Border per column header

Question

I want to create a table on wpf that each column header has a round corner this is what i got so far:

as you can see, i have the desired outcome with a little un desired outcome. the undesired out come is that the all data grid header itself (not the columns) is getting the same border, i need to make it transparent, how can i do that?

this is the part of the style:

<DataGrid.ColumnHeaderStyle>
    <Style TargetType="{x:Type DataGridColumnHeader}">
        <Setter Property="SeparatorBrush"  Value="Transparent"/>
        <Setter Property="Margin" Value="2"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate>                                        
                        <Grid>
                            <Border CornerRadius="5 5 0 0" BorderThickness="1" BorderBrush="Black">
                                <TextBlock Text="{Binding }"/>
                            </Border>
                        </Grid>                                        
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</DataGrid.ColumnHeaderStyle>  

Answer 1

for some reasons, DataGrid has a blank DataGridColumnHeader in its template. that blank columns doesn't have DataContext value (null). So change border brush to transparent in a DataTrigger:

<ControlTemplate>
    <Grid>
        <Border CornerRadius="5 5 0 0" BorderThickness="1" >
            <TextBlock Text="{Binding}"/>
            <Border.Style>
                <Style TargetType="Border">
                    <Setter Property="BorderBrush" Value="Black"/>
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding}" Value="{x:Null}">
                            <Setter Property="BorderBrush" Value="Transparent"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </Border.Style>
        </Border>
    </Grid>
</ControlTemplate>

---

improved version, which uses ContentPresenter in header template and test Content in a trigger

<ControlTemplate TargetType="{x:Type DataGridColumnHeader}">
    <Grid>
        <Border CornerRadius="5 5 0 0" BorderThickness="1" >
            <ContentPresenter/>                                        
            <Border.Style>
                <Style TargetType="Border">
                    <Setter Property="BorderBrush" Value="Black"/>
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding Path=Content, RelativeSource={RelativeSource TemplatedParent}}" Value="{x:Null}">
                            <Setter Property="BorderBrush" Value="Transparent"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </Border.Style>
        </Border>
    </Grid>
</ControlTemplate>

header is not necessary a text, e.g.:

<DataGrid.Columns>
    <DataGridTextColumn Binding="{Binding Path=Name}">
        <DataGridTextColumn.Header>
            <Border Background="Cyan">
                <TextBlock Text="NAME" Margin="5"/>
            </Border>
        </DataGridTextColumn.Header>
    </DataGridTextColumn>
</DataGrid.Columns>