LINQ performance and pitfalls

LINQ is beautiful and you may tempted to do some cool queries, but beware, it might perform poorly. Learn from these examples.

Published 9 March 2021

Example 1 – Use ToLookup instead of where clause.

Original code before optimization

//this code is mapping data to a view model... 
foreach (var store in stockinfo.stores)
   store.stockOnlineStatus = ...
   store.stockOnlineText = ...
   //set inventory status on the stores that have inventory
   storeinventory.Where(w => == store.warehouseId)
      .Select(w => MapStockStatus(store, w)) 

LINQ Explained under the hood: First filters all with specific warehouseid, potentially many then Select will run the Map function, ToList is there to actually run/execute the query. So in short a LINQ to populate/map some stock data on store object.

Instead we use ToLookup and if statement

ToLookup is a flexible variant of Dictionary, read more further down.

var dictInv = storeinventory.ToLookup(i =>;//we want the lookup key to be warehouseId
foreach (var store in stockinfo.stores)
   store.stockOnlineStatus = ...
   store.stockOnlineText = ...
   //set inventory status on the stores that have inventory
   if (dictInv.Contains(store.warehouseId)) {
       MapStockStatus(store, lookupInv[store.warehouseId].ToFirstOrDefault());

Performance metrics

before: 7 sec

after: down to 80 ms to max 700 ms in all cases

Example 2 – Replace Where clause with ToLookup

Code before optimization

public StorePage GetStorePageByWarehouseId(int? warehouseId) {
return this.ListAllStores()
  .Where(s => s.WarehouseId == warehouseId)

The code was called foreach variant foreach stock warehouse.

Instead use cache and ToLookup

First i tried ListAllStores.FirstOrDefault(x => x.WarehouseId == warehouseId) which was faster but not optimal, finally used ToLookup like this:

public StorePage GetStorePageByWarehouseId(int? warehouseId)
  var lookupList = GetLookupAllStoresByWarehouseId(); // this saves up to 400ms on some products index when many variants 

  if (lookupList.Contains(warehouseId))
     return lookupList[warehouseId].FirstOrDefault();
  return null;

public ILookup<int?, StorePage> GetLookupAllStoresByWarehouseId()
   var stores = _cache.Get("LookupListAllStores") as ILookup<int?, StorePage>;

   if (stores == null)
       stores = ListAllStores().ToLookup(l => l.WarehouseId);
       _cache.Insert("LookupListAllStores", stores, new CacheEvictionPolicy(TimeSpan.FromSeconds(15), CacheTimeoutType.Absolute));
   return stores;

Since the function was called inside loops for potentially 100 warehouse per variant i wanted just to call the lookup maximum once per request since ToLookup has a overhead too. Cache to the rescue, even for only 15 seconds.

Performance metrics

before: 700ms to 1s

after: 480ms

ToLookup vs ToDictionary

ToLookup is more forgiving. ToLookup allows null as key. But you may also query a lookup on a key that doesn’t exist, and you’ll get an empty sequence. Do the same with a dictionary and you’ll get an exception.

A dictionary is a 1:1 map (each key is mapped to a single value), and a dictionary is mutable (editable) after the fact.

A lookup is a 1:many map (multi-map; each key is mapped to an IEnumerable<> of the values with that key), and there is no mutate on the ILookup<,> interface.

An overly simplified way of looking at it is that a Lookup<TKey,TValue> is roughly comparable to a Dictionary<TKey,IEnumerable<TValue>>


  • Don’t use where clause with FirstOrDefault, put the critera in the FirstOrDefault instead
  • if using FirstOrDefault with criteria in an loop, but it in a ToLookup dict, and query with contains
  • use cache, even for some seconds

SEO Terms

  • LINQ danger
  • LINQ Performance traps
  • LINQ poor performance
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About the author

Luc Gosso
– Independent Senior Web Developer
working with Azure and Episerver

Twitter: @LucGosso

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