c# - 为什么这个代码块说 "not all code paths return a value"？

Question

我写了下面的代码...但是我收到如下错误:

错误 1“LoginDLL.Class1.Login(string, string, string)”:并非所有代码路径都返回值

请帮帮我...

提前致谢...

我的代码如下...

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

如果 value == 3 会怎样？

你可以这样重写代码:

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
}