Why compiler treats System.Object as null when its set to False

Question

I wrote a generic method to retrieve single values from database (MSSQL Server).
I encountered into a case that I need to get a Boolean value from DB.

As you can see in the code below, a Object local field (IsExist) gets the result.

When the value in DB is False GenericScalar() method return False (as it should) and the condition: if (IsExist == null) in GetWanLineDisconnectionData() is true and the return block is executing, even though IsExist is False and not null.

Why is that?

How can I overcome this problem?

private void GetWanLineDisconnectionData()
{
    string q = "SELECT WanLineDiscconection FROM AdditionalProjectsData WHERE SpCall= " + "'" + spCall + "'";
    object IsExist = Orange.ProjectManagment.DAL.Database.GenericScalar<object>(q);
    if (IsExist == null) {
        return;
    }
    if (bool.Parse(IsExist) == true) {
        RadWanDiscYes.Checked = true;
    } else {
        RadWanDiscNo.Checked = true;
    }    
}

Database method:

public static T GenericScalar<T>(string query)
{
    OleDbConnection connection = new OleDbConnection(sqlConnString);
    connection.Open();

    OleDbCommand cmd = new OleDbCommand(query, connection);
    try
    {
        var result = cmd.ExecuteScalar();

        if (result == null)
        {
            return default(T);
        }
        else
        {
            return (T)result;
        }
    }

    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        CloseConnection(ref connection);
    }
}

EDIT:
maybe a few screen shoots will better demonstrate it: (note that: GetWanLineDisconnectionData() is written in VB.NET and GenericScalar() is written in C# on a different project in the solution):

1. in the beginning IsExist is nothing (null).

2. the query finds one row and the value of the bool column WanLineDiscconection is false and IsExist is set to false as it should be.

3. here is the problem, the program enters the if block and IsExist is not nothing (null).

Answer 1

The variable foo in object foo = false is definitely not null, so the premise in your title is incorrect.

ExecuteScalar() returns null when there are no rows. In that case, the method GenericScalar() return default(T), which for object will be null.

How to solve this depends on what your data looks like. You probably want to represent the result in a nullable int, or int? instead of object:

var exists = Orange.ProjectManagment.DAL.Database.GenericScalar<int?>(q);
RadWanDiscYes.Checked = exists.GetValueOrDefault() > 0;

See How does GetValueOrDefault work?, What is the default value of the nullable type "int?" (including question mark)?.

Aside from that: you generally also don't want to write handy wrappers around database queries, because you're reinventing the wheel poorly, opening up your application to SQL injection. That being said, there's a lot more going wrong in that method, including but not limited to not disposing your database connection and rethrowing exceptions without their stacktrace.

Answer 2

You´re mixing compile-time and runtime information. Whilst GenericScalar<object>(q) is an information that exists at compile-time the type returned from ExecuteScalar at compile-time is just object. You expect it to be of type boolean, which might or might not be true for your specific case. However this is not relevant to the compiler.

In short that means that T actually is object, not whatever you expect it to be from your database. And as CodeCaster allready mentioned the default-value for object is just null.

Si I´d suggest to use GenericScalar<bool> instead of GenericScalar<object> because you seem to know what your query actually returns - in your case a bool. Then default(T) evaluates to false, not to null.