LINQ To SQL Generic methods

In order to avoid too much coding in my DAO layer, I've been creating some generic LINQ to SQL CRUD methods.

Generic Delete method:

/// <summary>

/// Delete an entity

/// </summary>

/// <param name="entity">Entity to be deleted</param>

[System.ComponentModel.DataObjectMethod(System.ComponentModel.DataObjectMethodType.Delete)]

public void Delete(TEntity entity)

{

    try

    {

TEntity original = GetEntity(_getPrimaryKey().GetValue(entity, null));

DAODataContext.GetTable<TEntity>().DeleteOnSubmit(original);      

    }

    catch (Exception e)

    {

throw new DAOException(DAOException.EError.Delete, e);

    }

} 

Generic Attach method:

The generic attach method is used to attach existing entitie or new entitie, a test is executed to check if the entitie has to be inserted or updated:

/// <summary>

/// Attach an entity to the data context

/// If the entity is allready attached, exception is catched

/// </summary>

/// <param name="entity">Entity to attach</param>

public void Attach(TEntity entity)

{

PropertyInfo primaryKey = _getPrimaryKey();

if (_isNullOrEmpty(primaryKey.GetValue(entity, null)))

{

         //new object case

    DAODataContext.GetTable<TEntity>().InsertOnSubmit(entity);

}

else

{

    try

    {

//existing object case

DAODataContext.GetTable<TEntity>().Attach(entity);

    }

    catch (InvalidOperationException)

    {

// Do nothing for exception when object already attached to data context

    }

    catch (Exception e)

    {

throw new DAOException(DAOException.EError.Attach, e);

    }

}

}

Method to get the primary key of an entitie:

/// <summary>

        /// Gets the primary key of the current entity type

        /// </summary>

        /// <returns>Primary key property info</returns>

        private PropertyInfo _getPrimaryKey()

        {

            MetaTable mapping = DAODataContext.Mapping.GetTable(typeof(TEntity));

            MetaDataMember pkfield = mapping.RowType.DataMembers.SingleOrDefault(d => d.IsPrimaryKey);

            int count = mapping.RowType.DataMembers.Count(d => d.IsPrimaryKey);

            if (count < 1) throw new Exception(String.Format("Table {0} does not contain a Primary Key field", mapping.TableName));

            if (count > 1) throw new Exception(String.Format("Table {0} contains a composite primary key field", mapping.TableName));

            return typeof(TEntity).GetProperty(pkfield.MappedName);

        } 

Method to test if a field as a null value:

  /// <summary>

        /// Check if a type object is considered as null value depending on its type

        /// (<1 for int , Guid.Empty for Guid, null or empty for string)

        /// </summary>

        /// <param name="value">Value to be check</param>

        /// <returns>True if value is considered null</returns>

        private bool _isNullOrEmpty(object value)

        {

            if (value is int)

            {

                return (int)value < 1;

            }

            else if (value is Guid)

            {

                return value == null || (Guid)value == Guid.Empty;

            }

            else if (value is string)

            {

                return string.IsNullOrEmpty((string)value);

            }

            return false;

        }

Generic Get an Entitie method:

/// <summary>

        /// Generic search method to get one entity, loading child objects requested by the loadOptionPredicates

        /// </summary>

        /// <param name="loadOptions">Request to load child objects</param>

        /// <param name="predicate">Predicate to search for the entity</param>

        /// <returns>A found entity with child entities or a default value</returns>

        [System.ComponentModel.DataObjectMethod(System.ComponentModel.DataObjectMethodType.Select)]

        public TEntity FindEntity(DataLoadOptions loadOptions, Expression<Func<TEntity, bool>> predicate)

        {

            TEntity res = null;         

            try

            {

                if (loadOptions != null)

                {

                    if (DAODataContext.DeferredLoadingEnabled == true)

                        DAODataContext.DeferredLoadingEnabled = false;

                    DAODataContext.LoadOptions = loadOptions;

                }

                res = DAODataContext.GetTable<TEntity>().FirstOrDefault(predicate);

            }

            catch (Exception e)

            {

                throw new DAOException(DAOException.EError.FindEntitie, e);

            }

            return res;

        }

Generic New method:

/// <summary>

/// Returns a new attached instance the requested Entity with default values

/// </summary>

/// <returns></returns>

public TEntity New()

{

    TEntity newEntity = new TEntity();

    Attach(newEntity);

    return newEntity;

}