public static async Task HandleUpdateCommand <TCommand, TEntity>(
EventStoreClient eventStore,
Func <TEntity> getDefault,
Func <TEntity, object, TEntity> when,
Func <TCommand, TEntity, object> handle,
Func <TCommand, string> getId,
Func <TCommand, uint> getVersion,
TCommand command,
CancellationToken ct) where TEntity : notnull
{
var id = getId(command);
var entity = await eventStore.Find(getDefault, when, id, ct);
var @event = handle(command, entity);
await eventStore.Append(id, @event, getVersion(command), ct);
}