1

私はこのコードを持っています.ISTMはnullの割り当てに対して防御的にコーディングしています:

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"]);
    }
    . . .

...まだ、最後に試行された割り当て (pauv.Week2Usage へ) の特定の反復で、「オブジェクトを DBNull から他の型にキャストすることはできません」というメッセージが表示されます。

そのフィールドはクラスに表示されます。

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; }
}

...そして、コードはほとんどのレコードで正常に実行されます。

この例外の原因は何ですか?どうすればそれを防ぐことができますか?

4

3 に答える 3

4

原因は、データベースの NULL 値が CLRnullとしてではなく型のオブジェクトとして返されるDBNullため、実際にそれに対してテストする必要があるためです。安全なことは、状況に応じて、両方に対してテストしnull、オブジェクトが型であるかどうかをテストすることですDBNull

ちょっとしたヘルパー関数を書いてみましょう:

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

コードを次のように変更します。

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"]);
}

この種のテストを常に行う必要があるため、さらに単純化する DataRowEx クラスを借用することをお勧めします。

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);
    }
}

次に、コードは次のようになります。

pauv.Description = priceAndUsageVarianceRow.String("Description");
pauv.Week1Usage = priceAndUsageVarianceRow.Double("Week1Usage");
pauv.Week2Usage =  priceAndUsageVarianceRow.Double("Week2Usage");
于 2016-01-20T00:33:42.997 に答える
2

@Übercoderの回答にジェネリックを追加するだけです。

拡張メソッド。

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)));
}

サンプルコール;

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);
}
于 2016-01-20T03:01:08.797 に答える
1

最後のif条件を次のように変更します

if (!(priceAndUsageVarianceRow["Week2Usage"] is DBNull) && null != priceAndUsageVarianceRow["Week2Usage"])
{
    pauv.Week2Usage = Convert.ToDouble(priceAndUsageVarianceRow["Week2Usage"]);
}
于 2016-01-20T00:36:19.123 に答える