[CheckBox] Setting Checked = true doesn't work for Indeteminate state

[jmairboeck]

I'm not sure if this is a bug or not.

If a CheckBox's CheckState is Indeterminate, setting Checked = true (programmatically) doesn't work because the setter checks whether the new value differs from what the Checked getter returns (which is true for Indeterminate).

I expected that setting Checked = true would always be equivalent to setting CheckState = CheckState.Checked, so to actually change the state if it was Indeterminate before, even if the Checked getter already returns true before.

[LeafShi1]

This is expected behavior of the WinForms CheckBox control.
When ThreeState = false, the control internally supports only two states: Checked and Unchecked.
If you assign CheckState.Indeterminate in this mode, the framework automatically treats it as Checked.
The logical state becomes Checked, but the visual glyph may not repaint immediately unless the control is explicitly redrawn.
So even if the Indeterminate icon still appears on screen, the underlying state is already Checked.
This is a limitation of the Win32-based rendering rather than a functional bug.

[jmairboeck]

No, this is not a limitation of Win32. The CheckStates Indeterminate and Checked are distinct. I was referring to this check, where the Checked setter thinks that the value is already correct and skips the assignment if the CheckState is Indeterminate, because the Checked getter already returns true in this case.

I am not sure if this is by design (for consistency between the getter and setter of Checked), so I didn't create an issue for it right away. Also, there is an easy workaround by using CheckState = CheckState.Checked instead of Checked = true.

[jmairboeck]

In short, I am asking whether the Checked setter should be changed to something like this:

set
{
    var newCheckState = value ? CheckState.Checked : CheckState.Unchecked;
    if (newCheckState != CheckState)
    {
        CheckState = newCheckState;
    }
}

Although the check is probably even redundant, because the CheckState setter already checks for changes itself before doing anything.

[LeafShi1]

This change effectively prevents invalid operations when the control is in the Indeterminate state, aligning with developers' expectations (Checked = true will display a clear checked mark), improving the consistency of data binding/events, and reducing visual inconsistencies.

However, this may lead to behavioral changes for applications that rely on Checked = true being an invalid operation in the three-state mode.

If this is blocking your scenario, please let us know your constraints so we can assess a safe path (e.g., an opt‑in change or guidance).

[jmairboeck]

Thank you for the answer. Compatibility is a concern of course, so I think it is best to keep it like this. As I wrote above, there is an easy workaround using CheckState.