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C# convert int to Null

Stanton_Roux asked
Last Modified: 2012-05-09
I have a few drop down boxes which feed into a SQL query. The query is written so that any of the parameters can be Null and thus excluded from the query. If the selected value of any of the drop down boxes is 0 then i would like to set that parameter to null but I am getting compile time errors.
When I do Convert.ToInt32(null), it still returns 0.

pseudocode: if tddnRegionId.SelectedValue == 0 then make RegionID = null

Code below,
protected void btnGo_Click(Object sender, EventArgs e)
        int RegionIDNull = Convert.ToInt32(null);
        int RegionID; 

        RegionID = Convert.ToInt32(tddnRegion.SelectedValue) == 0 ? RegionIDNull : Convert.ToInt32(tddnRegion.SelectedValue);
        int Month = Convert.ToInt32(tddnMonth.SelectedValue);

        ResultsManager rm = new ResultsManager();
            DataSet ds = new DataSet();
            ds = rm.getEventDetailsForCalendar(RegionID, Month);

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