ComboBox in Silverlight

try this

if in view:

ComboBox1.SelectedIndex = -1 

or dou you want it in viewmodel,

then Have a property for selected item. and set it to null:

For eg:

public List<gender> GenderList
{get;set;}

//include notify property changed for that property..the above one is just outline

Then have a property

Public Gender SelectedGender
{get;set;}

Bind Itemsource to GenderList property and Selectedvalue property to SelectedGender.

Whenever You want to clear the selection Set SelectedGender =null. SInce Notifyproperty changed element is there, it will clear the selection