C# 使多个 else-if 语句更有效。方法？

Question

private void displayButton_Click(object sender, EventArgs e)
    {


        string connectionString;
        SqlConnection cnn;

        connectionString = "Server= myservername; Database= nba_database; Integrated Security=True";

        cnn = new SqlConnection(connectionString);

        cnn.Open();
       // MessageBox.Show("Connection Established!");

   

        // define variables
        SqlCommand command;
        
        String sql;

        // define SQL statement
        if (listBox1.SelectedIndex == 0)
        {
            sql = "SELECT LastName, FirstName " +
            "FROM Players, Teams " +
            "WHERE Players.TeamID = Teams.TeamID " +
            "AND Teams.Nickname = 'Hawks'";

            // command statement
            command = new SqlCommand(sql, cnn);


            textBox1.Text = command.ExecuteScalar().ToString();
            // Scalar returns the first column of the first row of the query


            cnn.Close();
            
            command.Dispose();
        }
        else if (listBox1.SelectedIndex == 1)
        {
            String sql2 = "SELECT FirstName, LastName FROM Players WHERE Team = 'Milwaukee Bucks'" +
                            "AND Number <= 24";

            

            // command statement
            command = new SqlCommand(sql2, cnn);

            SqlDataReader reader = command.ExecuteReader();


            // Get table values

            StringBuilder sb = new StringBuilder();
            while (reader.Read())
            {
                sb.AppendLine(reader.GetString(0).ToString() + " " + reader.GetString(1).ToString());
            }

            textBox1.Text = sb.ToString();


            cnn.Close();

            command.Dispose();
        }   

    }

我有这个连接到 SQL Server 数据库的代码。有一个列表框供用户选择，根据他们的选择，查询将运行并在文本框中返回查询结果。

就目前而言，感觉就像我有一堆重复的代码，而我在列表框中只有 2 个选项。我将需要添加更多选项，并且嵌套的 if else 语句将继续增长。

有没有更好的方法来做到这一点？我正在考虑使用方法，但是当我尝试时，我不知道如何从方法中返回值，以及如何将其用作查询的参数。

任何帮助/建议将不胜感激

Answer 1

好吧，最初switch 语句会整理它：

   var i = listbox1.SelectedIndex;

   switch (i)
    {
        case 1:
            // do stuff
            break;
        case 2:
            // do other stuff
            break;
        default:
            break;
    }

但这很快也会变得笨拙。

Answer 2

我会做这样的事情。

但是将 SelectIndexZero/SelectIndexOne 重命名为更具描述性的名称。

我会保留 IF 语句，不要更改大小写，因为您没有默认大小写。只有2个非常具体的案例。如果 0 是默认值，我会将其更改为大小写。

private void displayButton_Click(object sender, EventArgs e)
{
    string connectionString = "Server= myservername; Database= nba_database; Integrated Security=True";

    SqlConnection cnn = new SqlConnection(connectionString);

    cnn.Open();
    // MessageBox.Show("Connection Established!");

    // define variables
    SqlCommand command;
    
    // define SQL statement
    if (listBox1.SelectedIndex == 0)
    {
        SelectIndexZero(cnn, command);
    }
    else if (listBox1.SelectedIndex == 1)
    {
        SelectIndexOne(cnn, command);
    }

    Dispose(cnn, command);
}

private void SelectIndexZero(SqlConnection cnn, SqlCommand command)
{
    string sql = "SELECT LastName, FirstName " +
        "FROM Players, Teams " +
        "WHERE Players.TeamID = Teams.TeamID " +
        "AND Teams.Nickname = 'Hawks'";

    // command statement
    command = new SqlCommand(sql, cnn);

    textBox1.Text = command.ExecuteScalar().ToString();
    // Scalar returns the first column of the first row of the query
}  

private void SelectIndexOne(SqlConnection cnn, SqlCommand command)
{
    string sql = "SELECT FirstName, LastName FROM Players WHERE Team = 'Milwaukee Bucks'" +
                        "AND Number <= 24";

    // command statement
    command = new SqlCommand(sql, cnn);

    SqlDataReader reader = command.ExecuteReader();


    // Get table values

    StringBuilder sb = new StringBuilder();
    while (reader.Read())
    {
        sb.AppendLine(reader.GetString(0).ToString() + " " + reader.GetString(1).ToString());
    }

    textBox1.Text = sb.ToString();
}

private void Dispose(SqlConnection cnn, SqlCommand command)
{
    cnn.Close();
    command.Dispose();
}