Question

Possible Duplicate:
C# 'var' vs specific type performance

Are there any performance costs (in terms of type conversion, etc.) if I write the line

SqlConnection c = new SqlConnection(connectionString))

as

var c = new SqlConnection(connectionString))

Answer 1

No. The compiled IL is identical.

The only potential side effect is in the case of inheritance, if you’re variable definition is a base class, and you instantiate a subclass. If you do:

 BaseClass item = new DerivedClass();

This will potentially act differently than:

 var item = new DerivedClass();

This is because the second compiles to:

 DerivedClass item = new DerivedClass();

In most cases, it should behave identically (due to the Liskov substitution principle). However, if DerivedClass uses method hiding it is possible to have a change in behavior.