
|
|
|
|
Is it possible to create an instance of an object that has been passed through as a method parameter with a ref keyword ?
public class BasicClass{...}
public class NewClass: BasicClass{...}
...
Something(ref NewClass)
...
public BasicClass Something(ref BasicClass aclass)
{
return new aclass();
}
Regards, Desmond
|
|
|
|
|
You should look at making a class factory. In particular you should look at Reflection and the Activator.CreateInstance() method.
- Nick Parker
My Blog
|
|
|
|
|
Something like
return (BasicClass)Activator.CreateInstance(aclass.GetType()); should work.
Charlie
if(!curlies){ return; }
|
|
|
|
|
Could BasicClass implement ICloneable so that in method Something, you return aclass.Clone()?
|
|
|
|
|
General 
News 
Suggestion 
Question 
Bug 
Answer 
Joke 
Praise 
Rant 
Admin 
Submit an article or tip
