CODEWITHSUNDEEP
c#performancecasting
Kira Namida
Kira Namida

Reputation: 279

Casting an object to the same type & the effect on performance

A question regarding the performance of casting an object to the same type (I know that sounds odd at first but let me put it in context)

public T Get<T>(string key) where T : class
{
     var objToReturn = (T)_cache[key];
     if (objToReturn != null)
     {
         return objToReturn;
     }
     return null;
}

The above is a snippet of code for attempting to get an object of type T out of a CacheObject where the type is strongly typed by the caller.

Know seeing as the type of the object is strongly typed, I am wondering about the line:

var objToReturn = (T)_chache[key];

Will this actually invoke a cast and convert instance in the cache to another instance and return that or will the types be found to be the same to the cast simply ignored.

I ask as the ability to cast will be useful in later development to the application that this cache is being used in for getting derived types but I don't want to breed in a potentially major performance hit early on.

Thoughts and thank you.

Upvotes: 3

Views: 668

Answers (1)

Jon Skeet
Jon Skeet

Reputation: 1500535

The cast you've shown is a "natural" reference type cast. It doesn't change anything about the object - it just ensures that the object is of an appropriate type (either the one you're casting to or a subclass). If the object is of the wrong type, an exception is thrown. No extra objects are created, or anything like that.

It's not free of course - but it would be rare for this to be a performance bottleneck.

It's not clear why you've got this code though:

if (objToReturn != null)
{
    return objToReturn;
}
return null;

Why not just use:

return objToReturn;

? That will behave exactly the same way. Then your entire method can be reduced to just:

public T Get<T>(string key) where T : class
{
    return (T) _cache[key];
}

EDIT: Note that you should not just use as instead, unless you really want values of the wrong type to silently be returned as null. For example:

cache[key] = new object();
string x = cache[key] as string; // x is null
string y = (string) cache[key]; // Exception

Upvotes: 5

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