Linq查詢中的CASE或IF語句？

Question

是否有一種方法來包含一個CASE或條件語句內的LINQ查詢？我試圖根據傳入該方法的參數動態構建我的select語句。

這裏是我如何寫T中的SQL：

select * from Table1 
where Table1Id = 
     CASE WHEN LEN myParameter > 0 
     THEN myParamter 
     ELSE Table1.Table1Id 

，這裏是我的LINQ查詢與SQL CASE中猛：

var query = from a in Table1 
       where a.Table1Id== 
        CASE WHEN LEN myParameter > 0 
        THEN myParamter 
        ELSE Table1.Table1Id 
          && a.shortName == myParameter2 
          && a.Country == myParameter3 
          select a; 

       return query.ToList(); 

這樣的事情......

看起來我可以使用myParameter.Length()方法，但不知道從哪裏開始的語法？

好吧，這裏有雲，整個原始SQL語句的方法：

string nativeSQLQuery = string.Format("SELECT c.ArticleId , 
               + "c.ArticleTitle," 
               + c.Version, 
               + "c.shortName,c.Country,c.ArticleHeader,c.ArticleBody," 
               + "c.DateCreated,c.UserCreated,c.DateUpdated,c.UserUpdated," 
               + "c.isPublished" 
               + "c.isBreakingNews" 
              + " FROM ISES.vwArticles c " 
              + " WHERE c.ArticleTitle = " 
              + "  CASE WHEN LEN('{1}') > 0" 
              + "  THEN '{1}'" 
              + "  ELSE c.ArticleTitle" 
              + "  END " 
              + " AND c.shortName = " 
              + "  CASE WHEN LEN('{2}') > 0" 
              + "  THEN '{2}'" 
              + "  ELSE c.shortName" 
              + "  END " 
              + " AND c.Country = " 
              + "  CASE WHEN LEN('{3}') > 0" 
              + "  THEN '{3}'" 
              + "  ELSE c.Country" 
              + "  END " 
              + " AND c.articleid NOT IN " 
              + " (SELECT pa.articleid FROM ISES.tbPublicationArticles pa " 
              + " WHERE pa.publicationid={0})", publicationId, ModelSelectedArticleTitle, vRegionShortName, ModelSelectedCountryshortName); 

Answer 1

使用條件運算符：

var query = from item in table 
    let table1Id = myParameter.Length() > 0 ? myParameter : item.Table1Id 
    select item; 

Answer 2

有條件的過濾不必是查詢本身的一部分， LINQ語句使用延遲加載，因此您可以構建動態查詢

var query = someList; 
if (myParameter > 0) { 
    // dynamic filter 
    query = query.Where(x => x.Table1Id == myParameter); 
} 

Alt eratively寫成一個班輪

var query = myParameter > 0 ? someList : someList.Where(x => x.Table1Id == myParameter); 

---

現有代碼重新編寫

var query = _Context.vwArticles 
    .Where(x => a.ArticleTitle == modelSelectedArticleTitle && 
       a.shortName == regionShortName && 
       a.Country == modelSelectedCountryshortName); 
if (myParameter > 0) { 
    query = query.Where(x => x.Table1Id == myParameter); 
} 
return query.ToList();