Performance impact of using var type

Question

I have noted in c# that using var improves your performance, especially when declaring variables of a specific class. I have done multiple bench marks and have got the same result every time. I have also read similar questions on this site and others. but no one has actually commented on this. Can anyone please confirm or refute this. I have a class for image data, clsImageData and collection clsImageDataCollection. a function in collection class is SetCaseLowerCase(). Code is

private void SetCaseLowerCase()
{ 
   foreach (var item in this) 
   { 
      item.DestinationImageName = item.DestinationImageName.ToLower(); 
   } 
}

Now if I use clsImageData instead of var, it performs slower, I have checked it with 100000 iterations. but results is always the same. also with other examples too. Thanks

Edited I have tested this simple code and got 7/10 times better performance using var. Please tell me what am I doing wrong, I am using VS 2017, debug mode for any CPU, my processor is quad core. Thanks

            sw.Start();
            for (int i = 0; i < 10000000; i++)
            {
                var xyz = "test";
            }
            StopClock(sw, "Example var - ");
            sw.Start();
            for(int i = 0; i<10000000; i++)
            {
                string xyx2 = "test";
            }
            StopClock(sw, "Example type - ");

There is no performance impact or otherwise. Whoever told you that is quite frankly wrong — TheGeneral, Aug 06 '19 at 04:11
I think this link could help you more https://stackoverflow.com/questions/356846/will-using-var-affect-performance — Seyedraouf Modarresi, Aug 06 '19 at 04:13
It is only syntactic sugar. Variables defined with `var` are still strongly-typed and is determined at compile time. See here: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/var — Frank Fajardo, Aug 06 '19 at 04:15
You could add that information to your question, its impossible to read in a comment — TheGeneral, Aug 06 '19 at 04:20

score 1 · Answer 1 · 2019-08-06T04:41:24.270

var is compiled in a way, that makes the use of it equals to just declaring the variable/ type (except in cases it cant be done somehow else but using var ex: anonymous types). that means there are no performance changes at all. read more about var at https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/var

here is a demonstration, type the following code:

var list = new List<int>();

note that when you hover above list your IDE says:

(local variable) List<int> list

the reason for that is that var is converted to the declared type, meaning that var is actually cosmetic and its use can not impact any performance.

Performance impact of using var type

1 Answers1