linq 实现查询字符串拼接 : And 和 OR 两种方式

N年前我们是这样来 拼接查询字符串的:

     public string Test(string a, string b, string c,string d)
        {
            string sql = "SELECT * FROM Users WHERE 1=1";
            if (!string.IsNullOrEmpty(a))
            {
                sql += " AND name='" + a + "'";
            }
            if (!string.IsNullOrEmpty(b))
            {
                sql += " AND age='" + b+ "'";
            }
            if (!string.IsNullOrEmpty(c))
            {
                sql += " AND sex='" + c + "'";
            }
            if (!string.IsNullOrEmpty(d))
            {
                sql += " AND address='" + d + "'";
            }
            return sql.ToString();
        }

现在我们使用linq来实现上边的代码：

 public void Test(string a, string b, string c,string d)
        {
            QueryContext query = new QueryContext();
            var q = from u in query.Users
                     select u;
            if (!string.IsNullOrEmpty(a))
            {
                q = q.Where(p => p.name == a);
            }
            if (!string.IsNullOrEmpty(b))
            {
                q = q.Where(p => p.age == b);
            }
            if (!string.IsNullOrEmpty(c))
            {
                q = q.Where(p => p.sex == c);
            }
            if (!string.IsNullOrEmpty(d))
            {
                q = q.Where(p => p.address == d);
            }
            q.ToList();  //上边的所有if，只有到此处才会执行
        }

ps: 如果查询字符串内 不是AND，而是OR 的话

如：

     public string Test(string a, string b, string c,string d)
        {
            string sql = "SELECT * FROM Users WHERE 1=1";
            if (!string.IsNullOrEmpty(a))
            {
                sql += " OR name='" + a + "'";
            }
            if (!string.IsNullOrEmpty(b))
            {
                sql += " OR age='" + b+ "'";
            }
            if (!string.IsNullOrEmpty(c))
            {
                sql += " OR sex='" + c + "'";
            }
            if (!string.IsNullOrEmpty(d))
            {
                sql += " OR address='" + d + "'";
            }
            return sql.ToString();
        }

此种情况，通过linq可以这样实现：

    public void Test(string a,string b,string c,string d)
        {
                 QueryContext query = new QueryContext();
                 var q1 = from u in query.Users
                          where u.name== a && a != ""
                      || u.age == b && b != ""
                      || u.sex == c && c != ""
                      || u.address ==d && d !="" 
                          select u;
                 q1.ToList();
        }