try
{
    //...
}
catch (AggregateException ex)
{
    foreach (Exception innerEx in ex.InnerExceptions)
    {
        Debug.WriteLine(innerEx.ToString());
        // Do something considering the innerEx Exception
    }
}

var cts = new System.Threading.CancellationTokenSource();
var ct = cts.Token;
var t1 = Task.Factory.StartNew(
    () => GenerateAESKeysWithCharPrefix(ct, 'A'), ct);
t1.Wait();
for (int i = 0; i < t1.Result.Count; i++)
{
    Console.WriteLine(t1.Result[i]);
}
private static List<String> GenerateAESKeysWithCharPrefix(
    System.Threading.CancellationToken ct, char prefix)
{
    var aesM = new AesManaged();
    var keysList = new List<String>();
    for (int i = 1; i <= NUM_AES_KEYS; i++)
    {
        aesM.GenerateKey();
        byte[] result = aesM.Key;
        string hexString = ConvertToHexString(result);
        if (hexString[0] == prefix)
        {
            keysList.Add(hexString);
        }
        if (ct.IsCancellationRequested)
        {
            ct.ThrowIfCancellationRequested();
        }
    }
    return keysList;
}

var cts = new System.Threading.CancellationTokenSource();
var ct = cts.Token;
var t1 = Task.Factory.StartNew(
    () => GenerateAESKeysWithCharPrefix(ct, 'A'), ct);
var t2 = t1.ContinueWith((t) =>
{
    for (int i = 0; i < t.Result.Count; i++)
    {
        Console.WriteLine(t.Result[i]);
    }
});