你可以使用条件运算符:

employee.FirstName = sqlreader["indexFirstName"] != DBNull.Value ? sqlreader[indexFirstName].ToString() : "";

你应该使用as运算符和?? ?操作符用于默认值。值类型需要被读取为空，并给出默认值。

employee.FirstName = sqlreader[indexFirstName] as string;
employee.Age = sqlreader[indexAge] as int? ?? default(int);

as操作符处理转换，包括检查DBNull。

我尽了最大努力从DataTable中重新实现Field方法。https://learn.microsoft.com/en-us/dotnet/api/system.data.datarowextensions.field

如果您尝试转换DBNull，它将抛出。值转换为非空类型。否则它将转换为DBNull。值为null。

我还没有完全测试过。

public static T Field<T>(this SqlDataReader sqlDataReader, string columnName)
{
    int columnIndex = sqlDataReader.GetOrdinal(columnName);
    if (sqlDataReader.IsDBNull(columnIndex))
    {
        if (default(T) != null)
        {
            throw new InvalidCastException("Cannot convert DBNULL value to type " + typeof(T).Name);
        }
        else
        {
            return default(T);
        }
    }
    else
    {
        return sqlDataReader.GetFieldValue<T>(columnIndex);
    }
}

用法:

string fname = sqlDataReader.Field<string>("FirstName");
int? age = sqlDataReader.Field<int?>("Age");
int yearsOfExperience = sqlDataReader.Field<int?>("YearsEx") ?? 0;

此解决方案较少依赖于供应商，并与SQL, OleDB和MySQL Reader一起工作:

public static string GetStringSafe(this IDataReader reader, int colIndex)
{
    return GetStringSafe(reader, colIndex, string.Empty);
}

public static string GetStringSafe(this IDataReader reader, int colIndex, string defaultValue)
{
    if (!reader.IsDBNull(colIndex))
        return reader.GetString(colIndex);
    else
        return defaultValue;
}

public static string GetStringSafe(this IDataReader reader, string indexName)
{
    return GetStringSafe(reader, reader.GetOrdinal(indexName));
}

public static string GetStringSafe(this IDataReader reader, string indexName, string defaultValue)
{
    return GetStringSafe(reader, reader.GetOrdinal(indexName), defaultValue);
}

下面是其他人可以根据@marc_s的答案使用的helper类:

public static class SQLDataReaderExtensions
    {
        public static int SafeGetInt(this SqlDataReader dataReader, string fieldName)
        {
            int fieldIndex = dataReader.GetOrdinal(fieldName);
            return dataReader.IsDBNull(fieldIndex) ? 0 : dataReader.GetInt32(fieldIndex);
        }

        public static int? SafeGetNullableInt(this SqlDataReader dataReader, string fieldName)
        {
            int fieldIndex = dataReader.GetOrdinal(fieldName);
            return dataReader.GetValue(fieldIndex) as int?;
        }

        public static string SafeGetString(this SqlDataReader dataReader, string fieldName)
        {
            int fieldIndex = dataReader.GetOrdinal(fieldName);
            return dataReader.IsDBNull(fieldIndex) ? string.Empty : dataReader.GetString(fieldIndex);
        }

        public static DateTime? SafeGetNullableDateTime(this SqlDataReader dataReader, string fieldName)
        {
            int fieldIndex = dataReader.GetOrdinal(fieldName);
            return dataReader.GetValue(fieldIndex) as DateTime?;
        }

        public static bool SafeGetBoolean(this SqlDataReader dataReader, string fieldName)
        {
            return SafeGetBoolean(dataReader, fieldName, false);
        }

        public static bool SafeGetBoolean(this SqlDataReader dataReader, string fieldName, bool defaultValue)
        {
            int fieldIndex = dataReader.GetOrdinal(fieldName);
            return dataReader.IsDBNull(fieldIndex) ? defaultValue : dataReader.GetBoolean(fieldIndex);
        }
    }

private static void Render(IList<ListData> list, IDataReader reader)
        {
            while (reader.Read())
            {

                listData.DownUrl = (reader.GetSchemaTable().Columns["DownUrl"] != null) ? Convert.ToString(reader["DownUrl"]) : null;
                //没有这一列时，让其等于null
                list.Add(listData);
            }
            reader.Close();
        }