Expression<Func<T, bool>>

public static Expression<Func<T, bool>> True<T>() { return f => true; }

public static Expression<Func<T, bool>> False<T>() { return f => false; }

/// <summary>

        /// 条件或    

     /// </summary>   

      /// <typeparam name="T"></typeparam>  

       /// <param name="expr1"></param>   

      /// <param name="expr2"></param>   

      /// <returns></returns>  

       public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,         Expression<Func<T, bool>> expr2)         {           

  var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());  

           return Expression.Lambda<Func<T, bool>>             (Expression.Or(expr1.Body, invokedExpr), expr1.Parameters);  

       }

        /// <summary>    

     /// 条件且

        /// </summary>

        /// <typeparam name="T"></typeparam>

        /// <param name="expr1"></param>

        /// <param name="expr2"></param>   

      /// <returns></returns>    

     public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,         Expression<Func<T, bool>> expr2)         {            

var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());      

       return Expression.Lambda<Func<T, bool>>  

           (Expression.And(expr1.Body, invokedExpr), expr1.Parameters);   

      }