检查sql databse中是否存在记录返回错误结果

Question

I'm working on my project in Asp.net (C#), I have one textbox to get a string from the user (by button click), And like all the strings that i get from the user - (username, email, etc), I check if the input string is exists in the database (SQL), if it exists, an error message is shown.

I use this code in .cs Code behind file:

string cn = cnTextBox.Value;
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["MyDB"].ConnectionString);
con.Open();
SqlCommand cmd = new SqlCommand("SELECT [CN] FROM [dbo].[MyTable] WHERE CN=@CN");   
cmd.Parameters.AddWithValue("@CN", cn);
cmd.Connection = con;
SqlDataReader dr = cmd.ExecuteReader();
bool theSame = false;
if (dr.HasRows())
    while (dr.Read())
    {            
        string value = dr["CN"].ToString();
        if (cn == value)
            theSame = true;
    }

if (theSame)
    errorMessage.Style["display"] = "block";
else
    errorMessage.Style["display"] = "none";
if (!theSame)
    // code to insert the string to a new record in the database, works fine.

As you can see i use WHERE clause inside a SELECT (parameterized) query, in order to verify if the string is already exists in the database.

Note: The "CN" column in the database is of type varchar(50) .

The problem is, (never hppened before, and it is really not the first time i do that), if i have the string "xxxxx" in a record in the database (in the column CN) and the input string that i want to insert is "xxxxx n " (not the same length) or "xxxxn" (not the same chars), the above check returns that this input is already exists in the database and the error message is shown. And even more strange? the string is inserted to the database after all.

Note: It happens over and over again when i test my project using Internet Explorer 11, but never happened when i used another bowser like Firefox.

Is something wrong? Any suggestions?

Thanks for help!

Answer 1

Too long for a comment. Are you sure you are looking at the same database that you application is using?

Unrelated tips:

1. SqlConnection , SqlCommand and SqlDataReader are all IDisposable so each should be in a using block.
2. You may want to stop using AddWithValue .
3. You can pass the connection to the constructor of the SqlCommand .
4. Rather than dr["CN"].ToString(); prefer dr.GetString(0);
5. SQL string comparisons are case insensitive by default. C# string equality are case sensitive. SQL might find the record and your C# code could dismiss it as being different.
6. Any question including the phrase " [doing X] returns wrong results" misses the point that computers do exactly what you ask them to, so unless this is a freakish bug which millions of other users have not seen over decades of usage; then you need to realise it is returning the right results for what it has been asked. This shift in perception, along with use of the debugger is likely to help you diagnose the problem.

Answer 2

trim your value been passed. may be extra space gets added that make it differ like you said 'xxxx' and 'xxxxn', n may be extra space.