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System.Windows.Forms.ListBox lbPlatforms;

lbPlatforms.DataSource = Platform.SortByName(Platform.All);
lbPlatforms.SelectionMode = SelectionMode.MultiExtended; // oder MultiSimple
lbPlatforms.FormattingEnabled = true;
lbPlatforms.DisplayMember = "Name";
lbPlatforms.ValueMember = "Id";					

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foreach(int nIdx in lbPlatforms.SelectedIndices)
{
    Platform platform = lbPlatforms.Items[nIdx] as Platform;
    if(platform != null)
    {
        int nPlatformId = platform.Id;
    }
}					

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foreach(Platform platform in lbPlatforms.SelectedItems)
{
    int nPlatformId = platform.Id;
}