比较C＃中的两个列表，并使用LINQ或foreach根据条件拒绝或选择子列表

Question

I have two lists of same entity in C# (one parent list and another child list which can vary) which has lot of properties and out of those properties two are V_ID (not the unique ID of the entity) and value.

I need to compare between two lists and find out from the child list can be selected completely or rejected based on the condition : If the V_ID is equal between the two list and the value can be null or equal to the value of the parent's list.

I will not be able to convert the list to Dictionary as I will not be identify the item uniquely without the primary ID of the entity. Let me know if there is a way to do it via dictionary.

Please help me how can I achieve this and I have tried various ways including LINQ, for-each, etc as all the entities in the child list are either completely rejected or selected.

Also am new to C# and Linq and I have provide the code sample which is extracted out from my original source code and various scenarios examples.

public class MyEntity
{

public int ID {get; set;}

public int W_ID {get; set;}

public long ValueOfW {get; set;}

public string SampleName {get; set;}

public DateTime ModifiedBy {get; set;}

//Other Properties

}
 public List<long> MyMethod(List<MyEntity> parentInput, List<MyEntity> childInput)
{
 var parentList = new List<MyEntity>(); //Obtained from Source A which is the parent and never changes
 var childList = new List<MyEntity>();  //Obtained from Source B and converted to type MyEntity
 var mySelectedIds = new List<long>(); 

 if(parentInput.Any() && childInput.Any())
 {
    parentList = parentInput;
    childList = childInput;
    foreach(var parent in parentList)
    {
        foreach(var child in childList)
        {
            if(parent.W_ID == child.W_ID && (parent.ValueOfW == child.ValueOfW || parent.ValueOfW == 0))
                {
                    mySelectedIds.Add(child.ID);
                }
        }
    }
 }
    return mySelectedIds; //Need to do some operation after getting the IDs
}

Data Sample:

Parent List

     W_ID  ValueOfW 
     ----------
     10  100
     20  200
     30  300

Child List A:

   W_ID  ValueOfW 
    ----------
    10  100
    20  200
    30  NULL
  Expected Output: The child list A should be selected 
  as IDs are matching and the value for 30 is NULL

Child List B

   W_ID  ValueOfW 
   ----------
    10  100
    20  200
    30  301

  Expected Output: The child list B should be selected 
  as IDs are matching and but the value for 30 is not equal

Child List C

   W_ID  ValueOfW 
    --------
    10  100
    20  200
    30  300
    40  400
   Expected Output: The child list C should be selected 
  as IDs are matching and values match even though there is 
  extra item in the list.

Answer 1

The first thing I would do for the sake of simplicity is make the comparison its own function. This isn't critical, but it will help with readability. I don't know what the condition describes, so I'll just call it TheCondition . (Also, if there's a bug in the condition itself it may be easier to spot this way.)

public bool TheCondition(MyEntity parent, MyEntity child)
{
    return parent.W_ID == child.W_ID 
        && (parent.ValueOfW == child.ValueOfW || parent.ValueOfW == 0)
}

Then, to get the elements you want from the child list

var selected = childList.Where(child => 
    parentList.Any(parent => TheCondition(parent, child) == true)); 

Specifying == true just to make it easier to read. In "real life" I would just use

var selected = childList.Where(child => 
    parentList.Any(parent => TheCondition(parent, child)));

since that function already returns a boolean.

Now selected is an IEnumerable<MyEntity> . To get a list of ids,

var ids = selected.Select(entity => entity.ID).ToList();

What we're saying is 'select from the child list where the condition is true for that child and any parent from the parent list.' Using Any means that if it does find a match with one of the parents it doesn't need to do a comparison with all of the other parents.

Then in the last step we're saying 'select all of the IDs from the previous list of entities' which returns an IEnumerable<int> and then create a new List<int> from those values.

---

This could even be a little bit clearer if we break it up some more. I wouldn't necessarily do this every time, but it can make it easier to follow. (I struggled with this at first - maybe you'll catch on faster than I did.)

public bool TheCondition(MyEntity parent, MyEntity child)
{
    return parent.W_ID == child.W_ID 
        && (parent.ValueOfW == child.ValueOfW || parent.ValueOfW == 0)
}

public bool ChildMatchesAnyParent(MyEntity child, IEnumerable<MyEntity> parents)
{
    return parents.Any(parent => TheCondition(parent, child);
}

var selected = childList.Where(child => ChildMatchesAnyParent(child, parentList));

var ids = selected.Select(entity => entity.ID).ToList();

You could also chain these together list this, combining them into one statement.

var ids = childList.Where(child => ChildMatchesAnyParent(child, parentList))
    .Select(entity => entity.ID).ToList();

For me I needed to understand how it worked separately, and then it's easier to start combining them together. But sometimes for the sake of readability it's still good to separate them out.

Answer 2

One way this can be accomplished is with a LINQ query language statement.

var childArray = child.ToArray();               // Convert to an array to enumerate once

return !(from e in parent                       // Get all the parents
    from c in childArray                        // cross join with children
    where e.W_ID == c.W_ID                      // filter those with only matching ID
    where e.Value != c.Value && c.Value != null // filter those based on your condition
    select e).Any();                           

Note the boolean NOT ( ! ). If there are any results left after filtering, the child should be rejected.