What is more efficient: Dictionary TryGetValue or ContainsKey+Item?

c# performance dictionary
Rado picture Rado · Feb 21, 2012 · Viewed 185.8k times · Source

From MSDN's entry on Dictionary.TryGetValue Method:

This method combines the functionality of the ContainsKey method and the Item property.

If the key is not found, then the value parameter gets the appropriate default value for the value type TValue; for example, 0 (zero) for integer types, false for Boolean types, and null for reference types.

Use the TryGetValue method if your code frequently attempts to access keys that are not in the dictionary. Using this method is more efficient than catching the KeyNotFoundException thrown by the Item property.

This method approaches an O(1) operation.

From the description, it's not clear if it is more efficient or just more convenient than calling ContainsKey and then doing the lookup. Does the implementation of TryGetValue just call ContainsKey and then Item or is actually more efficient than that by doing a single lookup?

In other words, what is more efficient (i.e. which one performs less lookups):

Dictionary<int,int> dict;
//...//
int ival;
if(dict.ContainsKey(ikey))
{
  ival = dict[ikey];
}
else
{
  ival = default(int);
}

or

Dictionary<int,int> dict;
//...//
int ival;
dict.TryGetValue(ikey, out ival);

Note: I am not looking for a benchmark!

Answer

Reed Copsey picture Reed Copsey · Feb 21, 2012

TryGetValue will be faster.

ContainsKey uses the same check as TryGetValue, which internally refers to the actual entry location. The Item property actually has nearly identical code functionality as TryGetValue, except that it will throw an exception instead of returning false.

Using ContainsKey followed by the Item basically duplicates the lookup functionality, which is the bulk of the computation in this case.

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