Error "Object reference not installed"

Question

I have the error "The reference to the object is not installed in the instance of the object." by the following method:

        private void alSave_Click(object sender, EventArgs e)
    {
        _alRecord.WriteXml(@".\alRecord.xml", XmlWriteMode.WriteSchema);
    }

and I don’t know what I can do ... here is the code:

        private string alFile = @".\alRecord.xml";

    public DataTable _alRecord;
    private DataTable alRecord
    {
        get
        {
            if (_alRecord == null)
            {
                _alRecord = new DataTable();
                if (File.Exists(alFile))
                { _alRecord.ReadXml(alFile); }
                else
                { InitDataTable2(_alRecord); }
            }
            return _alRecord;
        }
    }

    private void InitDataTable2(DataTable table)
    {
        table.TableName = "AlTable";
        table.Columns.Add("ID", typeof(int));
        table.Columns.Add("sun", typeof(bool));
        table.Columns.Add("mon", typeof(bool));
        table.Columns.Add("tue", typeof(bool));
        table.Columns.Add("wed", typeof(bool));
        table.Columns.Add("thu", typeof(bool));
        table.Columns.Add("fri", typeof(bool));
        table.Columns.Add("sat", typeof(bool));
        table.Columns.Add("doors", typeof(string));
        table.Columns.Add("from1", typeof(DateTime));
        table.Columns.Add("to1", typeof(DateTime));
        table.Columns.Add("from2", typeof(DateTime));
        table.Columns.Add("to1", typeof(DateTime));
        for (int i = 0; i < 99; i++)
        {
            var row = alRecord.NewRow();
            row["ID"] = i;
            row["sun"] = false;
            row["mon"] = false;
            row["tue"] = false;
            row["wed"] = false;
            row["thu"] = false;
            row["fri"] = false;
            row["sat"] = false;
            row["doors"] = "";
            row["from1"] = "00:01";
            row["to1"] = "23:59";
            row["from2"] = "00:01";
            row["to2"] = "23:59";
            alRecord.Rows.Add(row);
        }
    }
    private void alSave_Click(object sender, EventArgs e)
    {
        _alRecord.WriteXml(@".\alRecord.xml", XmlWriteMode.WriteSchema);
    }

Answer 1

private void alSave_Click(object sender, EventArgs e) 
    { 
        _alRecord.WriteXml(@".\alRecord.xml", XmlWriteMode.WriteSchema); 
    } 

_alRecordonly loads when you reference it by property name alRecord, so it should be written as:

private void alSave_Click(object sender, EventArgs e) 
    { 
        alRecord.WriteXml(@".\alRecord.xml", XmlWriteMode.WriteSchema); 
    } 

Answer 2

You are using a method called lazy initialization. _alRecord is not assigned until alRecord is called.

Your alSave_Click method uses the wrong alRecord ... it should use one that does not have an underscore prefix. I.e.

private void alSave_Click(object sender, EventArgs e) 
{ 
    alRecord.WriteXml(@".\alRecord.xml", XmlWriteMode.WriteSchema); 
} 

, alReord ( ).

public DataTable _alRecord; //Normally **private** members would have an underscore prefix    
private DataTable alRecord  //And public members would have no prefix.

, _alRecord , . .

. , , , _alRecord, ; , , .

private void alSave_Click(object sender, EventArgs e) 
{ 
  alRecord;
  _alRecord.WriteXml(@".\alRecord.xml", XmlWriteMode.WriteSchema); 
} 

Answer 3

Edit:

public DataTable m_alRecord;// don't use it
private DataTable alRecord
{
    get
    {
        if (m_alRecord == null)
        {
            m_alRecord = new DataTable();
            if (File.Exists(alFile))
            { m_alRecord.ReadXml(alFile); }
            else
            { InitDataTable2(m_alRecord); }
        }
        return _alRecord;
    }
    set { m_alRecord = value; }
}