<ComboBox Name="drpInstallation" SelectionChanged="drpInstallation_SelectionChanged" />

List<string> installations = new List<string>();
installations.Add("Testing 123");
installations.Add("Antoher test");
installations.Add("Yeah");
drpInstallation.ItemsSource = installations;

<ComboBox Name="drpInstallation" SelectionChanged="drpInstallation_SelectionChanged">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <ComboBoxItem Content="{Binding Installation}" />
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

ICollection<InstallationClass> installations = a list of the installation class;
drpInstallation.ItemsSource = installations;

public class InstallationClass
{
    public int PK;
    public string Installation;
}

<ComboBox Name="drpInstallation" SelectionChanged="drpInstallation_SelectionChanged">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <TextBox Text="{Binding Installation}" />
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>