在C＃中将不接受空对象

Question

When I run my code it tells me that the object of adr is null, and thats true, but why wont it work when it works in a duplicate of the same method, with the exeption of insert instead of select.

the code looks like this:

public City doesExist(string postnr, string navn, City city, SqlConnection con)
{
    DatabaseConnection.openConnection(con);
    using (var command = new SqlCommand("select Id from [By] where Postnummer='" + postnr + "' and Navn='" + navn + "'", con))
    {
        command.Connection = con;
        SqlDataReader reader = command.ExecuteReader();
        if (reader.Read())
        {
            city.id = reader.GetInt32(0);
            city.postnr = postnr;
            city.navn = navn;
            reader.Close();

            return city;
        }

        reader.Close();
        return null;
    }
}

public City create(string postnr, string navn, City city, SqlConnection con)
{
    DatabaseConnection.openConnection(con);
    using (var command = new SqlCommand("insert into [By] (Postnummer, Navn) values ('" + postnr + "', '" + navn + "'); select @@identity as 'identity';", con))
    {
        object ID = command.ExecuteScalar();

        city.id = Convert.ToInt32(ID);
        city.postnr = postnr;
        city.navn = navn;
        return city;
    }
}

the call looks like this:

City city = new City();
city = city.doesExist(zip, by, city, connection); // this works fine
if (city == null)
{
     // I know that city is null
     // tried inserting City city = new City(); same error
     city = city.create(zip, by, city, connection); // this is where the null error occours
}

Answer 1

Well yes, look:

if (city == null)
{
    // If we've got in here, we know that city is a null reference, but...
    city = city.create(...);
}

You're calling a method on a reference which is definitely null. That's guaranteed to throw a NullReferenceException .

You almost certainly want to make your create method static (and rename it to comply with normal .NET naming conventions), and call it as

city = City.Create(...);

You'll also need to remove the city parameter from the method call, and instead create a new City object inside your method. For example:

public static City Create(string postnr, string navn, SqlConnection con)
{
    DatabaseConnection.openConnection(con);
    using (var command = new SqlCommand
         ("insert into [By] (Postnummer, Navn) values (@postnr, @navn); "+
          "select @@identity as 'identity';", con))
    {
        command.Parameters.Add("@postnr", SqlDbType.NVarChar).Value = postnr;
        command.Parameters.Add("@navn", SqlDbType.NVarChar).Value = navn;
        object ID = command.ExecuteScalar();

        City = new City();
        city.id = Convert.ToInt32(ID);
        city.postnr = postnr;
        city.navn = navn;
        return city;
    }
}

Note how I've changed your code to use parameterized SQL too. You really, really, shouldn't put values directly into your SQL statement like that - it opens up your system to SQL injection attacks and makes various conversions messy.

Additionally, I would recommend creating a new SqlConnection (and closing it) for each database operation.

Frankly it's a bit odd for doesExist to be an instance method, too... and again, for it to take a city parameter.

I would suggest changing the design of this so that you have a CityRepository (or something similar) which knows the connection string, and exposes:

// I'd rename these parameters to be more meaningful, but as I can't work out what they're
// meant to mean now, it's hard to suggest alternatives.
public City Lookup(string postnr, string nav)

public City Create(string postnr, string nav)

The repository would know the relevant connection string, and would be responsible for all the database operations. The City type would know nothing about databases.

Answer 2

You are trying to call a method on object class that has not been initialized/is null.

You need to make City.create a static member.

public static City create(string postnr, string navn, City city, SqlConnection con)
{
    DatabaseConnection.openConnection(con);
    using (var command = new SqlCommand("insert into [By] (Postnummer, Navn) values ('" + postnr + "', '" + navn + "'); select @@identity as 'identity';", con))
    {
        object ID = command.ExecuteScalar();

        city.id = Convert.ToInt32(ID);
        city.postnr = postnr;
        city.navn = navn;
        return city;
    }
}

And use it like :

if(city==null) 
{
    City city = City.create(... );
}

Answer 3

the better way :

 static public City create(string postnr, string navn, SqlConnection con)
 {
     DatabaseConnection.openConnection(con);
     using (var command = new SqlCommand("insert into [By] (Postnummer, Navn) values ('" + postnr + "', '" + navn + "'); select @@identity as 'identity';", con))
     {
         object ID = command.ExecuteScalar();

         City city = new City();
         city.id = Convert.ToInt32(ID);
         city.postnr = postnr;
         city.navn = navn;
         return city;
     }

     return null;
 }

Create method must be static, and no need to have city in argument. Call it with :

 if (city == null)
 {
      city = City.Create(.....);
 }