"Property or indexer of type RepeatedField cannot be assigned to — it is read only" - A Protobuf Decoding Headache
Problem: You're working with Google Protocol Buffers (Protobuf) and encounter the error "Property or indexer of type RepeatedField cannot be assigned to — it is read only." This error indicates that you're trying to modify a field declared as RepeatedField directly, which is not permitted in Protobuf.
Understanding the Issue:
Imagine you're building a system that stores information about books. You want to store multiple authors for each book, so you use a RepeatedField to hold the authors' names. While you can access the individual elements within a RepeatedField, you can't directly assign a new value to the entire field. Think of it like a read-only list - you can read the contents but not directly overwrite the entire list.
Scenario and Code Example:
Let's say you have a Protobuf message definition like this:
message Book {
string title = 1;
repeated string authors = 2;
}
And you attempt to modify the authors field like this:
Book book = new Book();
book.authors = new List<string> {"Author1", "Author2"}; // Error!
This code will result in the "Property or indexer of type RepeatedField cannot be assigned to — it is read only" error.
The Right Approach: Using Add and AddRange
Protobuf's RepeatedField is designed for flexibility and efficient data management. To modify the contents of a RepeatedField, you should use the methods provided:
Add(T item): Adds a single item to theRepeatedField.AddRange(IEnumerable<T> items): Adds a collection of items to theRepeatedField.
Here's the corrected code:
Book book = new Book();
book.authors.Add("Author1");
book.authors.Add("Author2");
This approach ensures that the RepeatedField is managed correctly and avoids the read-only error.
Why is RepeatedField Read-Only?
The read-only nature of RepeatedField is due to the underlying implementation of Protobuf. RepeatedField is designed to handle efficiently any number of elements, even zero, and to guarantee data consistency across different platforms. Directly assigning a new value could potentially lead to unexpected behavior and data corruption.
Additional Considerations:
- Efficiency: Using
AddandAddRangeis often more efficient than creating a new list and assigning it to theRepeatedField, as it allows the underlying data structures to be optimized. - Data Integrity: This design ensures that changes to
RepeatedFieldare always tracked and managed in a way that preserves data integrity.
Conclusion:
Understanding the read-only nature of RepeatedField is crucial when working with Protobuf. By utilizing the provided methods like Add and AddRange, you can modify RepeatedField data effectively while maintaining data integrity and leveraging the efficient data handling capabilities of Protobuf.
