c# - 检查数据库记录映射中的空值

Question

如何检查附加代码中的数据库空值？请理解我是一个新的 C# convert...

此代码所做的是获取一个 IDataReader 对象并将其转换并映射到一个强类型对象列表。但我发现，当读取器中返回空列时，它会完全出错。

转换器

internal class Converter<T> where T : new()
{
    // Declare our _converter delegate
    readonly Func<IDataReader, T> _converter;
    // Declare our internal dataReader
    readonly IDataReader dataReader;

    // Build our mapping based on the properties in the class/type we've passed in to the class
    private Func<IDataReader, T> GetMapFunc()
    {
        // declare our field count
        int _fc = dataReader.FieldCount;
        // declare our expression list
        List<Expression> exps = new List<Expression>();
        // build our parameters for the expression tree
        ParameterExpression paramExp = Expression.Parameter(typeof(IDataRecord), "o7thDR");
        ParameterExpression targetExp = Expression.Variable(typeof(T));
        // Add our expression tree assignment to the exp list
        exps.Add(Expression.Assign(targetExp, Expression.New(targetExp.Type)));
        //does int based lookup
        PropertyInfo indexerInfo = typeof(IDataRecord).GetProperty("Item", new[] { typeof(int) });
        // grab a collection of column names from our data reader
        var columnNames = Enumerable.Range(0, _fc).Select(i => new { i, name = dataReader.GetName(i) }).AsParallel();
        // loop through all our columns and map them properly
        foreach (var column in columnNames)
        {
            // grab our column property
            PropertyInfo property = targetExp.Type.GetProperty(column.name, BindingFlags.Public | BindingFlags.Instance | BindingFlags.IgnoreCase);
            // check if it's null or not
            if (property != null)
            {
                // build our expression tree to map the column to the T
                ConstantExpression columnNameExp = Expression.Constant(column.i);
                IndexExpression propertyExp = Expression.MakeIndex(paramExp, indexerInfo, new[] { columnNameExp });
                UnaryExpression convertExp = Expression.Convert(propertyExp, property.PropertyType);
                BinaryExpression bindExp = Expression.Assign(Expression.Property(targetExp, property), convertExp);
                // add it to our expression list
                exps.Add(bindExp);
            }
        }
        // add the originating map to our expression list
        exps.Add(targetExp);
        // return a compiled cached map
        return Expression.Lambda<Func<IDataReader, T>>(Expression.Block(new[] { targetExp }, exps), paramExp).Compile();
    }

    // initialize
    internal Converter(IDataReader dataReader)
    {
        // initialize the internal datareader
        this.dataReader = dataReader;
        // build our map
        _converter = GetMapFunc();
    }

    // create and map each column to it's respective object
    internal T CreateItemFromRow()
    {
        return _converter(dataReader);
    }
}

映射器

    private static IList<T> Map<T>(DbDataReader dr) where T : new()
    {
        try
        {
            // initialize our returnable list
            List<T> list = new List<T>();
            // fire up the lamda mapping
            var converter = new Converter<T>(dr);
            while (dr.Read())
            {
                // read in each row, and properly map it to our T object
                var obj = converter.CreateItemFromRow();
                // add it to our list
                list.Add(obj);
            }
            // reutrn it
            return list;
        }
        catch (Exception ex)
        {
            // make sure this method returns a default List
            return default(List<T>);
        }
    }

我只是不太明白键入对象的列在这里发生的位置，所以我会尝试自己做...但我只是不知道它在哪里。

我知道这可能不会有太大帮助，但我得到的错误是:

无法将“System.DBNull”类型的对象转换为类型“System.String”。

它发生在

internal T CreateItemFromRow()
    {
        return _converter(dataReader); //<-- Here
    }

注意事项

如果我用 ISNULL(column, '') 将查询本身中的列包装起来，则不会发生这种情况，但我相信您可以理解这肯定不是解决方案

Answer 1

问题在于 convertExp = Expression.Convert(propertyExp, property.PropertyType) 行。您不能期望将 DbNull 值转换为框架类型中的等效值。当您的类型是值类型时，这尤其令人讨厌。一种选择是检查从 db 读取的值是否为 DbNull.Value，如果是，您需要自己找到一个兼容的值。在某些情况下，人们可以接受 C# 中这些类型的默认值。如果你必须这样做

property = value == DBNull.Value ? default(T): value;

通用实现看起来像(就转换器类中的 foreach 而言):

foreach (var column in columns)
{
    var property = targetExp.Type.GetProperty(
        column.name,
        BindingFlags.Public | BindingFlags.Instance | BindingFlags.IgnoreCase);
    if (property == null)
        continue;

    var columnIndexExp = Expression.Constant(column.i);
    var propertyExp = Expression.MakeIndex(
        paramExp, indexerInfo, new[] { columnIndexExp });
    var convertExp = Expression.Condition(
        Expression.Equal(
            propertyExp, 
            Expression.Constant(DBNull.Value)), 
        Expression.Default(property.PropertyType), 
        Expression.Convert(propertyExp, property.PropertyType));
    var bindExp = Expression.Assign(
        Expression.Property(targetExp, property), convertExp);
    exps.Add(bindExp);
}

现在这相当于

property = reader[index] == DBNull.Value ? default(T): reader[index];

您可以通过将读取器分配给一个变量并在条件检查中使用它的值来避免重复查找读取器。所以这应该稍微好一点，但有点复杂:

foreach (var column in columns)
{
    var property = targetExp.Type.GetProperty(
        column.name,
        BindingFlags.Public | BindingFlags.Instance | BindingFlags.IgnoreCase);
    if (property == null)
        continue;

    var columnIndexExp = Expression.Constant(column.i);
    var cellExp = Expression.MakeIndex(
        paramExp, indexerInfo, new[] { columnIndexExp });
    var cellValueExp = Expression.Variable(typeof(object), "o7thPropValue");
    var convertExp = Expression.Condition(
        Expression.Equal(
            cellValueExp, 
            Expression.Constant(DBNull.Value)), 
        Expression.Default(property.PropertyType), 
        Expression.Convert(cellValueExp, property.PropertyType));
    var cellValueReadExp = Expression.Block(new[] { cellValueExp },
        Expression.Assign(cellValueExp, cellExp), convertExp);
    var bindExp = Expression.Assign(
        Expression.Property(targetExp, property), cellValueReadExp);
    exps.Add(bindExp);
}

这样进行条件检查:

value = reader[index];
property = value == DBNull.Value ? default(T): value;