如何防止“無法將對象從DBNull強制轉換為其他類型”？

Question

我有以下代碼，在ISTM中，我針對分配空值進行防御性編碼：

foreach (DataRow priceAndUsageVarianceRow in _dtUsage.Rows)
{
    //var pauv = new PriceAndUsageVariance
    //{
    //    Description = priceAndUsageVarianceRow["Description"].ToString(),
    //    Week1Usage = Convert.ToDouble(priceAndUsageVarianceRow["Week1Usage"]),
    //    Week2Usage = Convert.ToDouble(priceAndUsageVarianceRow["Week2Usage"]),
    //    UsageVariance = Convert.ToDouble(priceAndUsageVarianceRow["UsageVariance"]),
    //    Week1Price = Convert.ToDecimal(priceAndUsageVarianceRow["Week1Price"]),
    //    Week2Price = Convert.ToDecimal(priceAndUsageVarianceRow["Week2Price"]),
    //    PriceVariance = Convert.ToDecimal(priceAndUsageVarianceRow["PriceVariance"]),
    //    PriceVariancePercentage = Convert.ToDouble(priceAndUsageVarianceRow["PriceVariancePercentage"])
    //};
    // Got exception with the code above; trying to prevent it with this:
    var pauv = new PriceAndUsageVariance();
    pauv.Description = String.Empty;
    pauv.Week1Usage = 0.0;
    pauv.Week2Usage = 0.0;
    pauv.UsageVariance = 0.0;
    pauv.Week1Price = 0.00M;
    pauv.Week2Price = 0.00M;
    pauv.PriceVariance = 0.00M;
    pauv.PriceVariancePercentage = 0.0;
    if (null != priceAndUsageVarianceRow["Description"])
    {
        pauv.Description = priceAndUsageVarianceRow["Description"].ToString();
    }
    if (null != priceAndUsageVarianceRow["Week1Usage"])
    {
        pauv.Week1Usage = Convert.ToDouble(priceAndUsageVarianceRow["Week1Usage"]);
    }
    if (null != priceAndUsageVarianceRow["Week2Usage"])
    {
        pauv.Week2Usage = Convert.ToDouble(priceAndUsageVarianceRow["Week2Usage"]);
    }
    . . .

...但我得到的是，在最后一次嘗試的分配（到pauv.Week2Usage）的特定迭代中，“ 無法將對象從DBNull強制轉換為其他類型 ”。

該字段確實出現在班級中：

public class PriceAndUsageVariance
{
    public String Description { get; set; }
    public Double Week1Usage { get; set; }
    public Double Week2Usage { get; set; }
    public Double UsageVariance { get; set; }
    public Decimal Week1Price { get; set; }
    public Decimal Week2Price { get; set; }
    public Decimal PriceVariance { get; set; }
    public Double PriceVariancePercentage { get; set; }
}

...並且大多數記錄的代碼運行良好。

是什么導致此異常，我該如何防范？

Answer 1

原因是數據庫NULL值不是作為CLR null值返回，而是作為DBNull類型的對象返回，因此您真的想對此進行測試。 根據情況，安全的做法是同時測試null和對象是否為DBNull類型

給自己寫一個小助手函數：

public static class DbNullExt
{
    public static bool IsNullData(this object obj)
    {
        return obj == null || obj is DBNull;
    }
}

然后修改您的代碼：

if (!IsNullData(priceAndUsageVarianceRow["Description"]))
{
    pauv.Description = priceAndUsageVarianceRow["Description"].ToString();
}
if (!IsNullData(priceAndUsageVarianceRow["Week1Usage"]))
{
    pauv.Week1Usage = Convert.ToDouble(priceAndUsageVarianceRow["Week1Usage"]);
}
if (!IsNullData(priceAndUsageVarianceRow["Week2Usage"]))
{
    pauv.Week2Usage = Convert.ToDouble(priceAndUsageVarianceRow["Week2Usage"]);
}

當您需要一直進行這種測試時，您可能想借用DataRowEx類，這進一步簡化了事情：

public static class DataRowEx
{
    public static string String(this DataRow row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return null;

        return obj.ToString();
    }

    public static Int32 Int32(this DataRow row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return 0;

        return (Int32)obj;
    }

    public static Decimal Decimal(this DataRow row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return 0;

        return Convert.ToDecimal(obj);
    }

    public static Double Double(this DataRow row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return 0;

        return Convert.ToDouble(obj);
    }

    public static Single Single(this DataRow row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return 0;

        return Convert.ToSingle(obj);
    }

    public static bool Bool(this DataRow row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return false;

        if (obj is int)
            return (int) obj != 0;

        return (bool)obj;
    }

    public static DateTime DateTime(this DataRow row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return System.DateTime.MinValue;

        return (DateTime)obj;
    }

    public static object ToType(this DataRow row, Type targetType, string columnName)
    {
        if (targetType == typeof(Int32))
            return row.Int32(columnName);

        if (targetType == typeof(bool))
            return row.Bool(columnName);

        if (targetType == typeof(DateTime))
            return row.DateTime(columnName);

        if (targetType == typeof (Decimal))
            return row.Decimal(columnName);

        if (targetType == typeof(Single))
            return row.Double(columnName);

        if (targetType == typeof(Double))
            return row.Double(columnName);

        if (targetType == typeof(string))
            return row.String(columnName);

        return row.String(columnName);
    }

    public static string String(this DataRowView row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return null;

        return obj.ToString();
    }

    public static Int32 Int32(this DataRowView row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return 0;

        return (Int32)obj;
    }

    public static Decimal Decimal(this DataRowView row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return 0;

        return (Decimal)obj;
    }

    public static Double Double(this DataRowView row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return 0;

        return (Double)obj;
    }

    public static Single Single(this DataRowView row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return 0;

        return (Single)obj;
    }

    public static bool Bool(this DataRowView row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return false;

        return (bool)obj;
    }

    public static DateTime DateTime(this DataRowView row, string columnName)
    {
        object obj = row[columnName];

        if (obj is DBNull)
            return System.DateTime.MinValue;

        return (DateTime)obj;
    }

    public static object ToType(this DataRowView row, Type targetType, string columnName)
    {
        if (targetType == typeof(Int32))
            return row.Int32(columnName);

        if (targetType == typeof(bool))
            return row.Bool(columnName);

        if (targetType == typeof(DateTime))
            return row.DateTime(columnName);

        if (targetType == typeof(Decimal))
            return row.Decimal(columnName);

        if (targetType == typeof(Double))
            return row.Double(columnName);

        if (targetType == typeof(Single))
            return row.Single(columnName);

        return row.String(columnName);
    }
}

然后您的代碼變為：

pauv.Description = priceAndUsageVarianceRow.String("Description");
pauv.Week1Usage = priceAndUsageVarianceRow.Double("Week1Usage");
pauv.Week2Usage =  priceAndUsageVarianceRow.Double("Week2Usage");

Answer 2

只需在@Übercoder的答案中添加泛型即可。

您的擴展方式；

public static T GetValueOrDefault<T>(this IDataRecord row, string fieldName)
{
    int ordinal = row.GetOrdinal(fieldName);
    return row.GetValueOrDefault<T>(ordinal);
}

public static T GetValueOrDefault<T>(this IDataRecord row, int ordinal)
{
    return (T)((row.IsDBNull(ordinal) ? default(T) : row.GetValue(ordinal)));
}

樣品通話；

DataReader reader = //your database call
var employees =  new List<Employee>();
while (reader.Read())
{
    var employee = new Employee
    {
        Id = reader.GetValueOrDefault<int>("EmpId"),
        Name = reader.GetValueOrDefault<string>("Name")
    };
    employees.Add(employee);
}

Answer 3

將最后一個if條件更改為

if (!(priceAndUsageVarianceRow["Week2Usage"] is DBNull) && null != priceAndUsageVarianceRow["Week2Usage"])
{
    pauv.Week2Usage = Convert.ToDouble(priceAndUsageVarianceRow["Week2Usage"]);
}