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WPF : how to customize ComboBox's dropdown list selecting item style?

I want to customize ComboBox's dropdown items' selection style, like the blue selection rect below:
but I can't find what kind of style is controlling this item. so far i doubt maybe ContentTemplateSelector property of ContentPresenter do the trick but I don't know how to customize ContentTemplateSelector's style.
so anyone please tell me how to customize the selection item style in dropdown panel?
edit: I want to know the mouse-hover style on dropdown item.

Right-click on the ComboBox in design mode in Visual Studio or in Blend and choose Edit Additional Templates->Edit Generated Item Container (ItemContainerStyle)->Edit a Copy.
This will copy the default template of the ComboBoxItem containers into your XAML markup and you can then edit it as per your requirements.
Look for the resources with the x:Keys ComboBoxItem.ItemsviewHover.Background, ComboBoxItem.ItemsviewSelectedHover and ComboBoxItem.ItemsviewHoverFocus.Background and change the colour of these.
There is no property of the ComboBox that you can simply set to change the selection colour on Windows 8 and later. Please refer to the following blog post for more information: https://blog.magnusmontin.net/2014/04/30/changing-the-background-colour-of-a-combobox-in-wpf-on-windows-8/.

System.Window.Forms.CheckBox databinding won't get hit

I have a problem with System.Window.Forms.CheckBox databinding. I generate a checkbox and then map it's value to a property on a BindingSource via
Checkbox → Properties → DataBindings → Checked → BindingSource - PropertyValue
I then have a CheckedChanged event that get's triggered when i click on the checkbox. However on click the BindingSource property is not updated. Because of this I have to use checkbox Design -> Name to get the value of the checkbox. Should the binding be done in another way? I can map strings this way with no problem but for some reason a checkbox bool seems harder to map.

When you setup data-binding, the default value for Data Source Update Mode is OnValidation which means Data source is updated when the control property is validated. So in the current state if you move the focus to another control the data-binding will update the data source property.
If you want data source update whenever the value of the control property changes, you should change the setting to OnPropertyChanged.
You can change the setting using designer this way:
Select the control and in property grid, expand (DataBindings) and click ... in from of (Advanced) to open Formatting and Advanced Binding.
In the window, from the left Property list, select the property which you want to perform advanced setting for it.
At right pane, from Data Source Update Mode select OnPropertyChanged. To learn more about other available values, take a look at DataSourceUpdateMode documentations.

Read only combobox?

How do I make a combobox read only? I remember back when I used VS2005 you could have a read only combobox in the default style. Reason I don't want to use DropDownList is because it looks ugly on forms that don't use a default color. Here is an example.
I want one like in the left of this picture.
Were you can not enter in your own text. But I don't want the style in the first image to the right.

Set the drop down style to DropDownList and the flatStyle to Flat:
this.ComboBox1.DropDownStyle = ComboBoxStyle.DropDownList;
this.comboBox1.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
You can select always the element[0] (if exists) to look like the example you provided.
The Visual Studio combo draws a border OnMouseHover, you can also do this if you want.

Change the drop down style, it's a property.
See: http://msdn.microsoft.com/en-us/library/system.windows.forms.combobox.dropdownstyle.aspx

C# How to set default value of a string collection in Design

In WinForm, I have a combobox with DropDownStyle set to DropDownList (so can't enter a Text). In the properties window, there is the Items property which is a string collection. I enter all my values.
But now, I would like to set one of these value by default (instead to have the empty entry at run-time). I know how to do this via coding, but I am pretty sure (damn memory) that it was possible to set one of the value in the string collection as default by adding a special symbole in front of the line.
Anybody know that symbole? Or my memory is playing me trick and it is not possible to do it via the designer?

Doesn't look like it can be done when using a DropDownList. From here it is suggested that you can set the text property to the default value you want, but this will only works in a DropDown rather than DropDownList style.

I'm sorry but that is not possible within the Designer only, since the Text property is used for this feature and that property is ignored/cleared when the using a DropDownList.
If you don't mind having your data values outside of the Designer, you could probably use DataBinding to accomplish this since the DisplayMember and ValueMember properties of ComboBox can be used in the Designer and would set the display value. I don't normally use DataBinding so unfortunately I cannot provide code examples - perhaps another user can chime in?

data binding update when value changes, not when tabbing out

Is there a property or setting to force a bound control in Winforms to update the object it is bound to when the input value, either in a textbox or whatever, actually changes?
And not after the control is tabbed out of.

Assuming you have already bound the textbox to something:
Select the textbox in the designer
Click property "databindings"
Click property "advanced"
In this dialog look for the "datasource update mode"
Select onpropery changed instead of onvalidate
Now the datasource gets updated on every change of the text property of the textbox.