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1. Declare the ObservableCollection property as private or protected, to restrict access to it from outside the class.
2. Define a public readonly property that returns a copy of the original ObservableCollection. This way, consumers can access the data but cannot modify the original collection.
3. Use the ToList() method of ObservableCollection to create a new List<t> that contains the same elements as the ObservableCollection, and return the List as an IReadOnlyList<t>. This provides read-only access to the elements of the collection.
4. Optionally, you can implement the IEnumerable<t> interface on the read-only property to allow consumers to iterate over the elements using foreach.

Example code:

public class MyClass {
    private ObservableCollection<string> myCollection = new ObservableCollection<string>();

    public IReadOnlyList<string> MyReadOnlyCollection {
        get { return myCollection.ToList().AsReadOnly(); }
    }
}

Note: While this approach provides a read-only view of the collection, it does not protect against modification of the original objects within the collection. If you need to ensure that the elements of the collection cannot be modified, you will need to use immutable objects or create your own read-only collection class that enforces immutability.