Page.GetFormulas

    var pages = doc.Pages;
    var page = pages.Add();
    page.NameU = "PGF";

    var shape = page.DrawRectangle(1, 1, 4, 3);
    shape.get_CellsU("Width").Formula = "=(1.0+2.5)";
    shape.get_CellsU("Height").Formula = "=(0.0+1.5)";

    // BUILD UP THE REQUEST
    var items = new[]
    {
        new {   shapeid = (short) shape.ID,
                section = (short) IVisio.VisSectionIndices.visSectionObject, 
                row     = (short) IVisio.VisRowIndices.visRowXFormOut, 
                cell    = (short) IVisio.VisCellIndices.visXFormWidth },

        new {   shapeid = (short) shape.ID,
                section = (short) IVisio.VisSectionIndices.visSectionObject, 
                row     = (short) IVisio.VisRowIndices.visRowXFormOut, 
                cell    = (short) IVisio.VisCellIndices.visXFormHeight }
    };

    // MAP THE REQUEST TO THE STRUCTURES VISIO EXPECTS
    short[] SID_SRCStream = new short[items.Length * 4];
    for (int i = 0; i < items.Length; i++)
    {
        SID_SRCStream[i * 4 + 0] = items[i].shapeid;
        SID_SRCStream[i * 4 + 1] = items[i].section;
        SID_SRCStream[i * 4 + 2] = items[i].row;
        SID_SRCStream[i * 4 + 3] = items[i].cell;
    }

    // EXECUTE THE REQUEST
    System.Array formulas_sa;
    page.GetFormulasU(SID_SRCStream, out formulas_sa);

    // MAP OUTPUT BACK TO SOMETHING USEFUL 
    string [] formulas = new string[formulas_sa.Length];
    formulas_sa.CopyTo(formulas, 0);

    // DISPLAY THE INFORMATION
    shape.Text = string.Format("Formulas={0},{1}", formulas[0], formulas[1]);

Shape.GetFormulas

public static void Shape_GetFormulas(IVisio.Document doc)
{
    var pages = doc.Pages;
    var page = pages.Add();
    page.NameU = "SGF";

    var shape = page.DrawRectangle(1, 1, 4, 3);
    shape.get_CellsU("Width").Formula = "=(1.0+2.5)";
    shape.get_CellsU("Height").Formula = "=(0.0+1.5)";

    // BUILD UP THE REQUEST
    var items = new[]
    {
        new {   section = (short) IVisio.VisSectionIndices.visSectionObject, 
                row     = (short) IVisio.VisRowIndices.visRowXFormOut, 
                cell    = (short) IVisio.VisCellIndices.visXFormWidth },

        new {   section = (short) IVisio.VisSectionIndices.visSectionObject, 
                row     = (short) IVisio.VisRowIndices.visRowXFormOut, 
                cell    = (short) IVisio.VisCellIndices.visXFormHeight }
    };

    // MAP THE REQUEST TO THE STRUCTURES VISIO EXPECTS
    short[] SRCStream = new short[items.Length * 3];
    for (int i = 0; i < items.Length; i++)
    {
        SRCStream[i * 3 + 0] = items[i].section;
        SRCStream[i * 3 + 1] = items[i].row;
        SRCStream[i * 3 + 2] = items[i].cell;
    }

    // EXECUTE THE REQUEST
    System.Array formulas_sa;
    shape.GetFormulasU(SRCStream, out formulas_sa);

    // MAP OUTPUT BACK TO SOMETHING USEFUL 
    var formulas = new string[formulas_sa.Length];
    formulas_sa.CopyTo(formulas, 0);

    // DISPLAY THE INFORMATION
    shape.Text = string.Format("Formulas={0},{1}", formulas[0], formulas[1]);
}