动态构建表达式树辅助方法

 public static class ExpressionHelp
    {
        private static Expression<T> Combine<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            MyExpressionVisitor visitor = new MyExpressionVisitor(first.Parameters[0]);
            Expression bodyone = visitor.Visit(first.Body);
            Expression bodytwo = visitor.Visit(second.Body);
            return Expression.Lambda<T>(merge(bodyone, bodytwo), first.Parameters[0]);
        }
        public static Expression<Func<T, bool>> ExpressionAnd<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Combine(second, Expression.And);
        }
        public static Expression<Func<T, bool>> ExpressionOr<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Combine(second, Expression.Or);
        }
    }
    public class MyExpressionVisitor : ExpressionVisitor
    {
        public ParameterExpression _Parameter { get; set; }

        public MyExpressionVisitor(ParameterExpression Parameter)
        {
            _Parameter = Parameter;
        }
        protected override Expression VisitParameter(ParameterExpression p)
        {
            return _Parameter;
        }

        public override Expression Visit(Expression node)
        {
            return base.Visit(node);//Visit会根据VisitParameter()方法返回的Expression修改这里的node变量
        }
    }