无效的转换异常从smallint转换为Int32时？

Question

The code below fills the properties of my model from the DataReader .I made some properties Nullable and it isn't working since then.

foreach (var property in _properties)
{
    property.SetValue(model, null, null);

    if (columnsInDataReader.Contains(property.Name.ToLower()))
    {
        if (!(_dataReader[property.Name] == DBNull.Value))
            property.SetValue(model, _dataReader[property.Name]);
    }
}

The property is Int32? and the database column is smallint and the conversion fails.

Answer 1

Even if an implicit conversion from short to int? do exists, the issue here arises from the fact that the data reader returns a boxed short , for which you need an unboxing conversion . In this specific case, the conversion is from object to a value type ( int? ).

The paragraph that matches your situation is the last of the cited section: since the object returned is a boxed short and not a boxed int , the cast fails.

As a concrete example, compare these two snippets:

short src = 42;
int? dst = (int?)src;

object src = (short)42;
int? dst = (int?)src;

While the first one succeeds, the second throws an InvalidCastException .

In your case, no explicit cast occurs, however the call to SetValue applies similar conversion rules, and therefore fails.

Summing up, in order to have the call succeed, you must choose the type of the property among those that are castable at runtime from the type of the extracted object, such as short and short? .