如何通过文本框（Windows窗体），C＃检查我的数据库中是否存在主键

Question

I have a textbox and a button in a windows form application. I want to check if the primary key (persId) exists in my sql database/dataset (made with Visual studio) when I enter a number in the textbox and press the button. I dont know how to compare the text with persId from the database.

If the persId exists I want to fill two textboxes in a new form and show the persId and persName.

I am new to programming in C# so I have probably missed something. I looked at how to check if value exists in database from textbox c# but could not find an answer.

Thanks in advance!

    public void searchPersId(string persId)
    {
        SqlConnection conn = new SqlConnection();
        SqlCommand myCommand = new SqlCommand("SELECT persId FROM Customers  WHERE persId = @persId", conn); 
        myCommand.Parameters.AddWithValue("@persId", persId);


        if (textBox1.Text = myCommand  ) //I dont know how to compare the values of textbox with myCommand..
        {
            //Show values (persId and persName) in two textBoxes in a new form. 
        }

        else
        {
            MessageBox.Show("The ID does not exist.");
        }


    }

Answer 1

First, use the using -statement for everything implementing IDisposable like the connection to dispose unmanaged resources and to close the connection, even in case of an error.

Then you have to open the connection and to use ExecuteReader to get a datareader to check if there's at least one record with that ID, you can use reader.HasRows . You also have to select the persName if you want it as mentioned.

using(var conn = new SqlConnection())
using(var myCommand = new SqlCommand("SELECT persId, persName FROM Customers  WHERE persId = @persId", conn))
{
    myCommand.Parameters.AddWithValue("@persId", persId);
    conn.Open();
    using(var rd = myCommand.ExecuteReader())
    {
        bool personExists = rd.HasRows;
        if(personExists)
        {
            // advance the reader to the first record, presuming there is only one, otherwise use a loop while(rd.Read)
            rd.Read();
            string persName = rd.GetString(1); // second field
            // ...
        }
        else
        {
            MessageBox.Show("The ID does not exist.");
        }
    }
}

Answer 2

You can also use ExecuteScalar

public void searchPersId(string persId)
    {
        SqlConnection conn = new SqlConnection();
        SqlCommand myCommand = new SqlCommand("SELECT persName FROM Customers  WHERE persId = @persId", conn);
        myCommand.Parameters.AddWithValue("@persId", persId);

        object personName = myCommand.ExecuteScalar();

        if(!string.IsNullOrEmpty(personName.ToString()))
        //if (textBox1.Text = myCommand) //I dont know how to compare the values of textbox with myCommand..
        {
            //Show values (persId and persName) in two textBoxes in a new form. 
            textBox2.Text = personName.ToString();
        }
        else
        {
            MessageBox.Show("The ID does not exist.");
        }
    }

Answer 3

First you have to Execute the command.

SqlDataReader dr = myCommand.ExecuteReader(CommandBehavior.CloseConnection);



if (dr.HasRows)
{
    // ... if it has rows then you know it match
} 
else 
{
   // ... data doesn't exists
}

Then you can compare the result.