Unboxing -1 and casting to Nullable <int> using generics gives an InvalidCastException

Question

In this SO post, I found a generic extension method that returns the default value if the value read SqlDataReaderis equal to nullor DBNull.Value, and the value is correctly converted otherwise. I implemented it as follows:

public static T GetValueOrDefault<T>(this SqlDataReader reader, string columnName, T defaultValue = default(T))
{
  object val = reader[columnName];
  if (val == null || val == DBNull.Value)
  {
    return defaultValue;
  }

  return (T)val;
}

The following examples of methods in the above message do not use the correct syntax, but, nevertheless, suggest that this should also work with NULL types such as int?.

However, I read the null column intfrom the database as follows:

MyProperty = reader.GetValueOrDefault<int?>("SomeColumnName")

, val - -1, T - int?, InvalidCastException. , unboxing casting (val object, short), , ? , , .

Answer 1

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public static T GetValueOrDefault<T>(this SqlDataReader reader, string columnName, T defaultValue = default(T))
{
  object val = reader[columnName];
  if (val == null || val == DBNull.Value)
  {
    return defaultValue;
  }

  return (T)Convert.ChangeType(val, typeof(T));
}