System.Win32.OpenFileDialog dialog = new System.Win32.OpenFileDialog();
dialog.AddExtension = true;
dialog.CheckPathExists = true;
dialog.DefaultExt = ".xaml";
dialog.Filter = "Xaml files (.xaml)|*xaml|All files|*.*";
dialog.FilterIndex = 0;

Boolean? result = dialog.ShowDialog();
if (result.HasValue && result.Value)
{
   // business logic
}

Filter = "Xaml files (.xaml)|*xaml|All files|*.*",

Filter = "Xaml files (.xaml)|*.xaml|All files|*.*",

System.Win32.OpenFileDialog dialog = new System.Win32.OpenFileDialog()
{
    AddExtension = true,
    CheckPathExists = true,
    DefaultExt = ".xaml",
    Filter = "Xaml files (.xaml)|*xaml|All files|*.*",
    FilterIndex = 0
};

private void FileOpen()
{
    Boolean? result = dialog.ShowDialog();
    if (result.HasValue && result.Value)
    {
        // business logic
    }
}