optimistic-concurrency.cs

public virtual async Task<T> SaveOrUpdate(T data)
{
    var document = await GetById(data.Id);

    if (document == null)
    {
        await Insert(data);
    }
    else
    {
        data.ModificationDate = DateTime.UtcNow;
        var ac = new AccessCondition { Condition = data.ETag, Type = AccessConditionType.IfMatch };

        try
        {
            var output = await Client.ReplaceDocumentAsync(
                UriFactory.CreateDocumentUri(DatabaseId, CollectionId, document.Id),
                data, new RequestOptions
                {
                    AccessCondition = ac
                });

            document.ETag = output.Resource.ETag;
            data.ETag = output.Resource.ETag;
        }
        catch (DocumentClientException ex)
        {
            if (ex.StatusCode == HttpStatusCode.PreconditionFailed)
            {
                throw new OptimisticConcurrencyException(
                    $"Impossible to replace document '{data.Id}' from collection '{CollectionId}'. Precondition failed.", ex);
            }
        }
    }

    return document;
}

public async Task<T> SaveOrUpdateOptimistically(string id, Action<T> modifier)
{
    var doc = await GetById(id);

    try
    {
        modifier(doc);
        await SaveOrUpdate(doc);

        return doc;
    }
    catch (OptimisticConcurrencyException)
    {
        return await SaveOrUpdateOptimistically(id, modifier);
    }
}