“ExecuteScalar：连接属性尚未初始化。”答案

【问题标题】："ExecuteScalar: Connection property has not been initialized."“ExecuteScalar：连接属性尚未初始化。”
【发布时间】：2017-11-25 14:26:58
【问题描述】：

我是 C# 新手，目前在不调试的情况下运行此程序时遇到一些问题。

这是我的项目的登录页面。我创建了一个基于服务的数据库，我想连接到数据库中表“表”中的用户名和密码数据。但是，我遇到了“ExecuteScalar：连接属性尚未初始化”的问题。当我运行这段代码时。

谁能帮我解决这个问题？

    private void button1_Click(object sender, EventArgs e)
    {
        SqlConnection conn = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=c:\users\hp\documents\visual studio 2015\Projects\PersonalFinancialSoftware\PersonalFinancialSoftware\Login.mdf;Integrated Security=True");
        SqlCommand cmd = new SqlCommand();

        cmd.CommandText = "SELECT COUNT (*) FROM Table where UserID = @userid AND Password = @password";
        cmd.Parameters.AddWithValue("@userid", textBox1.Text);
        cmd.Parameters.AddWithValue("@password", textBox2.Text);
        object result = cmd.ExecuteScalar();

        conn.Open();
        string useridlogin = Convert.ToString(result);
        conn.Close();

        if (useridlogin != " ")
        {
            Home_Page homepage = new Home_Page();
            homepage.Show();
        }
        else
        {
            MessageBox.Show("Invalid ID or password, please try again!", "Info", MessageBoxButtons.OK);
        }
    }

【问题讨论】：

标签： c#

【解决方案1】：

我可以看到您在打开 SQL 数据库连接之前执行了 ExecuteScalar 方法，导致您遇到错误。

在 ExecuteScalar 方法之前打开连接，就完成了。

private void button1_Click(object sender, EventArgs e)
{
    SqlConnection conn = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=c:\users\hp\documents\visual studio 2015\Projects\PersonalFinancialSoftware\PersonalFinancialSoftware\Login.mdf;Integrated Security=True");
    conn.Open();
    SqlCommand cmd = new SqlCommand();

    cmd.CommandText = "SELECT COUNT (*) FROM Table where UserID = @userid AND Password = @password";
    cmd.Parameters.AddWithValue("@userid", textBox1.Text);
    cmd.Parameters.AddWithValue("@password", textBox2.Text);
    object result = cmd.ExecuteScalar();

    string useridlogin = Convert.ToString(result);
    conn.Close();

    if (useridlogin != " ")
    {
        Home_Page homepage = new Home_Page();
        homepage.Show();
    }
    else
    {
        MessageBox.Show("Invalid ID or password, please try again!", "Info", MessageBoxButtons.OK);
    }
}

【讨论】：

【解决方案2】：

您在 ExecuteScalar 之后打开了连接，这就是您遇到错误，您必须在 ExecuteScalar 尝试此代码之前打开连接

private void button1_Click(object sender, EventArgs e)
    {
        SqlConnection conn = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=c:\users\hp\documents\visual studio 2015\Projects\PersonalFinancialSoftware\PersonalFinancialSoftware\Login.mdf;Integrated Security=True");
        SqlCommand cmd = new SqlCommand();
        conn.Open();
        cmd.CommandText = "SELECT COUNT (*) FROM Table where UserID = @userid AND Password = @password";
        cmd.Parameters.AddWithValue("@userid", textBox1.Text);
        cmd.Parameters.AddWithValue("@password", textBox2.Text);
        object result = cmd.ExecuteScalar();

        string useridlogin = Convert.ToString(result);
        conn.Close();

        if (useridlogin != " ")
        {
            Home_Page homepage = new Home_Page();
            homepage.Show();
        }
        else
        {
            MessageBox.Show("Invalid ID or password, please try again!", "Info", MessageBoxButtons.OK);
        }
    }

在您的代码 SQL 查询中查找连接。但是你在 ExecuteScalar 之后打开了它，所以这会给你一个错误。

【讨论】：