添加 IF 语句

Question

我想在这段代码中添加一个 if 语句，上面写着..

如果我单击此按钮，它将检查 ListU.SelectedValue 是否为空，如果为空，则显示“请在继续之前选择名称”的消息框，如果不为空，则代码运行。

我该怎么做？

这是按钮单击的代码。 （我知道，我的代码需要一些参数，我们可以暂时忽略它）

private void button4_Click(object sender, EventArgs e)
{
   //update code//
   SqlConnection conn = new SqlConnection();
   conn.ConnectionString = "Data Source=PEWPEWDIEPIE\\SQLEXPRESS;Initial Catalog=master;Integrated Security=True";
   conn.Open();

   SqlDataAdapter daCount = new SqlDataAdapter("select iCount from ComDet where cName = @cName", conn);
   daCount.SelectCommand.Parameters.Add("@cName", SqlDbType.VarChar).Value = ListU.SelectedValue;

   DataTable dtC = new DataTable();
   daCount.Fill(dtC);
   DataRow firstRow = dtC.Rows[0];

   string x = firstRow["iCount"].ToString();
   int y = Int32.Parse(x);
   int z = y + 1;

   SqlCommand cmdC = conn.CreateCommand();
   cmdC.CommandText = "Update ComDet set iCount = " + z + ", ViewTime = '" + lblTime.Text + "', LastView = '" + txtUser2.Text + "' Where cName = '" + ListU.SelectedValue.ToString() + "'";

    cmdC.ExecuteNonQuery();
    conn.Close();

    var ufdet = new UserFullDetail(ListU.SelectedValue.ToString());
    ufdet.ShowDialog();
}

Answer 1

private void button4_Click(object sender, EventArgs e)
{
   if(ListU.SelectedValue == null || ListU.SelectedValue.ToString() == string.Empty)
   {
       MessageBox.Show("Select something from the listbox, please");
       return;
   }
   .....

Answer 2

这样试试

private void button4_Click(object sender, EventArgs e)
{
if(ListU.SelectedValue == null || ListU.SelectedValue.ToString() == string.Empty)
   {
       MessageBox.Show("Select something from the listbox, please");      
   }
else
   {
   //update code//
   SqlConnection conn = new SqlConnection();
   conn.ConnectionString = "Data Source=PEWPEWDIEPIE\\SQLEXPRESS;Initial Catalog=master;Integrated Security=True";
   conn.Open();

   SqlDataAdapter daCount = new SqlDataAdapter("select iCount from ComDet where cName = @cName", conn);
   daCount.SelectCommand.Parameters.Add("@cName", SqlDbType.VarChar).Value = ListU.SelectedValue;

   DataTable dtC = new DataTable();
   daCount.Fill(dtC);
   DataRow firstRow = dtC.Rows[0];

   string x = firstRow["iCount"].ToString();
   int y = Int32.Parse(x);
   int z = y + 1;

   SqlCommand cmdC = conn.CreateCommand();
   cmdC.CommandText = "Update ComDet set iCount = " + z + ", ViewTime = '" + lblTime.Text + "', LastView = '" + txtUser2.Text + "' Where cName = '" + ListU.SelectedValue.ToString() + "'";

    cmdC.ExecuteNonQuery();
    conn.Close();

    var ufdet = new UserFullDetail(ListU.SelectedValue.ToString());
    ufdet.ShowDialog();
    }
}

Answer 3

我更喜欢这个：

private void button4_Click(object sender, EventArgs e)
{
    if (ListU.SelectedItem == null)
    {
        MessageBox.Show("Please pick a name before continuing.");
    }
    else
    {
        // Run the code.
    }
}

这使得 else 条件显式化，而不是像 Steve 的示例中那样隐含。

另一种方法是仅在 ListU 中选择某些内容时才启用 button4，例如，

private void ListU_SelectedIndexChanged(object sender, System.EventArgs e)
{
    button4.Enabled = ListU.SelectedItem != null;
}

Answer 4

试试这个：

private void button4_Click(object sender, EventArgs e)
{
  if(ListU.SelectedValue == null || string.IsNullOrEmpty(Convert.ToString(ListU.SelectedValue)))
  {
    MessageBox.Show("Select something from the listbox.");
    return;
  }
.....