I am filtering a List (childList) based on the contents of another List (parentList).

The parentList can contain 0-n entities where items of the childList are referenced by an ID (btw. ID is in this case a string, you could also call it "Name" or whatever you like). I want to filter the childList in a way that the result is all elements that have at least one entry in the parentList.

The obvious solution is:

List<childItem> resultList = new List<childItem>();    

foreach(var childItem in childList){
    foreach(var parentItem in parentList){
        if(childItem.ID.Equals(parentItem.ChildID,StringComparison.InvariantCultureIgnoreCase){
            resultList.Add(childItem); //found entity
            break; // search for next childItem
        } 
}

What I have done by now is:

// Group by childID and create List with all relevant IDs
var groupedParentList = parentList.AsParallel().GroupBy(p => p.ChildID).Select(group => group.First()).ToList();
        
// Filter childList on groupedParentList
var result = childList.AsParallel().Where(child => groupedParentList.AsParallel().Count(parent => parent.childID.Equals(child.ID, StringComparison.InvariantCultureIgnoreCase)) > 0).ToList();

But it's lasting rather long. The parentList contains about 75k entries and the childList about 4.5k entries. I just can't believe it actually takes time - imo this should be done in almost no time?

Maybe I am on the wrong track or am I missing some obvious performance-stopper?

Btw.: I have chosen the names parent- and childList just as an example. It's actaully no parent-child-relation.

I am filtering a List (childList) based on the contents of another List (parentList).

The parentList can contain 0-n entities where items of the childList are referenced by an ID (btw. ID is in this case a string, you could also call it "Name" or whatever you like). I want to filter the childList in a way that the result is all elements that have at least one entry in the parentList.

The obvious solution is:

List<childItem> resultList = new List<childItem>();    

foreach(var childItem in childList){
    foreach(var parentItem in parentList){
        if(childItem.ID.Equals(parentItem.ChildID,StringComparison.InvariantCultureIgnoreCase){
            resultList.Add(childItem); //found entity
            break; // search for next childItem
        } 
}

What I have done by now is:

// Group by childID and create List with all relevant IDs
var groupedParentList = parentList.AsParallel().GroupBy(p => p.ChildID).Select(group => group.First()).ToList();
        
// Filter childList on groupedParentList
var result = childList.AsParallel().Where(child => groupedParentList.AsParallel().Count(parent => parent.childID.Equals(child.ID, StringComparison.InvariantCultureIgnoreCase)) > 0).ToList();

But it's lasting rather long. The parentList contains about 75k entries and the childList about 4.5k entries. I just can't believe it actually takes time - imo this should be done in almost no time?

Maybe I am on the wrong track or am I missing some obvious performance-stopper?

Btw.: I have chosen the names parent- and childList just as an example. It's actaully no parent-child-relation.

Additional Question: Could it be faster to direcly check if the parentList contains the childList-Item? Something a lá:

var test = childList.RemoveWhere(child => !parentList.Contains(item => item.childID.Equals(child.ID));

So I would just skip the grouping? Any suggestions/ideas?