How to change the CornerRadius of the Combobox WPF

I want to use a ComboBox with different CornerRadius, how can I change that simply? I've tried with Style and ControlTemplate, but without any success.

I don't know if this is simple, but creating a ControlTemplate based on the default ComboBox should do the trick. Here is an example:

    <Style x:Key="ComboBoxTextBoxStyle" TargetType="{x:Type TextBox}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type TextBox}">
                    <Grid>
                        <Border CornerRadius="5,0,0,5"
                            BorderThickness="1"
                            Background="{TemplateBinding Background}"
                                BorderBrush="Black">
                            <ScrollViewer x:Name="PART_ContentHost"/>
                        </Border>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style TargetType="{x:Type ComboBox}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ComboBox}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition/>
                            <ColumnDefinition MaxWidth="18"/>
                        </Grid.ColumnDefinitions>
                        <TextBox Name="PART_EditableTextBox"
                                 Style="{StaticResource ComboBoxTextBoxStyle}"
                                 Padding="5,0,0,0"
                                 Height="{TemplateBinding Height}"/>
                        <ToggleButton Grid.Column="1" Margin="0"
                                     Height="{TemplateBinding Height}"
                                     Style="{StaticResource ComboBoxButtonStyle}"
                                     Focusable="False"
                                     IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                                      ClickMode="Press">
                            <Path Grid.Column="1"
                                  HorizontalAlignment="Center"
                                  VerticalAlignment="Center"
                                  Data="M 0 0 L 4 4 L 8 0 Z"
                                  Fill="DodgerBlue" />
                        </ToggleButton>
                        <ContentPresenter Name="ContentSite"
                                      Content="{TemplateBinding SelectionBoxItem}"
                                      ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                                      ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
                                      VerticalAlignment="Center"
                                      HorizontalAlignment="Left"
                                      Margin="5,0,0,0"/>
                        <Popup Name="Popup"
                               Placement="Bottom"
                               IsOpen="{TemplateBinding IsDropDownOpen}"
                               AllowsTransparency="True" 
                               Focusable="False"
                               PopupAnimation="Slide">
                            <Grid Name="DropDown"
                                  SnapsToDevicePixels="True"                
                                  MinWidth="{TemplateBinding ActualWidth}"
                                  MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                <Border 
                                    x:Name="DropDownBorder"
                                    BorderThickness="1"
                                    CornerRadius="5"
                                    Background="Azure"
                                    BorderBrush="Black"/>
                                <ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True">
                                    <StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
                                </ScrollViewer>
                            </Grid>
                        </Popup>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

You will need to define the VisualStates/Triggers in the Style if required