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    <p>not sure, but if you remove the <code>await</code> - does it help ?</p> <p>EDIT:</p> <p>I am no expert on C# 5 but what I gather that <code>await</code> wait for the launched task(s) to finish... it is a way to synchronize so the after the <code>await</code> the result be accessed without further checking whether the task(s) already finished... From the post I think that <code>await</code> is not needed and that it somehow "blocks" the <code>OnPropertyChange</code> call from the insise the launched Task.</p> <p>EDIT 2 - another try:</p> <pre><code>if (dlg.ShowDialog() == true) { string FN = dlg.FileName; Task.Factory.StartNew(() =&gt; ReadExcelFile(FN)); } </code></pre> <p><strong>EDIT 3 - solution (without C# 5 though):</strong></p> <p>I created a fresh WPF app, put 2 buttons (<code>button1</code> => select excel file, <code>button2</code> => Save) in the designer... I removed all "<code>OnPropertyChanged</code>" calls (I used <code>this.Dispatch.Invoke</code> instead)... <code>RelayCommand</code> is 1:1 from <a href="http://msdn.microsoft.com/en-us/magazine/dd419663.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/magazine/dd419663.aspx</a> ... following is the relevant changed source:</p> <pre><code>private void SelectExcelFile() { var dlg = new Microsoft.Win32.OpenFileDialog(); dlg.DefaultExt = ".xls|.xlsx"; dlg.Filter = "Excel documents (*.xls, *.xlsx)|*.xls;*.xlsx"; if (dlg.ShowDialog() == true) { Task.Factory.StartNew(() =&gt; ReadExcelFile(dlg.FileName)); } } private List&lt;int&gt; _fileContents = new List&lt;int&gt;(); public List&lt;int&gt; FileContents { get { return _fileContents; } set { if (value != _fileContents) { _fileContents = value; this.Dispatcher.Invoke ( new Action (delegate() { button2.IsEnabled = true; button2.Command = SaveCommand; }),null); } } } private void button1_Click(object sender, RoutedEventArgs e) { button2.IsEnabled = false; button2.Command = null; SelectExcelFileCommand.Execute(null); } private void button2_Click(object sender, RoutedEventArgs e) { SaveCommand.Execute(null); } </code></pre> <p>all problems described by the OP are gone: the Excel reading is on another thread... the UI does not freeze... the <code>Savecommand</code> gets enabled if the Excelreading is successfull...</p> <p>EDIT 4:</p> <pre><code> this.Dispatcher.Invoke(new Action(delegate() { CommandManager.InvalidateRequerySuggested(); }), null); </code></pre> <p>you can use this instead of the <code>IsEnabled</code>... causes the <code>CanExecuteChanged</code> event to be fired without "rebuilding" the <code>SaveCommand</code> (which causes the <code>CanExecuteChanged</code> event to be unregistered and then reregistered)</p>
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    1. POUsing an OleDbDataAdapter on a background thread causes my UI to not update correctly?
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    1. USYahia
    1. USYahia
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    1. COI already said it fixes the problem, but it also stops the application from reading the excel file on another thread, so it locks up while it is reading the file. The whole point of putting it on another thread was to stop it from freezing while reading files so users could continue to enter data.
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    2. COStartNew() already accomplishes what you want (starting it on another thread)... await just says "wait till the Task is finished... try my EDIT 2...
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    3. COI need the result of the excel file to know if the Save command can execute or not though. If the result is an invalid or empty list, the Save command cannot be executed. Also, the `Task.Factory.StartNew()` returns a `Task<List<int>>` object, not a `List<int>`. It is meant to be used with `await`
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