如何检查记录是否已存在于SQL数据库中？

Question

I am trying to check if the value exists in a database column. I know this question asked several times but none of them is solving my problem. I have tried it in many ways. Anyway my following code through an exception;

"An unhandled exception of type 'System.ArgumentException' occurred in System.Data.dll Additional information: No mapping exists from object type System.Windows.Forms.TextBox to a known managed provider native type."

This problem is resolved of exception that was a mistake. but it still not working although the record is in database but it is not restricting me.

Can anyone tell me whats wrong with my code, the code is;

private void btn_Register_Click(object sender, EventArgs e)
    {
        lbl_Available.Visible = false;
        lbl_Empty.Visible = false;
        bool hasValue1 =  string.IsNullOrWhiteSpace(txt_PinCode.Text);
        bool hasValue2 =  string.IsNullOrWhiteSpace(txt_Name.Text);
        if (!hasValue1 && !hasValue2)
        {
            string pincode = txt_PinCode.Text;
            if (UserExists(pincode))
            {
               lbl_Available.Visible = true;
               return;
            }
            else
            {
                this.Hide();
                RegCluePoint frm = new RegCluePoint();
                frm.lbl_PinCode.Text = txt_PinCode.Text;
                frm.lbl_Name.Text = txt_Name.Text;
                frm.Show();
            }                    
        }
        else
        {
            lbl_Empty.Visible = true;
        }
    }
private bool UserExists(string pincode)
    {
        //pincode = txt_PinCode.Text
        //SqlCommand cmd = new SqlCommand();
        cmd.Connection = conn;
        cmd.CommandText = "SELECT PINCODE from UserInput where PINCODE = '@pincode'";
        cmd.Parameters.AddWithValue("PINCODE", pincode);
        if (conn.State != ConnectionState.Open)
        conn.Open();

        string pinString = (string)cmd.ExecuteScalar();
        conn.Close();

        return (pinString != null);
    } 

Answer 1

您发送txt_PinCode ，一个文本框对象，而不是一个字符串（ txt_PinCode.Text在此行中） cmd.Parameters.AddWithValue("PINCODE", txt_PinCode);

Answer 2

You are not passing the pincode that is passed to the method, instead you are passing the textbox itself (not even the value of the textbox).

private bool UserExists(string pincode)
{
    //pincode = txt_PinCode.Text
    //SqlCommand cmd = new SqlCommand();
    cmd.Connection = conn;
    cmd.CommandText = "SELECT PINCODE from UserInput where PINCODE = '@pincode'";
    cmd.Parameters.AddWithValue("@pincode", pincode);
    if (conn.State != ConnectionState.Open)
    conn.Open();

    //Execute scalar returns the first column of the first row in the result set
    string pinString = (string)cmd.ExecuteScalar();
    conn.Close();

    return (pinString != null);
} 

Answer 3

You are trying to pass the textbox itself, but not text. Change the code

cmd.Connection = conn;
cmd.CommandText = "SELECT PINCODE from UserInput where PINCODE = '@pincode'";
cmd.Parameters.AddWithValue("PINCODE", txt_PinCode.Text);

Answer 4

In this line of code you are sending the textbox object onstead of text.

cmd.Parameters.AddWithValue("PINCODE", txt_PinCode);

Change it to:

cmd.Parameters.AddWithValue("PINCODE", txt_PinCode.Text);