How to bind a nullable bool to a checkbox?
bool? foo = null;
checkBox1.CheckState = foo.HasValue ?
    (foo == true ? CheckState.Checked : CheckState.Unchecked) :
    CheckState.Indeterminate;

public static CheckState ToCheckboxState(this bool booleanValue)
    {
        return booleanValue.ToCheckboxState();
    }

    public static CheckState ToCheckboxState(this bool? booleanValue)
    {
        return booleanValue.HasValue ?
               (booleanValue == true ? CheckState.Checked : CheckState.Unchecked) :
               CheckState.Indeterminate;
    }

if(foo.HasValue)
{
    if(foo == true)
       checkBox1.CheckState = CheckState.Checked;
    else
       checkBox1.CheckState = CheckState.Unchecked;
}
else
    checkBox1.CheckState  = CheckState.Indeterminate;

checkBox1.CheckState = GetCheckState(foo);

public CheckState GetCheckState(bool? foo)
{
    if(foo.HasValue)
    {
        if(foo == true)
           return CheckState.Checked;
        else
           return CheckState.Unchecked;
    }
    else
        return CheckState.Indeterminate

}

public static void SetCheckedNull(this CheckBox c, bool? Value)
{
    if (!c.ThreeState)
        c.Checked = Value == true;
    else
        c.CheckState = Value.HasValue ?
            (Value == true ? CheckState.Checked : CheckState.Unchecked) :
            CheckState.Indeterminate;
}

checkBox1.Checked = someBool;

checkBox2.SetCheckedNull(someNullableBool);