Don't test whether a HashSet contains a key if you want to remove it.
Instead, just straight try to remove it, halving your work.

if(list.Contains(value)) {
    list.Remove(value);
}

Becomes

list.Remove(value);

Don't test whether a HashSet contains a key if you want to remove it:

if(list.Contains(value)) {
    list.Remove(value);
}

Instead, just straight try to remove it:

list.Remove(value);

The replacement is cleaner, shorter, and more efficient. What's not to like?

Don't test whether a HashSet contains a key if you want to remove it.
Instead, just straight try to remove it, halving your work.

if(list.Contains(value)) {
    list.Remove(value);
}

Becomes

list.Remove(value);

Don't test whether a HashSet contains a key if you want to remove it.
Instead, just straight try to remove it, halving your work.

if(list.Contains(value)) {
    list.Remove(value);
}

Becomes

list.Remove(value);