如果更改组合框值，则更改文本框值（System.FormatException：'输入字符串格式不正确。'）

Question

如果我更改组合框索引，我将尝试自动加载文本框值 但不明白如果我加载我的表单会收到此错误？

我处理我的 null，如果我忽略这一行，一切都很好！

这是我的表格

---

这是我的仓库

区域存储库

public string GetAreaNamebyAreaID(int areaID)
{
    var result = db.btbArea.SingleOrDefault(g => g.AreaID == areaID);

    if (result == null)
        return string.Empty;

    return result.AreaName;
}

HowzehRepository

public string GetHowzehNamebyHoezehID(int howzehID)
{
    var result = db.btbHowzeh.SingleOrDefault(g => g.HowzehID == howzehID);
    if (result == null)
        return string.Empty;

    return result.HowzehName;
}

Paygah 存储库

public string GetPaygahNamebyPaygahID(int paygahID)
{
    var result = db.btbPaygah.SingleOrDefault(g => g.PaygahID == paygahID);
    if (result == null)
        return string.Empty;

    return result.PaygahName;
}

如果我更改组合框索引，我会尝试加载我的文本框值

private void frmAreasManage_Load(object sender, EventArgs e)
{
    //Load AreaComboBox Source from AreaTable
    using (UnitOfWork db = new UnitOfWork())
    { 
        cmbAreaNumber.DataSource = db.AreaRepository.Get();
        cmbAreaNumber.DisplayMember = "AreaNumber";
        cmbAreaNumber.ValueMember = "AreaID";
    }
}

private void cmbAreaNumber_SelectedIndexChanged(object sender, EventArgs e)
{
    string selectedValue = cmbAreaNumber.SelectedValue.ToString();
    using (UnitOfWork db = new UnitOfWork())
        if (!string.IsNullOrEmpty(selectedValue))
        {
            {
                //Load HowzehhComboBox From HowzehTable Filter By AreaID
                cmbHowzehNumber.DataSource = db.HowzehRepository.GetNameIDByFilter(selectedValue);

                cmbHowzehNumber.DisplayMember = "HowzehNumber";
                cmbHowzehNumber.ValueMember = "HowzehID";

                //Get AreaName from AreaTable Filter By AreaID
                txtAreaName.Text = db.AreaRepository.GetAreaNamebyAreaID(Convert.ToInt32(selectedValue));
            }
        }
}

private void cmbHowzehNumber_SelectedIndexChanged(object sender, EventArgs e)
{
    string selectedValue = cmbHowzehNumber.SelectedValue.ToString();
    using (UnitOfWork db = new UnitOfWork())
        if (!string.IsNullOrEmpty(selectedValue))
        {
            //Load PaygahComboBox From PaygahTable Filter By HowzehID
            cmbPaygahNumber.DataSource = db.PaygahRepository.GetNameIDByFilter(selectedValue);

            cmbPaygahNumber.DisplayMember = "PaygahNumber";
            cmbPaygahNumber.ValueMember = "PaygahID";
            //Get HowzehName from HowzehTable Filter By HowzehID
            txtHowzehName.Text = db.HowzehRepository.GetHowzehNamebyHoezehID(Convert.ToInt32(selectedValue));
        }
}

private void cmbPaygahNumber_SelectedIndexChanged(object sender, EventArgs e)
{
    using (UnitOfWork db = new UnitOfWork())
    {
        //Get HowzehName from HowzehTable Filter By HowzehID
        txtPaygahName.Text = db.PaygahRepository.GetPaygahNamebyPaygahID(Convert.ToInt32(selectedValue));
    }
}

Answer 1

TL;DR

要解决您的问题，请在方法frmAreasManage_Load 中进行下一次更改：

private void frmAreasManage_Load(object sender, EventArgs e)
{
    //Load AreaComboBox Source from AreaTable
    using (UnitOfWork db = new UnitOfWork())
    { 
        // At first assign properties DisplayMember and ValueMember.
        cmbAreaNumber.DisplayMember = "AreaNumber";
        cmbAreaNumber.ValueMember = "AreaID";

        // And then assign DataSource property of the cmbAreaNumber.
        cmbAreaNumber.DataSource = db.AreaRepository.Get();
    }
}

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说明

为什么我们需要在DataSource 属性之前分配ValueMember？

当我们分配DataSource 属性时，分配集合的第一项变为SelectedValue 的ComboBox。这会触发ComboBox.SelectedIndexChanged 事件。

如果我们在ValueMember 之前分配DataSource（因此是ValueMember = null），那么CombobBox.SelectedValue 就是集合本身的第一项。在您的情况下，SelectedValue 是btbArea 类型的对象。因此，当我要求您在 cmbAreaNumber_SelectedIndexChanged 处理程序中使用下一个代码时

txtAreaName.Text = db.AreaRepository.GetAreaNamebyAreaID(((btbArea) cmbAreaNumber.SelectedValue).AreaID);

它解决了第一次加载的问题。但后来你得到了System.InvalidCastException: 'Unable to cast object of type 'System.Int32' to type 'Gim.DataLayer.btbArea'。发生此错误是因为 ValueMember 属性被分配给值 AreaID 并且从那一刻起，ComboBox.SelectedValue 就是 btbArea 的属性 int AreaID 的值。

如果我们分配ValueMember = "AreaID"，然后分配DataSource，那么每当引发ComboBox.SelectedIndexChanged 事件时，SelectedValue 就是属性AreaID 的值。因此，为了防止SelectedValue 成为btbArea 类型的对象（ValueMember = null 时的情况），我们必须首先分配ValueMember，然后再分配DataSource。