How to detect modifier key states in WPF?

Is there some global constructs that I can use whenever I need to access whether the Control, Shift, Alt buttons are down? For instance inside MouseDown event of a TreeView.

If so how?

Use class Keyboard. Using Keyboard.IsKeyDown you can check if Control, Shift, Alt is down now.

For Shift:

if (Keyboard.IsKeyDown(Key.LeftShift) || Keyboard.IsKeyDown(Key.RightShift))
{ /* Your code */ }

For Control:

if (Keyboard.IsKeyDown(Key.LeftCtrl) || Keyboard.IsKeyDown(Key.RightCtrl))
{ /* Your code */ }

For Alt:

if (Keyboard.IsKeyDown(Key.LeftAlt) || Keyboard.IsKeyDown(Key.RightAlt))
{ /* Your code */ }

There's also:

// Have to get this value before opening a dialog, or user will have released the control key
if ((Keyboard.Modifiers & ModifierKeys.Control) == ModifierKeys.Control)
{

}

    private bool IsShiftKey { get; set; }

    private void OnPreviewKeyDown(object sender, KeyEventArgs e)
    {
        IsShiftKey = Keyboard.Modifiers == ModifierKeys.Shift ? true : false;

        if ((Key.Oem3 == e.Key || ((IsShiftKey && Key.Oem4 == e.Key) || (IsShiftKey && Key.Oem6 == e.Key) || (IsShiftKey && Key.Oem5 == e.Key)) && (validatorDefn as FormatValidatorDefinition).format == "packedascii"))
        {
           e.Handled = true;
        }
    }

This is how I handle it (using PreviewKeyDown), let's say we are looking for Alt + R...

private void OnPreviewKeyDown(object sender, KeyEventArgs e)
{
    if ((Keyboard.IsKeyDown(Key.LeftAlt) || Keyboard.IsKeyDown(Key.RightAlt)
       && e.SystemKey == Key.R)
    {
       //do whatever
    }
}

Maybe someone can clear up why I had to use e.SystemKey and not just e.Key, maybe due to the modifier? but this has worked flawlessly for me when searching for modifier+key.

Partly borrowing from @Josh, and somewhat similar to @Krushik, and also referencing a question about the <https://stackoverflow.com/questions/16668798/difference-between-keyeventargs-systemkey-and-keyeventargs-key> (answering why Josh has to use SystemKey); wherein, with modifier keys (such as Alt), e.Key returns Key.System, and thus the 'real' key is within e.SystemKey.

A way around this, is to first fetch the 'real' key, and then do your conditional:

private void OnPreviewKeyDown(object sender, KeyEventArgs e)
{
    // Fetch the real key.
    var key = e.Key == Key.System ? e.SystemKey : e.Key;

    if ((Keyboard.IsKeyDown(Key.LeftAlt) || Keyboard.IsKeyDown(Key.RightAlt))
        && key == Key.Return)
    {
        // Execute your code.
    }
}

and as well:

if My.Computer.Keyboard.ShiftKeyDown then ...

My.Computer.Keyboard.CtrlKeyDown

My.Computer.Keyboard.AltKeyDown