Different behavior in pattern matching when using var or explicit type

Question

Consider the following, at first glance absurd, pattern match:

string s = null;
if (s is string ss) //false
if (s is string) //false

Both is will return false. However if we use var the behavior changes completely:

string s = null;
if (s is var ss) //true!?!

If you hover over var in VS2017, the type is string but the behavior of is is completely different. The compiler is doing something radically different even though the inferred type is the same. How can this be? Is this a bug? Is the null type somehow bubbling out?

Answer 1

The C# language reference confirms the behaviour is intended.

A pattern match with the var pattern always succeeds. Its syntax is

expr is var varname

Where the value of expr is always assigned to a local variable named varname. varname is a static variable of the same type as expr.

Note that if expr is null, the is expression still is true and assigns null to varname.

Source: MSDN - C# Language Reference

---

The var pattern

The var pattern just copies the source variable into a new named variable which you can then build a case block expression with like below

string s = null;
var collection = new string[] { "abb", "abd", "abc", null};

switch (s)
{
    case "xyz":
        Console.WriteLine("Is xyz");
        break;

    case var ss when (collection).Contains(s):
        Console.WriteLine("Is in list");
        break;

    default:
        Console.WriteLine("Failed!");
        break;

}

Output:

> Is in list