C# 操作员'=='；无法应用于类型为'；char'；和'；字符串'；

我有一个注册应用程序，里面有“KeyChar”事件，它工作得很好！但是，当我在这个应用程序中给出相同的代码行时，它会给出

运算符'=='/'！='无法应用于“char”和“string”类型的操作数。

似乎无法理解为什么它在其他应用程序中工作，但在这里不工作！ 非常感谢您的帮助

 private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
    {
        SqlConnection DBConnection = new SqlConnection("Data Source=DATABASE;Initial Catalog=imis;Integrated Security=True");
        SqlCommand cmd = new SqlCommand();

        Object returnValue;

        string txtend = textBox1.Text;
        //string lastChar = txtend.Substring(txtend.Length - 1);

        if (e.KeyChar == "L")
        {
            DBConnection.Open();
        }
        if (DBConnection.State == ConnectionState.Open)
        {
            if (textBox1.Text.Length != 7) return;
            {
                //cmd.CommandText = ("SELECT last_name +', '+ first_name +'\t ('+major_key+')\t' from name where id =@Name");
                cmd.CommandText = ("SELECT last_name +', '+ first_name from name where id =@Name");
                cmd.Parameters.Add(new SqlParameter("Name", textBox1.Text.Replace(@"L", "")));
                cmd.CommandType = CommandType.Text;
                cmd.Connection = DBConnection;
                // sqlConnection1.Open();
                returnValue = cmd.ExecuteScalar() + "\t (" + textBox1.Text.Replace(@"L", "") + ")";
                DBConnection.Close();

                if (listBox1.Items.Contains(returnValue))
                {
                    for (int n = listBox1.Items.Count - 1; n >= 0; --n)
                    {
                        string removelistitem = returnValue.ToString();
                        if (listBox1.Items[n].ToString().Contains(removelistitem))
                        {
                            listBox1.Items.RemoveAt(n);
                            //listBox1.Items.Add(removelistitem+"    " +TimeOut+ Time);
                        }

                    }
                }
                else
                    listBox1.Items.Add(returnValue);

                System.IO.StreamWriter SaveFile = new System.IO.StreamWriter(fullFileName);
                foreach (object item in listBox1.Items)
                    SaveFile.WriteLine(item.ToString());
                SaveFile.Flush();
                SaveFile.Close();

                textBox1.Clear();

                if (listBox1.Items.Count != 0) { DisableCloseButton(); }
                else
                {
                    EnableCloseButton();
                }
                Current_Attendance_Label.Text = "Currently " + listBox1.Items.Count.ToString() + " in attendance.";
                e.Handled = true;
            }
        }
        /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
        else
        //////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
        {
            returnValue = textBox1.Text.Replace(@"*", "");

            if (e.KeyChar == "*") return;
            {
                if (listBox1.Items.Contains(returnValue))
                {
                    for (int n = listBox1.Items.Count - 1; n >= 0; --n)
                    {
                        string removelistitem = returnValue.ToString();
                        if (listBox1.Items[n].ToString().Contains(removelistitem))
                        {
                            //listBox1.Items.RemoveAt(n);
                        }
                    }
                }
                else
                    listBox1.Items.Add(returnValue);
                textBox1.Clear();
                System.IO.StreamWriter SaveFile = new System.IO.StreamWriter(fullFileName);
                foreach (object item in listBox1.Items)
                    SaveFile.WriteLine(item.ToString());
                SaveFile.Flush();
                SaveFile.Close();
                if (listBox1.Items.Count != 0) { DisableCloseButton(); }
                else
                {
                    EnableCloseButton();
                }
                Current_Attendance_Label.Text = "Currently " + listBox1.Items.Count.ToString() + " in attendance.";
                e.Handled = true;
            }
        }
    }

---

使用字符。单引号

”

定义

字符

，而双引号

“

定义

字符串

：

if (e.KeyChar == 'L')

---

真不敢相信我错过了！非常感谢！