Generate code with recursion - wpf

Question

I'm having issue with creating product code in recursion.
What I want to do is:
I enter code 1000
-If code exist in database regenerateCode(string code)
-else insert into database
Code:
(...)
if(codeExist)
CodeTB.Text = regenerateCode(string toParse); //toParse = 1000
string regenerateCode(string toParse)
{
string helper = "";
int parseCode = int.Parse(toParse);
helper = new string('0', 4 - parseCode.ToString().Length);
helper += parseCode + 1;
using (SqlConnection conn = new SqlConnection(cString.c_String))
{
conn.Open();
using (SqlCommand cmd = new SqlCommand("SELECT Product.productID FROM Product " +
"WHERE Product.PLU = '" + helper + "' ", conn))
{
using (SqlDataReader rdr = cmd.ExecuteReader())
{
if (rdr.HasRows)
{
// if code still exist in database, regenerate it
regenerateCode(helper);
}
else
{
//else return code
return helper;
}
}
}
}
return helper;
}
Actually it works fine with example:
1000 (exist)
1001 (exist)
1002 (not exist, insert), but
when code = 1002, it goes into line with else {return helper;} and have no idea why going again to regenerateCode() method..
Any ideas?

Answer 1

It happens because you do nothing with the return value. You pass helper to the recursion, but do not place the return value anywhere. your method should look like this:
string regenerateCode(string toParse)
{
string helper = "";
int parseCode = int.Parse(toParse);
helper = new string('0', 4 - parseCode.ToString().Length);
helper += parseCode + 1;
using (SqlConnection conn = new SqlConnection(cString.c_String))
{
conn.Open();
using (SqlCommand cmd = new SqlCommand("SELECT Product.productID FROM Product " +
"WHERE Product.PLU = '" + helper + "' ", conn))
{
using (SqlDataReader rdr = cmd.ExecuteReader())
{
// Return the next code that doesn't exist
return rdr.HasRows ? regenerateCode(helper) : helper;
}
}
}
}
Also, remember that while string are reference types, they are immutable. which means that a string that is passed as an argument would not change like a normal array. see: How are strings passed in .NET?