sql server -在一个方法中使用ArrayList绑定两个sql语句.c#)

您忘记像这样关闭阅读器:

// Call Close when done reading.
reader.Close();

尝试使用这种语法连接，它更安全:

    using (SqlConnection connection =
               new SqlConnection(connectionString))
    {
        SqlCommand command =
            new SqlCommand(queryString, connection);
        connection.Open();
        SqlDataReader reader = command.ExecuteReader();
        // Call Read before accessing data.
        while (reader.Read())
        {
            ReadSingleRow((IDataRecord)reader);
        }
        // Call Close when done reading.
        reader.Close();
    }

我认为这段代码将工作良好。这段代码是不言自明的。您正在创建一个SqlDataReader对象而没有处理之前的对象。

public void bindStudentID()
{
    try
    {
        ArrayList a = new ArrayList();
        con.Open();
        SqlCommand cmd = new SqlCommand("SELECT studentId FROM tbStudent",     con);
        SqlDataReader dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            a.Add(dr["studentId"]);
        }
        comboBox1.DataSource = a;
        ArrayList aa = new ArrayList();
        SqlCommand cmdd = new SqlCommand("SELECT SemesterID FROM tbSemester     Where StudentID='" + comboBox1.Text + "'", con);
        dr = null;
        dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            aa.Add(drr["SemesterID"]);
        }
        comboBox2.DataSource = aa;
        con.Close();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
    finally
    {
    }
}

如果我帮助了，请标记为答案

检查这个，

SqlDataReader drr = cmd.ExecuteReader();

应该是

SqlDataReader drr = cmdd.ExecuteReader();

您正在使用cmd而不是cmdd命令

MSDN显示:

在使用完DataReader对象后，应该始终调用Close方法。如果您的命令包含输出参数或返回值，则在关闭DataReader之前它们将不可用。请注意，当一个数据阅读器打开时，连接仅由该数据阅读器使用。在原始数据读取器关闭之前，您不能为连接执行任何命令，包括创建另一个数据读取器。

所以基于MSDN，试试这个:

public void bindStudentID()
{
//instantiating a ArrayList object does not need to be included inside Try/Catch block
//try
//{
    ArrayList a = new ArrayList();
try
{
    //Before opening the connection make sure that connection's current state is not open, otherwise you will get exception
    //con.Open();
 if (con.State != ConnectionState.Closed)
 {
     con.Close();
 }
 con.Open();
    SqlCommand cmd = new SqlCommand("SELECT studentId FROM tbStudent",     con);
    SqlDataReader dr = cmd.ExecuteReader();
    while (dr.Read())
    {
        a.Add(dr["studentId"]);
    }
    comboBox1.DataSource = a;
 //You should close data reader before using it again or defining new one
    dr.Close();
    ArrayList aa = new ArrayList();
 //No need to define new SqlCommand object and you can use the prev one
    cmd = new SqlCommand("SELECT SemesterID FROM tbSemester     Where StudentID='" + comboBox1.Text + "'", con);
 //No need to define new SqlDataREader object and you can use the prev one
    dr = cmd.ExecuteReader();
    while (dr.Read())
    {
        aa.Add(dr["SemesterID"]);
       //I think this should be stated out of the while block            
       //comboBox2.DataSource = aa;
    }
   comboBox2.DataSource = aa;
     //You should close data reader before using it again or defining new one
    dr.Close();
//Close the connection in finally block        
//con.Close();
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}
finally
{
 //You should also close the connection even if an exception raised
        con.Close();
}
}