C#: Option to validate duplicate fields during json deserialization

[searoz]

The issue

Proto:

syntax = "proto3";

package test;

message User {
  string first_name = 1 [json_name = "first_name"];
  string last_name = 2;
}

Code:

using Google.Protobuf;
using Test;

const string json =
    """
    {
        "first_name": "John",
        "first_name": "Alan",
        "last_name": "Doe",
        "lastName": "Smith"
    }
    """;

var user = JsonParser.Default.Parse<User>(json);
Console.WriteLine($"{user.FirstName} {user.LastName}");

Output:

Alan Smith

Why is this a problem?

This is a pretty unexpected behavior. Even using json_name cannot fully prevent one from using duplicate fields in json

What about implementations in other languages?

protobuf-go features a specific check for duplicate fields

What does this PR do?

This PR introduces an optional setting IgnoreDuplicateFields (true by default). If set to false, it makes JsonParser validate duplicate fields. Coming back to out example:

Code:

using Test;

const string json =
    """
    {
        "first_name": "John",
        "first_name": "Alan",
        "last_name": "Doe",
        "lastName": "Smith"
    }
    """;

var user = JsonParser.Default.WithIgnoreDuplicateFields(false).Parse<User>(json);
Console.WriteLine($"{user.FirstName} {user.LastName}");

Output:

Google.Protobuf.InvalidProtocolBufferException: Multiple values specified for field first_name

[google-cla]

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[tonyliaoss]

Hello @searoz, we can't start reviewing your PR unless the CLA has been signed.