Set DataGridHeaderStyle in code behind

Hello,

I've set the DataGridHeaderStyle in the XAML-Code:

<DataGrid Name="dtgrd_contacts" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="0,73,0,27" CanUserAddRows="False" CanUserDeleteRows="False" IsReadOnly="True" AlternationCount="2" MouseDoubleClick="dtgrd_contacts_MouseDoubleClick"><DataGrid.ColumnHeaderStyle><Style TargetType="{x:Type DataGridColumnHeader}"><Setter Property="HorizontalContentAlignment" Value="Stretch" /></Style></DataGrid.ColumnHeaderStyle></DataGrid>

An this code is wrking fine...

But I need this code in the code behind and I have tried this:

DataGrid tmpDatagrid = dtgrd_contacts;
Style tmpStyle = new Style(typeof(DataGridColumnHeader));
tmpStyle.Setters.Add(new Setter(DataGridColumnHeader.HorizontalAlignmentProperty, HorizontalAlignment.Stretch));
tmpDatagrid.ColumnHeaderStyle = tmpStyle;

But this code doesn't work and I have no idea why. In my opinion it should be the same as in the XAML-Code...

Please help me. What is wrong?

Thank you!