Error creating a method in a legacy WFA application and polymorphism

Question

I created a WFA project that will serve as a basis for the others, follows the description:

1. I created a Formbase that has public control variables, such as operator name, date and time of access to the systems;

2. I created a Formbasemain, which instance Formbase, putting in this a menu, status bar, toolbar;

3. I created a Formbasequery, which instance Formbase, in this I put a toolbar, a Datagridview , I installed a DAL class that I created to connect to the database (postgres, at first).

So far everything works legal.

My difficulty is in the Formbasequery class that I have a method to get the SQL string as follows:

    public virtual string getSCriptConsulta()
    {
        sScriprtSQL = "";

        return sScriprtSQL;
    }

In the Formcomestados class, which instance Formbasequery has a class that defines SQL:

    public override string getSCriptConsulta() 
    {
        return "select ufcodigo as codigo, " +
               "       ufsigla as uf, " +
               "       ufnome as estado " +
               "from cadastros.tbestados " +
               "order by ufnome ";
    }

Everything is working well, bringing the list of states, but I want to change some properties of the columns, for this I created a class as follows:

namespace wfaPraticandoHeranca
{
    class EntidadeGridColumns
    {
        private string sFieldName = null;
        private string sHeaderText = null;
        private int iMinimumWidth = 0;
        private string sToolTipText = null;
        private int iWidth = 0;
        private bool bFroze = false;

        public string FieldName
        {
            get { return sFieldName; }
            set { sFieldName = value; }
        }

        public string HeaderText
        {
            get { return sHeaderText; }
            set { sHeaderText = value; }
        }

        public int MinimumWidth
        {
            get { return iMinimumWidth; }
            set { iMinimumWidth = value; }
        }

        public string ToolTipText
        {
            get { return sToolTipText; }
            set { sToolTipText = value; }
        }

        public int Width
        {
            get { return iWidth; }
            set { iWidth = value; }
        }

        public bool Frozen
        {
            get { return bFroze; }
            set { bFroze = value; }
        }
    }
}

In Formbasequery I created a class as follows:

    public virtual EntidadeGridColumns getGridColumnsProperties()
    {
        EntidadeGridColumns colProperties = new EntidadeGridColumns[0];

        return colProperties;
    }

This class is reporting me the error (with getGridColumnsProperties() underlined in blue):

Error 3 Inconsistent Accessibility: Return type 'wfaPraticandoHeranca.Entidadegridcolumns' is Less accessible than method 'wfaPraticandoHeranca.frmModeloConsultas.getGridColumnsProperties()'

In Formconstados I overwrote the class:

    public override EntidadeGridColumns setColumnsProperties()
    {
        EntidadeGridColumns colProperties = new EntidadeGridColumns[3];

        colProperties[0] = new EntidadeGridColumns();
        colProperties[0].FieldName = "codigo";
        colProperties[0].HeaderText = "Código";
        colProperties[0].MinimumWidth = 75;
        colProperties[0].Width = 75;
        colProperties[0].ToolTipText = "Código do cadastro do Estado...";
        colProperties[0].Frozen = true;

        colProperties[1] = new EntidadeGridColumns();
        colProperties[1].FieldName = "sigla";
        colProperties[1].HeaderText = "Sigla";
        colProperties[1].MinimumWidth = 60;
        colProperties[1].Width = 60;
        colProperties[1].ToolTipText = "UF do Estado...";
        colProperties[1].Frozen = false;

        colProperties[2] = new EntidadeGridColumns();
        colProperties[2].FieldName = "estado";
        colProperties[2].HeaderText = "Estado";
        colProperties[2].MinimumWidth = 250;
        colProperties[2].Width = 25;
        colProperties[2].ToolTipText = "Nme do Estado...";
        colProperties[2].Frozen = false;

        return colProperties;

    }

This class is reporting me the error (with "getGridColumnsProperties()" underlined in blue):

Error 2 Inconsistent Accessibility: Return type 'wfaPraticandoHeranca.Entidadegridcolumns' is Less accessible than method 'wfaPraticandoHeranca.frmConState.setColumnsProperties()'

Finally, these are the mistakes I’m having, if you can help me I’ll be grateful, because I have no idea how to fix this!

When I see the class EntidadeGridColumns cannot be used as a return of the method and I do not know how to do in C#.

Note: - Another error that is happening, since these are errors 2 and 3, the design of the Formconestados no longer opens reporting the error:

> Connection string argument missing! Parameter name: Host

Mas executando tudo funciona devidamente; já excluí as pastas BIN e OBJ mas não deu certo. Já verifiquei no "Npgsql" este parâmetro "Host" e não encontre nada, apenas "Server".

Forgive me, I don’t even know how to define the title for this situation.