Can I decompress and deserialize a file using streams?

JsonSerializer has methods to serialize from a JsonTextReader and to a StreamWriter, both of which can be created on top of any sort of stream, including a GZipStream. Using them, you can create the following extension methods:

public static partial class JsonExtensions
{
    // Buffer sized as recommended by Bradley Grainger, https://faithlife.codes/blog/2012/06/always-wrap-gzipstream-with-bufferedstream/
    // But anything smaller than 85,000 bytes should be OK, since objects larger than that go on the large object heap.  See:
    // https://docs.microsoft.com/en-us/dotnet/standard/garbage-collection/large-object-heap
    const int BufferSize = 8192;
    // Disable writing of BOM as per https://datatracker.ietf.org/doc/html/rfc8259#section-8.1
    static readonly Encoding DefaultEncoding = new UTF8Encoding(false);

    public static void SerializeToFileCompressed(object value, string path, JsonSerializerSettings settings = null)
    {
        using (var fs = new FileStream(path, FileMode.Create, FileAccess.Write, FileShare.Read))
            SerializeCompressed(value, fs, settings);
    }

    public static void SerializeCompressed(object value, Stream stream, JsonSerializerSettings settings = null)
    {
        using (var compressor = new GZipStream(stream, CompressionMode.Compress))
        using (var writer = new StreamWriter(compressor, DefaultEncoding, BufferSize))
        {
            var serializer = JsonSerializer.CreateDefault(settings);
            serializer.Serialize(writer, value);
        }
    }

    public static T DeserializeFromFileCompressed<T>(string path, JsonSerializerSettings settings = null)
    {
        using (var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read))
            return DeserializeCompressed<T>(fs, settings);
    }

    public static T DeserializeCompressed<T>(Stream stream, JsonSerializerSettings settings = null)
    {
        using (var compressor = new GZipStream(stream, CompressionMode.Decompress))
        using (var reader = new StreamReader(compressor))
        using (var jsonReader = new JsonTextReader(reader))
        {
            var serializer = JsonSerializer.CreateDefault(settings);
            return serializer.Deserialize<T>(jsonReader);
        }
    }
}

See Performance Tips: Optimize Memory Usage in the Json.NET documentation.

For those looking for an idea how to use the extensions from @dbc in uwp apps, I modified the code to this - where the StorageFile is a file you have access to write to.

public static async void SerializeToFileCompressedAsync(object value, StorageFile file, JsonSerializerSettings settings = null)
{
    using (var stream = await file.OpenStreamForWriteAsync())
        SerializeCompressed(value, stream, settings);
}

public static void SerializeCompressed(object value, Stream stream, JsonSerializerSettings settings = null)
{
    using (var compressor = new GZipStream(stream, CompressionMode.Compress))
    using (var writer = new StreamWriter(compressor))
    {
        var serializer = JsonSerializer.CreateDefault(settings);
        serializer.Serialize(writer, value);
    }
}

public static async Task<T> DeserializeFromFileCompressedAsync<T>(StorageFile file, JsonSerializerSettings settings = null)
{
    using (var stream = await file.OpenStreamForReadAsync())
        return DeserializeCompressed<T>(stream, settings);
}

public static T DeserializeCompressed<T>(Stream stream, JsonSerializerSettings settings = null)
{
    using (var compressor = new GZipStream(stream, CompressionMode.Decompress))
    using (var reader = new StreamReader(compressor))
    using (var jsonReader = new JsonTextReader(reader))
    {
        var serializer = JsonSerializer.CreateDefault(settings);
        return serializer.Deserialize<T>(jsonReader);
    }
}