static void Main(string[] args)
{
    DiskMap<string> testMap = new DiskMap<string>("C:path");

        for (int i = 0; i < 100000; i++)
        {
            testMap.Add("a" + i, "aa" + i);
        }
}

public class DiskMap<TValue>
{
    private string path;
    private int loadedDictionaryId;
    private Dictionary<string, TValue> loadedDictionary;

    public DiskMap(string path)
    {
        this.limit = limit;
        this.path = path;
        File.WriteAllText(path + "\0.txt", "{");
        File.WriteAllText(path + "\1.txt", "{");

        loadedDictionary = new Dictionary<string, TValue>();
        loadedDictionaryId = 0;
    }

    public void Add(string key, TValue value)
    {
        int dictionaryId = GetId(key); // returns 0 or 1 in this example
        if (dictionaryId == loadedDictId)
        {
            loadedDict.Add(key, value);
        }
        else
        {
            string filePath = this.path + "\" + dictionaryId + ".txt";
            File.AppendAllText(filePath, JsonConvert.SerializeObject(key) + ":" + JsonConvert.SerializeObject(value) + ","); // It breaks at this line
        }
    }
}