I have to store and return data on multitasking requests. If data is missing, I schedule loading and return null. I'm using some nubie scheduling by saving current jobs in a list. Also I need data request to be locked during writing. .NET 4.0

How can I schedule and lock properly?

private Dictionary<string, IDataSet> data;
private List<string> loadingNow;

public IDataSet GetData(string dataId)
{
    if (loadingNow.Contains(dataId))
        return null; // Currently loading. Return null

    if (data.ContainsKey(dataId))
        return data[dataId]; // Return data

    // Schedule loading async. Return null.
    loadingNow.Add(dataId);
    dataIoAsync.LoadDataAsync(dataParams);
    return null;
}

private void DataIoAsync_DataFileLoaded(object sender, DataFileLoadedAsyncEventArgs e)
{
    loadingNow.Remove(e.DataId);
    data.Add(e.DataId, e.DataSet);
    OnDataFileLoaded(e.DataId);
}

I have to store and return data on multitasking requests. If data is missing, I schedule loading and return null. I'm using some nubie scheduling by saving current jobs in a list. Also I need data request to be locked during writing. .NET 4.0

How can I schedule and lock properly?

private ConcurrentDictionary<string, IDataSet> data;
private List<string> loadingNow;

public IDataSet GetData(string dataId)
{
    if (loadingNow.Contains(dataId))
        return null; // Currently loading. Return null

    if (data.ContainsKey(dataId))
        return data[dataId]; // Return data

    // Schedule loading async. Return null.
    loadingNow.Add(dataId);
    dataIoAsync.LoadDataAsync(dataParams);
    return null;
}

private void DataIoAsync_DataFileLoaded(object sender, DataFileLoadedAsyncEventArgs e)
{
    loadingNow.Remove(e.DataId);
    data.TryAdd(e.DataId, e.DataSet);
    OnDataFileLoaded(e.DataId);
}

EDIT: Dictionary replaced with ConcurrentDictionary