linq 实现查询字符串拼接 : And 和 OR 两种方式

普通查询方法：

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  
       }  

　　如果查询字符串内 不是AND，而是OR 的话

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();  
   }  

　　最终linq显示为：

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();  
    }