Question

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C# 'var' vs specific type performance

In term of performance does it make any difference to use var or IEnumerable in linq?
what about IEnumerable or IQueryable? how can we know which one is IEnumerable and which one is IQueryable?
for ex: in code below

 IEnumerable<CaseTable> test = from x in dataBase.CaseTables
                               where x.case_id.Equals("12")
                               select x;

Answer 1

Using var instead of IEnumerable<T> can make a performance difference in the case when the type of the expression on the right side is not IEnumerable<T>.

In your code above (assuming dataBase.CaseTables is IQueryable<CaseTable> backed by LINQ to SQL or some similar provider), if you wrote test.First(), you could retrieve lots of rows from the database to get only the first one.

If you used var or IQueryable<CaseTable>, you would retrieve only the first row.

This can make difference even without IQueryable<T>. For example, List<T>.GetEnumerator() returns a custom struct. Iterating using that is going to be faster than iterating through IEnumerator<T>. Although the difference is going to be negligible most of the time.

What this all means: yes, using var can make a difference: it can make your code faster, in some cases much faster.