使用的字符。单引号 定义字符，而双引号定义一个字符串：

 如果（e.KeyChar =='L '）
 

I have a registration application which has the "KeyChar" event inside it, and it works great ! but when i give the same lines of code in this application it gives me Operator '=='/'!=' cannot be applied to operands of type 'char' and 'string'

Can't seem to figure out why it works in the other application but not here! Any help is much appreciated !

 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;
            }
        }
    }

Use the character. Single quotes ' define a character, while double quotes " define a string:

if (e.KeyChar == 'L')