A new feature in C# 6.0 allows to declare variable inside TryParse
method.
Declaration expressions was cut from C# 6.0 and wasn’t shipped in the final release. You currently can’t do that. There is a proposal for it on GitHub for C# 7 (also see this for future reference).
Update (07/03/2017)
With the official release of C#7, the following code compiles:
string s = "42";
if (int.TryParse(s, out var result))
{
Console.WriteLine(result);
}