如何添加功能以将空值添加到组合框

Question

I have many class to set ComboBox datasource

- Position [DISPLAY, VALUE, ID, NAME]
- Division [DISPLAY, VALUE, ID, NAME]
- SubDivision [DISPLAY, VALUE, ID, NAME, DIVISIONID]
- ect.

and i binding data

 List<Position> list = new List<Position>;
 list.Add(...);
 cboPosision.DataSource = list;

How to create method for ComboBox to insert row Null data

 private void SetDataSource(this ComboBox cbo, object dataList, bool IncludeAll)
 {
  if(includeAll) { dataList.Add(null); } //Need Insert object {DISPLAY:"All", VALUE:null}
  cbo.DataSource = dataList;
 }

Answer 1

Here is one way to do it:
Create a common interface that all your combobox items must implement:

interface IComboBoxItem 
{
    string Display {get;set;}
    object Value {get;set;
}

Then use a generic extension method to set that list as a data source:

private void SetDataSource<T>(this ComboBox cbo, IList<T> dataList, bool IncludeAll) where T : new, IComboBoxItem 
{
    if(includeAll) 
    { 
        dataList.Add(new T() {Display = "All", Value = null});
    }
    cbo.DisplayMember = "Display";  // This corresponds to the display member of the data object
    cbo.ValueMember = "Value";  // this corresponds to the value member of the data object
    cbo.DataSource = dataList;
}

Note: code written directly here, there might be some typos.