Accessing UI (Main) Thread safely in WPF
C#.NetWpfMultithreadingDatagridC# Problem Overview
I have an application which updates my datagrid each time a log file that I'm watching gets updated (Appended with new text) in the following manner:
private void DGAddRow(string name, FunctionType ft)
{
ASCIIEncoding ascii = new ASCIIEncoding();
CommDGDataSource ds = new CommDGDataSource();
int position = 0;
string[] data_split = ft.Data.Split(' ');
foreach (AttributeType at in ft.Types)
{
if (at.IsAddress)
{
ds.Source = HexString2Ascii(data_split[position]);
ds.Destination = HexString2Ascii(data_split[position+1]);
break;
}
else
{
position += at.Size;
}
}
ds.Protocol = name;
ds.Number = rowCount;
ds.Data = ft.Data;
ds.Time = ft.Time;
dataGridRows.Add(ds);
rowCount++;
}
...
private void FileSystemWatcher()
{
FileSystemWatcher watcher = new FileSystemWatcher(Environment.CurrentDirectory);
watcher.Filter = syslogPath;
watcher.NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite
| NotifyFilters.FileName | NotifyFilters.DirectoryName;
watcher.Changed += new FileSystemEventHandler(watcher_Changed);
watcher.EnableRaisingEvents = true;
}
private void watcher_Changed(object sender, FileSystemEventArgs e)
{
if (File.Exists(syslogPath))
{
string line = GetLine(syslogPath,currentLine);
foreach (CommRuleParser crp in crpList)
{
FunctionType ft = new FunctionType();
if (crp.ParseLine(line, out ft))
{
DGAddRow(crp.Protocol, ft);
}
}
currentLine++;
}
else
MessageBox.Show(UIConstant.COMM_SYSLOG_NON_EXIST_WARNING);
}
When the event is raised for the FileWatcher, because it creates a separate thread, when I try to run dataGridRows.Add(ds); to add the new row, the program just crashes without any warning given during debug mode.
In Winforms, this was easily solved by utilizing the Invoke function but I am not sure how to go about this in WPF.
C# Solutions
Solution 1 - C#
You can use
Dispatcher.Invoke(Delegate, object[])
on the Application
's (or any UIElement
's) dispatcher.
You can use it for example like this:
Application.Current.Dispatcher.Invoke(new Action(() => { /* Your code here */ }));
or
someControl.Dispatcher.Invoke(new Action(() => { /* Your code here */ }));
Solution 2 - C#
The best way to go about it would be to get a SynchronizationContext
from the UI thread and use it. This class abstracts marshalling calls to other threads, and makes testing easier (in contrast to using WPF's Dispatcher
directly). For example:
class MyViewModel
{
private readonly SynchronizationContext _syncContext;
public MyViewModel()
{
// we assume this ctor is called from the UI thread!
_syncContext = SynchronizationContext.Current;
}
// ...
private void watcher_Changed(object sender, FileSystemEventArgs e)
{
_syncContext.Post(o => DGAddRow(crp.Protocol, ft), null);
}
}
Solution 3 - C#
Use [Dispatcher.Invoke(DispatcherPriority, Delegate)] to change the UI from another thread or from background.
Step 1. Use the following namespaces
using System.Windows;
using System.Threading;
using System.Windows.Threading;
Step 2. Put the following line where you need to update UI
Application.Current.Dispatcher.Invoke(DispatcherPriority.Background, new ThreadStart(delegate
{
//Update UI here
}));
> Syntax
>
> [BrowsableAttribute(false)]
> public object Invoke(
> DispatcherPriority priority,
> Delegate method
> )
>
> Parameters
>
> priority
>
> Type: System.Windows.Threading.DispatcherPriority
>
> The priority, relative to the other pending operations in the
> Dispatcher event queue, the specified method is invoked.
>
> method
>
> Type: System.Delegate
>
> A delegate to a method that takes no arguments, which is pushed onto
> the Dispatcher event queue.
>
> Return Value
>
> Type: System.Object
>
> The return value from the delegate being invoked or null if the
> delegate has no return value.
>
> Version Information
>
> Available since .NET Framework 3.0