System.InvalidCastException：“无法将对象从 DBNull 转换为其他类型。” 从数据库转换为 Object

Question

I have a problem when i try to set a DateTime from my datebase. i am having a DBnull tjek. and i have made my Datetime nullabel. cant figurer out why.

  public static List<ExportElements> GetExportElementsForCase(int caseNumber)
    {
        using (var dataAccess = new DbConnection<SqlConnection>())
        {
            DataTable dbExportElements = new DataTable();
            dataAccess.ExecuteCommand<DataTable>(command =>
            {
                command.CommandType = CommandType.StoredProcedure;
                command.CommandText = "GetElementsForEksport";

                var caseNr = command.CreateParameter();
                caseNr.ParameterName = "@caseNumberTolookUp";
                caseNr.Value = caseNumber;
                command.Parameters.Add(caseNr);

                var reader = command.ExecuteReader();
                //Create a new DataTable.
                DataTable resultToReturn = new DataTable("exportElements");

                //Load DataReader into the DataTable.
                resultToReturn.Load(reader);
                dbExportElements = resultToReturn;
                return resultToReturn;
            });

            List<ExportElements> exportMarterials = new List<ExportElements>();
            exportMarterials = (from DataRow dr in dbExportElements.Rows
                                select new ExportElements()
                                {
                                    CaseNumber = Convert.ToInt32(dr["CaseNumber"]),
                                    SubCaseNumber = Convert.ToInt32(dr["SubCaseNumber"]),
                                    ElementNumber = dr["ElementNumber"].ToString(),
                                    Side3To4 = Convert.ToDouble(dr["Side3To4"]),
                                    Side2To5 = Convert.ToDouble(dr["Side2To5"]),
                                    Side1To6 = Convert.ToDouble(dr["Side1To6"]),
                                    Weight = Convert.ToDouble(dr["Weight"]),
                                    Volume = Convert.ToDouble(dr["Volume"]),
                                    ProductNumber = Convert.ToInt32(dr["ProductNumber"]),
                                    DepartmentNumber = Convert.ToInt32(dr["DepartmentNumber"]),
                                    TurnElement = Convert.ToBoolean(dr["TurnElement"]),
                                    ErektionSeqence = Convert.ToInt32(dr["ErektionSeqence"]),
                                    ID = Convert.ToInt32(dr["ID"]),
                                    ReleasedDate = dr["ReleasedDate"] == DBNull.Value ? (DateTime?)null : Convert.ToDateTime(dr["ReleasedDate"]),
                                    ElementBasicPart = dr["ElementBasicPart"].ToString(),
                                    RevisionsLetter = dr["RevisionsLetter"].ToString(),
                                    ElementStatus = Convert.ToInt32(dr["ElementStatus"]),
                                    RevisionsText = dr["RevisionsText"].ToString(),
                                    EnvironmentalClass = Convert.ToInt32(dr["EnvironmentalClass"]),
                                    LedgeHeight1 = Convert.ToDouble(dr["LedgeHeight1"]),
                                    LedgeHeight2 = Convert.ToDouble(dr["LedgeHeight2"]),
                                    IsClosed = Convert.ToBoolean(dr["IsClosed"]),
                                    RevitTemplate = dr["revitTemplate"].ToString(),
                                }).ToList();

            return exportMarterials;
        }
    }

It is the releaseDate that fails.

The Model i am converting to from the DataRow.

public class ExportElements
{
    public int CaseNumber { get; set; }
    public int SubCaseNumber { get; set; }
    public string ElementNumber { get; set; }
    public double Side3To4 { get; set; }
    public double Side2To5 { get; set; }
    public double Side1To6 { get; set; }
    public double Weight { get; set; }
    public double Volume { get; set; }
    public int ProductNumber { get; set; }
    public int DepartmentNumber { get; set; }
    public bool TurnElement { get; set; }
    public int ErektionSeqence { get; set; }
    public int ID { get; set; }
    public DateTime? ReleasedDate { get; set; }
    public string ElementBasicPart { get; set; }
    public string RevisionsLetter { get; set; }
    public int ElementStatus { get; set; }
    public string RevisionsText { get; set; }
    public int EnvironmentalClass { get; set; }
    public double LedgeHeight1 { get; set; }
    public double LedgeHeight2 { get; set; }
    public bool IsClosed { get; set; }
    public string RevitTemplate { get; set; }
}

It works in other conversion with the same Datarow to DateTime conversion

I found the problem. it was not the DateTime? but the "ErektionSeqence" that was also DBNull witch i forgot to make a check for.

Answer 1

Instead of using all those Convert calls when you're not actually converting anything, I suggest that you use the Field method from LINQ to DataSet. It can handle nulls. Eg

NonNullableReferenceTypeProperty = row.Field<string>("Column1"),
NullableReferenceTypeProperty = row.Field<string>("Column2"),
NonNullableValueTypeProperty = row.Field<DateTime>("Column3"),
NullableValueTypeProperty = row.Field<DateTime?>("Column4"),