Nullable type as a generic parameter possible?

Disclaimer: This answer works, but is intended for educational purposes only. :) James Jones' solution is probably the best here and certainly the one I'd go with.

C# 4.0's dynamic keyword makes this even easier, if less safe:

public static dynamic GetNullableValue(this IDataRecord record, string columnName)
{
  var val = reader[columnName];

  return (val == DBNull.Value ? null : val);
}

Now you don't need the explicit type hinting on the RHS:

int? value = myDataReader.GetNullableValue("MyColumnName");

In fact, you don't need it anywhere!

var value = myDataReader.GetNullableValue("MyColumnName");

value will now be an int, or a string, or whatever type was returned from the DB.

The only problem is that this does not prevent you from using non-nullable types on the LHS, in which case you'll get a rather nasty runtime exception like:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: Cannot convert null to 'int' because it is a non-nullable value type

As with all code that uses dynamic: caveat coder.

public static T GetValueOrDefault<T>(this IDataRecord rdr, int index)
{
    object val = rdr[index];

    if (!(val is DBNull))
        return (T)val;

    return default(T);
}

Just use it like this:

decimal? Quantity = rdr.GetValueOrDefault<decimal?>(1);
string Unit = rdr.GetValueOrDefault<string>(2);

Just do two things to your original code – remove the where constraint, and change the last return from return null to return default(T). This way you can return whatever type you want.

By the way, you can avoid the use of is by changing your if statement to if (columnValue != DBNull.Value).

Change the return type to Nullable<T>, and call the method with the non nullable parameter

static void Main(string[] args)
{
    int? i = GetValueOrNull<int>(null, string.Empty);
}


public static Nullable<T> GetValueOrNull<T>(DbDataRecord reader, string columnName) where T : struct
{
    object columnValue = reader[columnName];

    if (!(columnValue is DBNull))
        return (T)columnValue;

    return null;
}