C# Filter a dictionary

For example:

using System.Linq;

var dictionary = new Dictionary<string, int>()
{
    ["apple"] = 5,
    ["banana"] = 8,
    ["cherry"] = 12
};

// filter
var filterList = dictionary.Where(kvp => kvp.Key.Length > 5);

// back to a dictionary
var newDictionary = filterList.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);

You can also use the dictionary constructor new Dictionary<string, int>(filterList) instead of ToDictionary() if you prefer.

This produces a new dictionary with the filtered items:

[banana, 8]
[cherry, 12]

Where() produces a list (actually an IEnumerable) of KeyValuePair objects. Most of the time, you’ll want the result as a dictionary, so you’ll need to use ToDictionary() to convert the list back into a dictionary.

Filter by Removing Items

Alternatively, you can remove unwanted items directly from the original dictionary. This modifies the dictionary in place, rather than creating a new one. The easiest way to do this is by using Where() to filter the items and then removing the items in a loop.

using System.Linq;

var dictionary = new Dictionary<string, int>()
{
    ["apple"] = 5,
    ["banana"] = 8,
    ["cherry"] = 12
};

// filter
var filterList = dictionary.Where(kvp => kvp.Key.Length <= 5);

// remove from original dictionary
foreach(var kvp in filterList)
{
    dictionary.Remove(kvp.Key);
}

This removes the items from the original dictionary, which now has only one item left:

[cherry, 12]

Before .NET Core 3.0 - Use .ToList() When Removing

In .NET Core 3.0 and later, you can remove items from a dictionary while looping over it.

However, in versions before .NET Core 3.0, attempting to do so directly would cause an InvalidOperationException: Collection was modified; enumeration operation may not execute.

If you're using a version prior to .NET Core 3.0, you should call .ToList() to avoid this exception when removing items:

// filter
var filterList = dictionary.Where(kvp => kvp.Key.Length <= 5);

// remove from original dictionary
foreach(var kvp in filterList.ToList())
{
    dictionary.Remove(kvp.Key);
}