Child 组合框 Return 如果 Child table 中不存在数据值组合框，则为 Null (LINQ)

Question

信息

我有3个table

tblCountry

tblStateName

tblState

tblState Child of tblStateName (StateName_ID) (为了更好的处理)

tblState tblCountry Child (Country_ID) (级联都知道)

我用 2 个组合框创建了 1 个表单

我将此代码用于加载 2 组合框 DataValue

    private void Form01_Load(object sender, EventArgs e)
    {
        //Load CountryComboBox Source from Table01
        using (UnitOfWork db = new UnitOfWork())
        {
            // At first assign properties DisplayMember and ValueMember.
            cmbCountry.DisplayMember = "Country";
            cmbCountry.ValueMember = "Country_ID";

            // And then assign DataSource property of the CountryComboBox .
            cmbCountry.DataSource = db.CountryRepository.Get();
        }
    }

    //Load StateComboBox From Table02
    private void cmbCountry_SelectedIndexChanged(object sender, EventArgs e)
    {
        string selectedValue = cmbCountry.SelectedValue.ToString();

        using (UnitOfWork db = new UnitOfWork())
            if (!string.IsNullOrEmpty(selectedValue))
            {
                // At first assign properties DisplayMember and ValueMember.
                cmbState.DisplayMember = "StateName";
                cmbState.ValueMember = "State_ID";
                // And then assign DataSource property of the cmbState.
                var result = (from state in db.StateRepository.GetNameIDByFilter(selectedValue)
                                              join stateName in db.StateNameRepository.Get() on state.StateName_ID equals stateName.Statename_ID

                                              select new
                                              {
                                                  State_ID = state.State_ID,
                                                  StateName = stateName.StateName
                                              }).ToList();

                cmbState.DataSource = result;
            }

加载表单后我看到了这个结果

如果我 Select Country01，我在 cmbState 中看到 4 个结果（很好，工作正常）

我的问题

但是如果我 Select 'Country02'（这个 Value 不是 child 在 tblState 中）但是我看到第一个 Value 从 tblState 像这样

我需要什么？

如何 select Country02 并在 cmbState 中查看 Null？或更好（ Return Null If Country_ID not Exist in State table ）并且 cmbState 为 Null

( SRY 我的英语不好)

Answer 1

如果result为null或空，请在重新加载数据后清除cmbState，如以下代码：

if(result == null || result.Count == 0)
{
 cmbState.DataSource = null;
 cmbState.DataBindings.Clear();
}else
{
 cmbState.DataSource = result;
}

希望对您有所帮助。

Child 组合框 Return 如果 Child table 中不存在数据值组合框，则为 Null (LINQ)

Child Cobobox Return Null if DataValue Combobox NotExist in Child table ( LINQ )

c#

linq

combobox

join

winforms