Am havg this error that says cannot explicitly call operator

Question

I think am on the right truck but i dont know where am messing up .Any advice is welcome.

 

Here is my code

private Worksheet wks(string string_1, ExpandableGridView expandableGridView_2)
        {
            IEnumerator enumerator = null;
            IEnumerator enumerator1 = null;
            Worksheet worksheet;
            Worksheet worksheet1 = new Worksheet();
            int num = 0;
            int num1 = 0;
            try
            {
                this.list_0.Clear();
                try
                {
                    enumerator = expandableGridView_2.Columns.GetEnumerator();
                    while (enumerator.MoveNext())
                    {
                        DataGridViewColumn current = (DataGridViewColumn)enumerator.Current;
                        if (!current.Visible)
                        {
                            continue;
                        }
                        this.list_0.Add(current.Index);
//the error appears on the below lines of code where it says get_Item
                        worksheet1.CellGrid.get_Item(num1, num).set_Value(current.HeaderText);
                        worksheet1.CellGrid.get_Item(num1, num).set_WrapText(true);
                        worksheet1.CellGrid.get_Item(num1, num).set_BackgroundColor(Color.Gray);
                        worksheet1.CellGrid.get_Item(num1, num).set_Color(Color.White);
                        worksheet1.CellGrid.get_Item(num1, num).set_VerticalAlignment(1);

                        if (num1 > 0)
                        {
                            worksheet1.CellGrid.get_Item(num1, num).set_RotationAngle(-90);
                        }
                        num1 = checked(num1 + 1);
                    }
                }