如何使用條件

Question

我們可以動態地在LINQ查詢條件的位置上？

例如：

class Result 
{ 
     string v1; 
     string v2; 
     string v3; 
} 

List<Result> result = (from r in results select r); 


//i want to do something like the following.... 

if(conditionA) 
{ 
    result = result appened (or v1 = xxx) 
} 
else if(conditionB) 
{ 
    result = result appened (or v2 = xxx) 
} 
else if(conditionC) 
{ 
    result = result appened (or v3 == xxx) 
} 

任何人都知道如何處理Linq中的條件????

釷

Answer 1

對於and關係的子句，你可以很容易地只是追加了.Where()過濾方法，因爲這樣的：

where conditionOriginal(r) and conditionDynamic(r) 

爲

var results = (from r in originalResults 
       where originalConditions(r) 
       select r); 
... 
if (conditionA) 
    results = results.Where(r => conditionDynamic(r)); 

要追加一個「或」類型的關係，但是，你不得不工會與原來的結果集，像這樣：

where conditionOriginal(r) or conditionDynamic(r) 

成爲

var results = (from r in originalResults 
       where conditionOriginal(r) 
       select r); 
... 
if (conditionB) 
    results = results.Union((from r in originalResults 
          where conditionDynamic(r) 
          select r)); 

或

if (conditionB) 
    results = results.Union(originalResults.Where(conditionDynamic(r))); 

Answer 2

的Where查詢運營商只需添加到您的查詢：

if(conditionA) 
{ 
    result = result.Where(r => r.v1 == xxx); 
} 
else if(conditionB) 
{ 
    result = result.Where(r => r.v2 == xxx); 
} 
else if(conditionC) 
{ 
    result = result.Where(r => r.v3 == xxx); 
} 

請注意，您results變量應被聲明爲IEnumerable<Result>，不List<Result>

Answer 3

你可以這樣做：

if (conditionA) 
{ 
    result = result.Where(p => p.v1 == xxx); // Just guessing at the intent here. 
} 
// Repeat as necessary... 

或者這樣：

if (conditionA) 
{ 
    result = from r in result where p.v1 == xxx select r; 
} 

Answer 4

由於LINQ的將推遲執行，你可以在那裏追加到您的查詢，並調用tolist到底執行：

var query = from r in results; 

//i want to do something like the following.... 

if(conditionA) 
{ 
    query = result.Where(x => x.v1 = xxx); 
} 
else if(conditionB) 
{ 
    query = result.Where(x => x.v2 = xxx); 
} 
else if(conditionC) 
{ 
    query = result.Where(x => x.v1 = xxx); 
} 

List<Result> result = query.ToList(); 

Answer 5

其他的答案是簡單的解決方案。如果你想有一個單一的執行你也可以使用表達式目錄樹：

http://blogs.msdn.com/abhinaba/archive/2006/03/01/541179.aspx http://www.devsource.com/c/a/Languages/Understanding-LINQ-Expression-Trees/1/

Answer 6

那麼，你可以隨時撥打where子句中的功能，並有建立自己的條件：

... 
public bool MeetsConditions(Result r, bool a, bool b) 
{ 
    bool result = false; 
    if(a) result = result || r.v1==xxx 
    if(b) result = result && r.v2==xxx 
    return result; 
} 
... 
var q = select from r in results where MeetsConditions(r, conditionA, conditionB) 

Answer 7

如果你想動態構建它，你可以使用 PredicateBuilder