如何防止“对象不能从DBNull转换到其他类型”?

I've got this code, where ISTM I'm defensively coding against assigning nulls:

我有这个代码，在ISTM中我防御性地编码反对分配空值:

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"]);
    }
    . . .

...yet I get, "Object cannot be cast from DBNull to other types" on a particular iteration on the last attempted assignment (to pauv.Week2Usage).

…然而，我在最近一次尝试的赋值(到pauv.Week2Usage)中得到一个特定迭代的“对象不能从DBNull转换到其他类型”。

That field does appear in the class:

这个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; }
}

...and the code runs fine for most of the records.

…对于大多数记录，代码运行良好。

What would cause this exception, and how can I guard against it?

什么会导致这个例外，我该如何防范呢?

3 个解决方案

#1

The cause is that Data base NULL values are returned not as CLR null but as an object of type DBNull so you really want to test against that. The safe thing, depending on circumstances would be to test against both null and whether the object is of type DBNull

原因是，数据基空值不作为CLR NULL返回，而是作为DBNull类型的对象返回，所以您确实想要对它进行测试。根据具体情况，最安全的方法是对null和对象是否是DBNull类型进行测试

Write yourself a little helper Function:

给自己写一个帮助函数:

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

and modify your code thus:

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"]);
}

As you need to do this sort of testing all the time you might want to borrow my DataRowEx class that simplfies things further:

当你需要一直做这种测试时，你可能会想要借用我的DataRowEx类来进一步简化:

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);
    }
}

Then your code becomes:

那么你的代码是:

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

#2

Just adding Generics to @Übercoder's answer.

只是在@Ubercoder的答案中添加泛型。

Your extension method;

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)));
}

Sample call;

示例调用;

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);
}

#3

Change the last if condition to

将最后一个if条件改为

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