Linq to SQL：使用int？在查詢

Question

參數所以我有一個查詢，看起來像這樣：

var clams = from clam in Clams 
      where clam.ClamTypeID == 143 && 
        clam.ParentClamID == null 
      select clam; 

沒有什麼太瘋狂了，返回我需要的結果。但是當我把它作爲一個int傳遞給可能的null值的函數時呢？我開始遇到問題。這裏是我的功能是什麼樣子：

public IQueryable<Clam> getAllClams(int clamTypeID, int? parentClamID) 
{ 
    return from clam in Clams 
      where clam.ClamTypeID == clamTypeID && 
        clam.ParentClamID == parentClamID 
      select clam; 
} 

該函數返回什麼 - 我查了PARAMS以確保它們是正確的，果然，parentClamID == NULL，仍然沒有結果。如果我改變clam.ParentClamID == parentClamID clam.ParentClamID == null它工作，所以我假設我使用int？不正確......任何想法是我在這裏做錯了嗎？

Answer 1

我以前有過這個問題。我找到工作的解決方法是這樣的：

public IQueryable<Clam> getAllClams(int clamTypeID, int? parentClamID) 
{ 
    return from clam in Clams 
      where clam.ClamTypeID == clamTypeID && 
        object.Equals(clam.ParentClamID, parentClamID) 
      select clam; 
} 

Answer 2

試試這個：

public IQueryable<Clam> getAllClams(int clamTypeID, int? parentClamID) 
{ 
    var result = from clam in Clams where clam.ClamTypeID == clamTypeID select clam; 

     if(parentClamID.HasValue) result = result.Where(c => c.ParentClamID == parentClamID); 
     else result = result.Where(c => c.ParentClamID == null); 

     return from clam in result select clam; 
} 

Answer 3

public IQueryable<Clam> getAllClams(int clamTypeID, int? parentClamID) 
{ 
    return from clam in Clams 
      where clam.ClamTypeID == clamTypeID && 
        clam.ParentClamID == parentClamID.HasValue ? parentClamID.Value : null 
      select clam; 
}