ComboBox的xaml绑定和后台代码绑定,求帮助？？

xaml中:
 <ComboBox SelectedValue="{Binding DepId}" SelectedValuePath="Id" DisplayMemberPath="Name"/>
后台代码： comboBox1.ItemsSource = DepartmentDAL.GetAll();
这样是成功的！但是<ComboBox  />不在xaml中绑定，完全在后台的话：
              comboBox1.ItemsSource = DepartmentDAL.GetAll();
            comboBox1.SelectedValue = "DepId";
            comboBox1.DisplayMemberPath = "Name";
            comboBox1.SelectedValuePath = "Id";
就会出现FormatException
GUID 应包含带 4 个短划线的 32 位数(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)。错误！！
求解？？？？？
bindingC#