Question

我们可以动态追加linq查询的条件吗？

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)
}

任何人都知道如何处理Linq ????

中的情况

的Th

Answer 1

如果要动态构建它，可以使用 PredicateBuilder

Answer 2

只需将Where查询运算符附加到您的查询中：

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);
}

请注意，您的results变量应声明为IEnumerable<Result>，而不是List<Result>

Answer 3

对于and子句的关系，您可以轻松添加.Where()过滤器方法，如下：

where conditionOriginal(r) and conditionDynamic(r)

作为

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

要附加'或'类型关系，您必须与原始结果集合并，如下所示：

where conditionOriginal(r) or conditionDynamic(r)

变为

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 4

你可以这样做：

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

或者这个：

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

Answer 5

由于Linq会延迟执行，你可以只追加查询的位置，并在最后执行tolist来执行：

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 6

其他答案是最简单的解决方案。如果你想要一次执行，你也可以使用表达式树：

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

Answer 7

好吧，你总是可以在where子句中调用一个函数，并在那里建立你的条件：

...
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)

如何使用条件

7 个答案: