如何将ComboBox文本设置为不在DataSource（C＃）中的值？

Question

I have multiple DropDown-style ComboBoxes on a form in which the user is able to specify which units are associated with a value (think meters, feet, etc.). To do this, I use an application string collection in Settings.Settings with some default unit types which is bound to the DataSource of the ComboBox as follows:

this.cboUnit.DataSource = 
    (System.Collections.Specialized.StringCollection)Properties.Settings.Default.Units;

The user is able to successfully pick a unit or specify a new one.

Now, when I save this.cboUnit.Text into some XML profile, I am able to handle both cases. Opening the XML profile and performing this.cboUnit.Text = "NonExistentUnit"; will fail and cause the first item in the collection to be selected instead.

How can I modify the ComboBox to support the kind of behavior I'm asking, without adding the item to the ComboBox or modifying the DataSource itself ?

There was a solution suggested in Using ComboBox.Text when combobox is tied to DataSource but I couldn't find official documentation to support this behavior. For that reason I prefer to avoid it.

Answer 1

I could not find any valid way to insert items into a bound DataSource. So instead I ended up doing the following:

BindingSource bindingSource = new BindingSource((System.Collections.Specialized.StringCollection)Properties.Settings.Default.Units, "");
if(!bindingSource.Contains(someSavedValue))
{
    bindingSource.Insert(0, someSavedValue));
}
this.cboUnit.DataSource = bindingSource;

This creates a new instance of the binding source that can be modified prior to binding it to a data source. I wrapped this code in a method for which I can pass in a string collection and the stored value ( savedUnit in this case) that returns a new BindingSource instance. This is especially useful in my case, as I can now simply write:

this.cboLengthUnit.DataSource = CreateBindingSource(unitsCollection, savedLengthUnit);
this.cboWidthUnit.DataSource = CreateBindingSource(unitsCollection, savedWidthUnit);
this.cboHeightUnit.DataSource = CreateBindingSource(unitsCollection, savedHeightUnit);

I wrote the code above from memory, so it may contain a mistake.