C# method listbox or combobox as parameter

Probably the simplest approach is Data Binding. For example:

List<string> items = new List<string> { "Item1", "Item2", "Item3" };
listBox1.DataSource = items;
comboBox1.DataSource = items;

If you want to use an explicit loop, then try something like this:

List<string> items = new List<string> { "Item1", "Item2", "Item3" };
FillListBoxOrComboBox( listBox1, items );
FillListBoxOrComboBox( comboBox1, items );

where

static void FillListBoxOrComboBox( dynamic control, IEnumerable<string> items )
{
 foreach( var i in items )
 {
 control.Items.Add( i );
 }
}

Also note that both of ListBox and ComboBox are derived from ListControl.

Hello @Saga ,

If you want both controls to be independent when traversing. You need to clone the list.

Add this class to your project.

static class Extensions  
{  
    public static IList<T> Clone<T>(this IList<T> listToClone) where T : ICloneable =>   
        listToClone.Select(item => (T)item.Clone()).ToList();  
}  

Then to populate the controls.

var firstNamesList = new List<string> { "Karen", "Jane", "Mike" };  
listBox1.DataSource = firstNamesList;  
comboBox1.DataSource = firstNamesList.Clone();  

Thank you both for your replies.

After more research, it seems that what I want done is not possible, but after tweaking the routine, I was able to successfully load a combo box or list box.

Here is the code:

private void LoadCoData(ListControl MyList)
{
    //Routine to load companies into list.

    //Clear data source.
    MyList.DataSource = null;

    //Clear list.  *****
    //MyList.Items.Clear();

    //Build the SQL to load the names.
    string sSQL = "select CompanyId,CompanyName from Companies where Co_Group = 17";

    //Get names.
    DataTable dtData = cDBFun.ExecSQLCommand(cDBFun.dbOptions.Query, sSQL).Tables[0];

    //Load the list.
    MyList.DisplayMember = "CompanyName";
    MyList.ValueMember = "CompanyId";
    MyList.DataSource = dtData;

}

My problem was with the Items.Clear() statement that's commented out. The Items collection was not found in the ListControl, so it was generating an exception. However, this statement is not necessary because the list clears whenever it gets a datasource.

I am interested in the techniques that you both provided and will be looking into these for future use, Again, I appreciate your help. Saga