In the db schema I’m currently working on, boolean values are represented by columns whose type is tinyint nullable (it’s SQL Server 2005). If such a value equals 1, then it is supposed to be True, 0 means False and if it is null then it is False, too.
This time I would like to show how to use NHibernate IUserType interface to enable automatic conversion from a database field to System.Boolean value
NHibernate.UserTypes.IUserType is a very useful interface when you are to handle some strange db schemas in your data access layer. As you can guess, the thing that we need is a class that implements this interface. You can see it below. The most important methods are NullSafeGet(…) and NullSafeSet(…) as they are responsible for conversion ‘logic’.

namespace MyNamespace
{
  public class ByteAsBool : IUserType
  {
      #region IUserType Members

      public object Assemble(object cached, object owner)
      {
          return cached;
      }

      public object DeepCopy(object value)
      {
          return value;
      }

      public object Disassemble(object value)
      {
          return value;
      }

      public int GetHashCode(object x)
      {
          if (x == null)
              return 0;
          return x.GetHashCode();
      }
      public bool IsMutable
      {
         get { return false; }
      }

      // represents conversion on load-from-db operations:
      public object NullSafeGet(System.Data.IDataReader rs, 
             string[] names, object owner)
      {
          var obj = NHibernateUtil.String.
                 NullSafeGet(rs, names[0]);
          if (obj == null)
              return false;
           byte b = 0;
          try
          {
              if (obj is string)
                  b = byte.Parse(obj as string);
              else
                  b = (byte)obj;
          }
          catch (Exception)
          {
              return false;
          }
          return b == 1;
      }

      // represents conversion on save-to-db operations:
      public void NullSafeSet(System.Data.IDbCommand cmd, 
             object value, int index)
      {
          if (value == null)
          {
              ((IDataParameter)cmd.Parameters[index]).Value = 
                    DBNull.Value;
          }
          else
          {
              var boolValue = (bool)value;
              ((IDataParameter)cmd.Parameters[index]).Value = 
                    boolValue ? (byte)1 : (byte)0;
          }
      }
      public object Replace(object original, object target, 
             object owner)
      {
          return original;
      }

      public Type ReturnedType
      {
          get { return typeof(bool); }
      }

      public NHibernate.SqlTypes.SqlType[] SqlTypes
      {
          get { return new[] { SqlTypeFactory.Byte }; }
      }
      #endregion

      bool IUserType.Equals(object x, object y)
      {
          return object.Equals(x, y);
      }
  }
}

The class ByteAsBool can be now used in NHibernate mappings in the following way:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
    assembly="SomeAssembly" namespace="MyNamespace.Types">
  <class name="SomeClass" table="Some_Table" >
  <!-- ... -->
    <property name="active" 
        column="ACTIVE" 
        type="MyNamespace.ByteAsBool, SomeAssembly" access="field" />
  <!-- ... -->
  </class>
</hibernate-mapping>

The mapped class is very simple and looks very nice, as there are no ugly bool? types, just simple System.Boolean:

  public class SomeClass : BaseEntity
  {
     protected bool active;
     // ...
  }