How to skip an element in a JsonArray, when the parsing of that element fails?

user25975796
October 7, 2024
69 views
0 votes
2 Answers

Below is the default code to parse a JsonArray, which works totally fine if there is of course no error or typo in the Json string:

public class Person
{
    public int Id { get; set; }
    public required string Name { get; set; }
}

Program code:

var jsonString = @"
[ 
    { ""Id"": 1, ""Name"": ""name1"" },
    { ""Id"": 2, ""Name"": ""name2"" },
    { ""Id"": 3, ""NameX"": ""name3"" },
    { ""Id"": 4, ""Name"": ""name4"" },
    { ""Id"": 5, ""Name"": ""name5"" }
]
";

List<Person> people = JsonSerializer.Deserialize<List<Person>>(jsonString) ?? [];
    
Console.WriteLine(people.Count);

But notice that in the jsonString, there is unfortunately a typo. Of course, this results in a runtime error. It is possible to wrap the Deserialize<> in a try-catch, but then I have 0 people in my people array.

This is a simplification of the situation I have of course, in my case I get the json from an external source. (Assume it is not possible to skip the required attribute for string Name in this example)

How can I change this code so that I basically have a TryDeserialize<>? I would like to parse every correct object, and skip (and log if that is possible) all the objects that aren’t correct? So that my final people array has 4 people in it (ids 1,2,4,5)?

Tags: .net-core c#json parsing system.text.json

Answers

For such task you would need to implement custom JSON converter for list of Person objects.

Here’s example implementation:

public class ListPfPeopleJsonConverter : JsonConverter<List<Person>>
{
    public override List<Person>? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var people = new List<Person>();
        reader.Read();

        while (reader.TokenType == JsonTokenType.StartObject)
        {
            if(TryReadPerson(ref reader, out var person))
            {
                people.Add(person);
            }
        }

        return people;
    }

    private static bool TryReadPerson(ref Utf8JsonReader reader, out Person person)
    {
        person = null;
        try
        {
            person = JsonSerializer.Deserialize<Person>(ref reader);
            return true;
        }
        catch
        {
            ConsumeJsonObject(ref reader);
            return false;
        }
        finally
        {
            // Move to next item.
            reader.Read();
        }
    }

    private static void ConsumeJsonObject(ref Utf8JsonReader reader)
    {
        // Consume invalid item to the "end of object"
        while (reader.TokenType != JsonTokenType.EndObject)
        {
            reader.Read();
        }
    }

    public override void Write(Utf8JsonWriter writer, List<Person> value, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}

And then the usage:

var jsonString = @"
    [ 
        { ""Id"": 1, ""Name"": ""name1"" },
        { ""Id"": 2, ""Name"": ""name2"" },
        { ""Id"": 3, ""NameX"": ""name3"" },
        { ""Id"": 4, ""Name"": ""name4"" },
        { ""Id"": 5, ""Name"": ""name5"" }
    ]
    ";

var options = new JsonSerializerOptions();
options.Converters.Add(new ListPfPeopleJsonConverter());

List<Person> people = JsonSerializer.Deserialize<List<Person>>(jsonString, options) ?? [];

Console.WriteLine(people.Count);

One option would be to use a JsonReader instead.

static void Main(string[] args)
{
    var jsonString = @"
    [ 
        { ""Id"": 1, ""Name"": ""name1"" },
        { ""Id"": 2, ""Name"": ""name2"" },
        { ""Id"": 3, ""NameX"": ""name3"" },
        { ""Id"": 4, ""Name"": ""name4"" },
        { ""Id"": 5, ""Name"": ""name5"" },
        { ""IdX"": 1, ""Name"": ""name6"" }
    ]
";

    List<Person> people = new List<Person>();
    var reader = new Utf8JsonReader(System.Text.Encoding.UTF8.GetBytes(jsonString));

    while (reader.Read())
    {
        if (reader.TokenType == JsonTokenType.StartObject)
        {
            int? id = null;
            string? name = null;

            while (reader.Read() && reader.TokenType != JsonTokenType.EndObject)
            {
                if (reader.TokenType == JsonTokenType.PropertyName)
                {
                    string? propertyName = reader.GetString();
                    reader.Read();

                    if (propertyName == "Id")
                        id = reader.GetInt32();
                    else if (propertyName == "Name")
                        name = reader.GetString();
                }
            }

            if (name != null && id != null)
                people.Add(new Person { Id = id.Value, Name = name });
        }
    }

    Console.WriteLine(people.Count);
}

public class Person
{
    public int Id { get; set; }
    public required string Name { get; set; }
}

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