/// <summary>
/// 获取实体集指定总和
/// </summary>
/// <param name="field">求和字段</param>
/// <param name="where">条件</param>
/// <returns></returns>
public decimal?GetSum(Expression <Func <T, decimal?> > field, Expression <Func <T, bool> > where = null)
{
return(_baseDal.GetSum(field, where));
}