组合框选中的项目为空，不显示DB值

我有一个组合框cboPrePartNum:

• 数据源- prepartBindingSource
• Display Member - PrePartNum
• Value Member - PrePartNum
• 选择值- engordbindingsource - prepartnum

当表单加载时，运行:

private void LoadComboBoxes()
{
    List<PartPrevious> partPreviousList;
    partPreviousList = PartDB.GetPartPreviousList();
    cboPrePartNum.DataSource = partPreviousList;
    cboPrePartNum.SelectedIndex = -1;
}

问题是当表单加载cboPrePartNum组合框不显示当前在engordbindingsource.prepartnum的值。

PartPrevious是这样的:

public class PartPrevious
{
    private string prepartnum;
    private string prerevision;
    private string prepartdescription;
    public PartPrevious() { }
    public string PrePartNum
    {
        get { return prepartnum; }
        set { prepartnum = value; }
    }
    public string PreRevision
    {
        get { return prerevision; }
        set { prerevision = value; }
    }
    public string PrePartdescription
    {
        get { return prepartdescription; }
        set { prepartdescription = value; }
    }
}

 cboPrePartNum.SelectedItem = <PartPrevioustype of object>;
 <PartPrevioustype of object> = cboPrePartNum.SelectedItem;

cboPrePartNum.DataTextField