Problem with [DefaultValue()] inheriting ComboBox

Question

Hello, for a project of mine I inherited a ComboBox to change its size behaviour. In addition to this i wanted, to speed up my forms' creation, to set the default DropDownStyle to ComboBoxStyle.DropDownList

To do this i used the [Default()] command overwriting the DropDownStyle property

[DefaultValue(ComboBoxStyle.DropDownList)]
public new ComboBoxStyle DropDownStyle
{
    get
    {
        return base.DropDownStyle;
    }
    set
    {
        base.DropDownStyle = value;
    }
}

Then i modified the default value in the Designer setting the DropDownStyle to ComboBoxStyle.DropDownList.

And here comes the problem... There is a small number of InheritedComboBox which i want to have ComboBoxStyle.DropDown because they need to work with

AutoCompleteMode = System.Windows.Forms.AutoCompleteMode.Append;
AutoCompleteSource = System.Windows.Forms.AutoCompleteSource.ListItems;

If i set it from the Designer it works fine, however, sometimes, after i rebuild the form, it throws an exception (also at design time) regarding the ComboBoxStyle. When i look to the FormName.Designer.cs file, i can find that for the specific InheritedComboBox there is no

DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDown

and I have to add it manually. This is a little boring because sometimes i just notice it at runtime, when the program throws an exception without showing the form and i cannot test every form every time i rebuild...

Do you have any idea why i get this strange behaviour?

Is there any way to fix it?

Thanks a lot for any answer!

Answer 1

Try setting this value to the property in the constructor of the inherited combobox too to the value you set with DefaultValue. This should probably fix your issue.

Answer 2

When you set the AutoCompleteMode or AutoCompleteSource property, I believe the designer is looking to the base ComboBox and not generating the line to set the DropDownStyle, since DropDown is the default value for ComboBox.

I was able to correct this by adding an AutoCompleteMode and AutoCompleteSource property to the inherited ComboBox, but also had to add in a line to set the base DropDownStyle because of the order in which the designer sets the properties.

Try something like this and see if it works for you:

public class MyComboBox : ComboBox
{
    public MyComboBox()
    {
        DropDownStyle = ComboBoxStyle.DropDownList;
        AutoCompleteMode = AutoCompleteMode.None;
        AutoCompleteSource = AutoCompleteSource.None;
    }

    [DefaultValue(ComboBoxStyle.DropDownList)]
    public new ComboBoxStyle DropDownStyle
    {
        get { return base.DropDownStyle; }
        set { base.DropDownStyle = value; }
    }

    [DefaultValue(AutoCompleteMode.None)]
    public new AutoCompleteMode AutoCompleteMode
    {
        get { return base.AutoCompleteMode; }
        set 
        {
            if (value != AutoCompleteMode.None)
                base.DropDownStyle = ComboBoxStyle.DropDown;

            base.AutoCompleteMode = value; 
        }
    }
    [DefaultValue(AutoCompleteSource.None)]
    public new AutoCompleteSource AutoCompleteSource
    {
        get { return base.AutoCompleteSource; }
        set 
        {
            if (value != AutoCompleteSource.None)
                base.DropDownStyle = ComboBoxStyle.DropDown;

            base.AutoCompleteSource = value; 
        }
    }
}