Get the selected checkboxs count on click in Datagridview

I need to get the count of the check boxes which are selected, in a datagridview.

I have added the check box column through code,
DataColumn colSelection = new DataColumn();
 colSelection.DataType = System.Type.GetType("System.Boolean");
 colSelection.ColumnName = "Select";
 this.objdatatable.Columns.Add(colSelection);
 this.dataGridViewResources.DataSource = this.objdatatable;

To get the count i have called a method on the cellContentClick event . Please refer the below code. The code gives me correct count when the check boxes are selected. But if i unCheck the check boxes the count is not correct (as chkActive.Selected  remains true even if i uncheck the check box)

any suggestions?

//method call on cell content click event 
private void dataGridViewResources_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
            if (e.ColumnIndex == 0)
            {
                string selectedCount = this.GetSelectedCount();
                lblSelectedCount.Text = selectedCount;
            }
        }

//method which returns the count of the check box selected
 private string GetSelectedCount()
        { 
            try
            {
                for (int i = 0; i <= dataGridViewResources.Rows.Count - 1; i++)
                {                  
                    DataGridViewCheckBoxCell chkActive = (DataGridViewCheckBoxCell)dataGridViewResources.Rows[i].Cells[0];
                    if (chkActive.Selected == true)
                    {                   
                        selectedCount = selectedCount + 1;                    
                    }                   
                }
            }
            catch (Exception ex)
            {                
            }
            return selectedCount.ToString();  
        }

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