Razor - Variable in Foreach

Question

In this code, I use variable num like a flag if equal 0 I use
td class="row_grid_Par" else if equal 1 I use td class="text-row_grid_Dis. Why this code doesn't work ?

    <tbody>
        @{int num = 0};
        @foreach (var CodRis in Model)
        {            
            <tr>
                <td>
                    @CodRis.CodRis
                </td>

                @if (num == 0)
                {
                    <td class="row_grid_Par">
                        @CodRis.DescRis
                    </td>
                    @{num = 1};
                }
                else
                {
                    <td class="text-row_grid_Dis">
                        @CodRise.DescRis
                    </td>
                    @{num = 0};
                }
            </tr>
        }
    </tbody>

Answer 1

  <tbody>
        @{int num = 0};
        @foreach (var CodRis in Model)
        {            
            <tr>
                <td>
                    @CodRis.CodRis
                </td>
                <td class='@(num==0?"row_grid_Par":"text-row_grid_Dis")'>
                   @CodRis.DescRis
                </td>     
            </tr>
           num=num==0?1:0;
        }
    </tbody>