Why does this code block say “not all code paths return a value”?

Question

I wrote following code...but i am getting Error like:

Error 1 'LoginDLL.Class1.Login(string, string, string)': not all code paths return a value

Please help me...

Thanks in advance...

My code is as given below...

public int Login(string connectionString,string username,string password)
{
    SqlConnection con=new SqlConnection(connectionString);
    con.Open();

    SqlCommand validUser = new SqlCommand("SELECT count(*) from USER where username=@username", con);
    validUser.Parameters.AddWithValue("@username", username);
    int value=Convert.ToInt32(validUser.ExecuteScalar().ToString());
    if (value == 1)
    {
        //check for password
        SqlCommand validPassword = new SqlCommand("SELECT password from USER where username=@username", con);
        validPassword.Parameters.AddWithValue("@username", username);
        string pass = validPassword.ExecuteScalar().ToString();
        if (pass == password)
        {
            //valid login
            return 1;
        }
        else
        {
            return 0;
        }
    }
    else if (value == 0)
    {
        return 2;

    }
}

Answer 1

What if value == 3 ?

You could rewrite the code like this:

public LoginResult Login(string connectionString, string username, string password)
{
    if (string.IsNullOrEmpty(username)
    {
        return LoginResult.InvalidUser;
    }
    else if (string.IsNullOrEmpty(password)
    {
        return LoginResult.InvalidPassword;
    }

    using (var connection = new SqlConnection(connectionString))
    {
        connection.Open();

        using (var command = connection.CreateCommand())
        {
            command.CommandText = "SELECT password from USER where username=@username";
            command.Parameters.AddWithValue("@username", username);
            var actualPassword = (string)command.ExecuteScalar();

            if (actualPassword == null)
            {
                return LoginResult.InvalidUser;
            }
            else if (password != actualPassword)
            {
                return LoginResult.InvalidPassword;
            }
            else
            {
                return LoginResult.Success;
            }
        }
    }
}

public enum LoginResult
{
    Success,
    InvalidPassword,
    InvalidUser
}

Answer 2

Because if the value variable were equal to 3 your method wouldn't return anything.

And just for kicks, here's a short rewrite of your code that I think would work splendid.

public int Login(string connectionString,string username,string password)
{
  using(var con = new SqlConnection(connectionString)) {
    con.Open();
    var cmdText = "SELECT password from USER where username=@username";
    using (var cmd = new SqlCommand(cmdText, con)) {

      cmd.Parameters.AddWithValue("@username", username);
      object passwordFromDb = userCmd.ExecuteScalar();
      if (passwordFromDb != null) {
          if (password == passwordFromDb.ToString()) {
            return 1;
          }
      }
    }
  }
  return 0;
}

You only query the database 1 time and you're able to get everything you need in order to see if it's a valid login attempt.

Answer 3

You get the error because it is possible for the function to end (that is, traverse a code path ) without returning a value. To fix the error, add an else clause to the end of your conditional:

    if (value == 1)
    {
      // ...
    }
    else if (value == 0)
    {
      // ...
    }
    else {
      // Return a value here.
    }

Answer 4

You may know that the result of your ExecuteScalar call is 0 or 1, but the compiler cannot know that in advance. Make your "else if" a standard else or provide another return value before the end of the method.

Answer 5

因为有可能value不是1或2，并且你的外部if那个分支没有return语句。

Answer 6

You don't have return at the end of your method. If method is not void and returning some value than compiler checks that it always returns a value. Your method may not return value in several cases.

Answer 7

It's recommended that always return your result with ONLY ONE way in your method.

public int Login(string connectionString,string username,string password)
{
    int result = 0; //Default result value.

    SqlConnection con=new SqlConnection(connectionString);
    con.Open();

    SqlCommand validUser = new SqlCommand("SELECT count(*) from USER where username=@username", con);
    validUser.Parameters.AddWithValue("@username", username);
    int value=Convert.ToInt32(validUser.ExecuteScalar().ToString());
    if (value == 1)
    {
        //check for password
        SqlCommand validPassword = new SqlCommand("SELECT password from USER where username=@username", con);
        validPassword.Parameters.AddWithValue("@username", username);
        string pass = validPassword.ExecuteScalar().ToString();
        if (pass == password)
        {
            //valid login
            result = 1;
        }
        //It is not necessary in this case
        //else
        //{
        //    result = 0;
        //}
    }
    else if (value == 0)
    {
        result = 2;

    }

    return result;
}