public class CalculateClaimsBondsChangedCommand : ICalculateCommand
{
    private readonly IDebenturChangesRepository debenturChangesRepository;
    private readonly IClaimsCalculatorService claimsCalculatorService;

    public CalculateClaimsBondsChangedCommand(
        IDebenturChangesRepository debenturChangesRepository,
        IClaimsCalculatorService claimsCalculatorService)
    {
        if (debenturChangesRepository == null) throw new ArgumentNullException("debenturChangesRepository");
        if (claimsCalculatorService == null) throw new ArgumentNullException("claimsCalculatorService");
        this.debenturChangesRepository = debenturChangesRepository;
        this.claimsCalculatorService = claimsCalculatorService;
    }

    public void Execute()
    {
        var newDebenturs = debenturChangesRepository.CreateForNewDebenturs();
        var changedDebenturs = debenturChangesRepository.CreateForChangedCalledIn();

        claimsCalculatorService.CalculateClaims(newDebenturs.Concat(changedDebenturs).Select(d => d.ClaimNo));

        foreach (var debentur in newDebenturs)
        {
            debenturChangesRepository.InsertDebenturChange(debentur);
        }
        foreach (var debentur in changedDebenturs)
        {
            debenturChangesRepository.UpdateDebenturChange(debentur);
        }
    }
}

public class DebenturChangesRepository : IDebenturChangesRepository
{
    private readonly string connectionString;

    public DebenturChangesRepository(string connectionString)
    {
        if (connectionString == null) throw new ArgumentNullException("connectionString");
        this.connectionString = connectionString;
    }

    public List<DebenturChange> CreateForNewDebenturs()
    {
        var sql = "Select ClaimNo, CalledIn " +
            "From Debentur " +
            "Where ClaimNo Not In (Select ClaimNo From DebenturChange) ";

        using (var connection = new SqlConnection(connectionString))
        {
            connection.Open();

            var command = connection.CreateCommand();
            command.CommandText = sql;

            using (var reader = command.ExecuteReader())
            {
                return GetDebentursFromReader(reader);
            }
        }
    }

    public List<DebenturChange> CreateForChangedCalledIn()
    {
        var sql = "Select d.ClaimNo, d.CalledIn " +
            "From Debentur d join DebenturChange c on d.ClaimNo = c.ClaimNo " +
            "Where IsNull(d.CalledIn, 0) <> IsNull(c.CalledIn, 0) ";

        using (var connection = new SqlConnection(connectionString))
        {
            connection.Open();

            var command = connection.CreateCommand();
            command.CommandText = sql;

            using (var reader = command.ExecuteReader())
            {
                return GetDebentursFromReader(reader);
            }
        }
    }

    private List<DebenturChange> GetDebentursFromReader(IDataReader reader)
    {
        var debenturs = new List<DebenturChange>();
        while (reader.Read())
        {
            debenturs.Add(ParseDebenturChange(reader));
        }

        return debenturs;
    }

    private DebenturChange ParseDebenturChange(IDataReader reader)
    {
        return new DebenturChange()
        {
            ClaimNo = (string)reader["ClaimNo"],
            CalledIn = ParseColumnToNullableInt(reader["CalledIn"])
        };
    }

    private int? ParseColumnToNullableInt(object o)
    {
        if (o == DBNull.Value) return null;
        return Convert.ToInt32(o);
    }

    public void InsertDebenturChange(DebenturChange debenturChange)
    {
        var sql = "Insert Into DebenturChange (ClaimNo, CalledIn) " +
            "Values (@ClaimNo, @CalledIn)";

        using (var connection = new SqlConnection(connectionString))
        {
            connection.Open();

            var command = connection.CreateCommand();
            command.CommandText = sql;
            command.Parameters.AddWithValue("ClaimNo", debenturChange.ClaimNo);
            command.Parameters.AddWithValue("CalledIn", ParseNullableIntToColumn(debenturChange.CalledIn));

            command.ExecuteNonQuery();
        }
    }

    public void UpdateDebenturChange(DebenturChange debenturChange)
    {
        var sql = "Update DebenturChange " +
            "Set CalledIn = @CalledIn " +
            "Where ClaimNo = @ClaimNo";

        using (var connection = new SqlConnection(connectionString))
        {
            connection.Open();

            var command = connection.CreateCommand();
            command.CommandText = sql;
            command.Parameters.AddWithValue("ClaimNo", debenturChange.ClaimNo);
            command.Parameters.AddWithValue("CalledIn", ParseNullableIntToColumn(debenturChange.CalledIn));

            command.ExecuteNonQuery();
        }
    }

    private object ParseNullableIntToColumn(int? value)
    {
        if (value == null) return DBNull.Value;
        return value.Value;
    }
}