IList對象創建中的System.InvalidCastException

Question

應該使用以下代碼將DataTable轉換為對象的IList 。 問題是，如果DataTable有一個空值，它將生成一個DBNull錯誤。

在將值分配給Deduction對象的屬性之前，是否可以檢查空值？

public IList<Deduction> PrepareDeductions() //Entry point
{
    return GetDeductionsToList();
}


public IList<Deduction > GetDeductionsToList()
{
    return CalculateDeductions().Rows.OfType<DataRow>().Select(CreateDeductions).ToList();
}

private DataTable CalculateDeductions()
{
    return _DataService.GetDeductions();
}

private static Deduction CreateDeductions(DataRow row)
{
    return new Deduction()
    {
        EmployeeID = Convert.ToInt32(row[0]),
        EPFAmount = Convert.ToDecimal(row[2]),
        AdvanceAmount = Convert.ToDecimal(row[3]),
        MealsAmount = Convert.ToDecimal(row[4]),
        LoanInstalmentAmount = Convert.ToDecimal(row[5]),
        UniformInstalmentAmount = Convert.ToDecimal(row[6]),
        InsuranceInstalmentAmount = Convert.ToDecimal(row[7]),
        FineAmount = Convert.ToDecimal(row[8]),
        DeathDonationAmount = Convert.ToDecimal(row[9]),
        WelfareAmount = Convert.ToDecimal(row[10]),
    };
}

Answer 1

您可以使用IsNull檢查是否為空； 參見http://msdn.microsoft.com/en-us/library/chk182xa.aspx

Answer 2

private static Deduction CreateDeductions(DataRow row)
{
    return new Deduction()
    {
        EmployeeID = row[0] == DBNull.Value ? 0 : Convert.ToInt32(row[0]),
        EPFAmount = row[2] == DBNull.Value ? 0 : Convert.ToDecimal(row[2]),
        AdvanceAmount = row[3] == DBNull.Value ? 0 : Convert.ToDecimal(row[3]),
        MealsAmount = row[4] == DBNull.Value ? 0 : Convert.ToDecimal(row[4]),
        LoanInstalmentAmount = row[5] == DBNull.Value ? 0 : Convert.ToDecimal(row[5]),
        UniformInstalmentAmount = row[6] == DBNull.Value ? 0 : Convert.ToDecimal(row[6]),
        InsuranceInstalmentAmount = row[7] == DBNull.Value ? 0 : Convert.ToDecimal(row[7]),
        FineAmount = row[8] == DBNull.Value ? 0 : Convert.ToDecimal(row[8]),
        DeathDonationAmount = row[9] == DBNull.Value ? 0 : Convert.ToDecimal(row[9]),
        WelfareAmount = row[10] == DBNull.Value ? 0 : Convert.ToDecimal(row[10]),
    };
}

Answer 3

不確定，但您可以檢查像

if (row[0] != System.DBNull.Value))
{
    EmployeeID = Convert.ToInt32(row[0]);
}
else
{
 EmployeeID = somethingelse;
}

Answer 4

您可以使用DataRow.Field擴展方法，該方法對DB中的可為空的值提供了更好的支持，並且不返回DBNull 。 您的屬性應該可以為空-例如int? ， decimal? 。

EmployeeID = row.Field<int?>(0);
EPFAmount = row.Field<decimal?>(2);
etc.