如何防范"Object cannot be cast from DBNull to other types"？

Question

我有这段代码，其中 ISTM 我正在防御性地编码以反对分配空值：

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"]);
    }
    . . .

。但是我得到，"对象不能从DBNull转换为其他类型的"在最后一次尝试赋值（到pauv。周2用法）。

该字段确实出现在类中：

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; }
}

。并且代码对于大多数记录运行良好。

什么会导致此异常，我该如何防范它？

Answer 1

原因是数据库 NULL 值不是作为 CLR null 返回，而是作为类型 DBNull 的对象返回，因此您确实希望针对它进行测试。根据具体情况，安全的做法是针对两个null以及对象是否属于DBNull类型

给自己写个小帮手 功能：

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

并因此修改您的代码：

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"]);
}

由于您需要一直进行此类测试，因此您可能想借用我的 DataRowEx 类来进一步简化事情：

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);
    }
}

然后你的代码变成：

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

Answer 2

只是将泛型添加到@Übercoder的答案中。

您的扩展方法;

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)));
}

样品呼叫;

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

将最后一个if条件更改为

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