如何在连接SQL的C#中的文本框中显示查询

Question

嗨，我在C#中工作，我想要一个计算平均值的按钮一些数字。

这是我的代码：

private void button5_Click(object sender, EventArgs e)
{
connect.Open();
string avg= "SELECT AVG(number) FROM info WHERE number = @number";
SqlCommand command= new SqlCommand(avg, connect);
SqlDataAdapter da = new SqlDataAdapter(command);
textBox8.Text = ; //I dont know what should I write here.
connect.Close();

我想通过按钮在文本框中显示数字的平均值。怎样我该怎么办？你能帮我吗？

编辑

我解决了这个问题：

private void button5_Click(object sender, EventArgs e)
{
connect.Open();
SqlCommand command= new SqlCommand("SELECT AVG(number) FROM info", connect);
SqlDataReader dr = command.ExecuteReader();
SqlDataAdapter da = new SqlDataAdapter(command);
if (dr.Read())
{
textBox8.Text = dr[0].ToString();
}
connect.Close();
}
}
}

Answer 1

代码不仅缺少查询结果的设置，还缺少执行查询所需的其他部分。

private void button5_Click(object sender, EventArgs e)
{
connect.Open();
string avg= "SELECT AVG(number) FROM info WHERE number = @number";
SqlCommand command= new SqlCommand(avg, connect);
// You need to add a parameter for @number and set its value 
command.Parameters.Add(@number, SqlDbType.Int).Value = valueForNumber;
// Now you need to execute the command to get back the average value
object result = command.ExecuteScalar();
// Finally you set the textbox with a conversion in string of the returned value
// but also check if there is really a return value
if(result != null)
textBox8.Text = result.ToString(); 
else
textBox8.Text = "Number Not found!";        
connect.Close();
}

但该代码还有其他问题
当您处理一次性物品时，您应该牢记危险信号。一次性物品"；通常"；包含应尽快释放的资源
连接和命令就是此类对象。在原始代码中，连接似乎是一个全局对象。像这样的代码应该避免这种情况

private void button5_Click(object sender, EventArgs e)
{
using(SqlConnection cnn = new SqlConnection(getTheConnectionString())
{
connect.Open();
string avg= "SELECT AVG(number) FROM info WHERE number = @number";
using(SqlCommand command= new SqlCommand(avg, cnn))
{
// You need to add a parameter for @number and set its value 
command.Parameters.Add("@number", SqlDbType.Int).Value = valueForNumber;
// Now you need to execute the command to get back the average value
object result = command.ExecuteScalar();

// Finally you set the textbox with a conversion in string of the returned value
// but also check if there is really a return value
if(result != null)
textBox8.Text = result.ToString(); 
else
textBox8.Text = "Number Not found!";
} // End using => SqlCommand disposed...
} // End using => SqlConnection closed and disposed
}

using块跟踪可丢弃对象，并确保当代码退出using块时，即使在块内部发生异常时，也能正确处理这些对象。