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
       }