Question

Good day,

I have a class that is used to store a value of Type T that I don’t know what the type will be until runtime. I want to unbox/cast, not sure what the correct term is, a specific type (in this case a nullable decimal) back to type object.

Please forgive my code layout:

The class snippet:

public abstract class Types
    {
        public class ValueField<T>
        {
            [XmlAttribute]
            public int TemplateID { get; set; }

            [XmlAttribute]
            public int FieldID { get; set; }

            [XmlIgnore]
            [ScriptIgnore]
            public TemplateApprovalField Field { get; set; }

            [XmlIgnore]
            [ScriptIgnore]
            public InstanceTemplateActivityValues Values { get; set; }

            [XmlAttribute]
            public T Value { get; set; }
        }
    }

The function snippet:

I am stuck at the line “values.Add((Types.ValueField)field);”, don’t know how to cast it. At that moment, var field is a Types.ValueField.

 Values = new Func<XmlDocument, List<Types.ValueField<object>>>(xml =>
 {
  List<Types.ValueField<object>> values = new List<Types.ValueField<object>>();

if (xml != null)
{
  foreach (XmlNode node in xml.SelectNodes("//Field"))
  {

    if (node.Attributes["Type"].Value == "Numeric")
      {
        var field = new Types.ValueField<decimal?>()
        {
          Field = ApprovalFields.Find(f => f.FieldID == int.Parse(node.Attributes["ID"].Value)),
          FieldID = int.Parse(node.Attributes["ID"].Value),
          TemplateID = int.Parse(node.SelectSingleNode("../@ID").Value)
        };
        field.Value = new Func<string, decimal?>(val =>
          {
            if (string.IsNullOrEmpty(val))
              return null;
            else
            {
              decimal parsed = 0;
              if (decimal.TryParse(val, out parsed))
                return parsed;
            }
            return null;
        })(node.InnerText);


      values.Add((Types.ValueField<object>)field); //This is where my problem occurs...
   }
 }


}
   return values;
   })(row["Form_Values"] != DBNull.Value ? 
      new XmlDocument() { InnerXml =  row["Form_Values"].ToString() } : null)

Any input will be greatly appreciated.

Regards
YP

Answer 1

You’re creating a ValueField<decimal?>. That isn’t a ValueField<object> – your ValueField<T> class isn’t covariant in T, and couldn’t be. (Classes can’t be covariant, and your API includes T in “in” positions too.)

To demonstrate why this mustn’t work:

 Value<decimal?> foo = new Value<decimal?>();
 Value<object> bar = foo; // Imagine this worked
 bar.Value = "hello";
 decimal? x = foo.Value;

What would you expect that to do? Everything other than the second line is above reproach, so we must make the second line fail, which it does.

The simple answer here is to create a Value<object> in the first place, instead of a Value<decimal?>.