C# – Why is it faster to check if dictionary contains the key, rather than catch the exception in case it doesn’t

cdictionaryperformance

Imagine the code:

public class obj
{
    // elided
}

public static Dictionary<string, obj> dict = new Dictionary<string, obj>();

Method 1

public static obj FromDict1(string name)
{
    if (dict.ContainsKey(name))
    {
        return dict[name];
    }
    return null;
}

Method 2

public static obj FromDict2(string name)
{
    try
    {
        return dict[name];
    }
    catch (KeyNotFoundException)
    {
        return null;
    }
}

I was curious if there is a difference in performance of these 2 functions, because the first one SHOULD be SLOWER than second one – given that it needs to check twice if the dictionary contains a value, while second function does need to access the dictionary only once but WOW, it's actually opposite:

Loop for 1 000 000 values (with 100 000 existing and 900 000 non existing):

first function: 306 milliseconds

second function: 20483 milliseconds

Why is that?

EDIT: As you can notice in comments below this question, the performance of second function is actually slightly better than first one in case there are 0 non existing keys. But once there is at least 1 or more non existing keys, the performance of second one decrease rapidly.

Best Answer

On the one hand, throwing exceptions is inherently expensive, because the stack has to be unwound etc.
On the other hand, accessing a value in a dictionary by its key is cheap, because it's a fast, O(1) operation.

BTW: The correct way to do this is to use TryGetValue

obj item;
if(!dict.TryGetValue(name, out item))
    return null;
return item;

This accesses the dictionary only once instead of twice.
If you really want to just return null if the key doesn't exist, the above code can be simplified further:

obj item;
dict.TryGetValue(name, out item);
return item;

This works, because TryGetValue sets item to null if no key with name exists.

Related Topic