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
       }
   }