Variable initialization in "if" statement

[foxesknow]

Currently I can write code like this to see if a string exists within another string

static void Test(string s)
{
    if (s.IndexOf("a") is var index && index >= 0)
    {
       Console.WriteLine("bingo");
    }
}

However, I was surprised to see that if I change the code to:

static void Test(string s)
{
    if (s.IndexOf("a") is var index)
    {
       Console.WriteLine("bingo");
    }
}

It compiles and has the effect of meaning that the if expression is always true. i.e. it is treated as:

static void Test(string s)
{
    int num = s.IndexOf("a");
    if (true)
    {
        Console.WriteLine("bingo");
    }
}

This strikes be as a bug. The compiler should raise an error saying that the expression does not evaluate to a boolean.

[HaloFour]

This strikes be as a bug. The compiler should raise an error saying that the expression does not evaluate to a boolean.

This is working as intended. Pattern expressions do evaluate to a bool and the var pattern is always true.

[foxesknow]

Thanks.

This variation, without the var, also always evaluates to true:

static void Test(string s)
{
    if (s.IndexOf("a") is {} index)
    {
       Console.WriteLine("bingo");
    }
}

I'm guessing that this also works as intended, but it really doesn't look right!

[HaloFour]

The {} pattern is a little weird. It's a degenerate form of the recursive property pattern, which normally looks like TypeName { Property1: pattern1, ... }. However, the compiler can infer the type if you omit it, and you are allowed to have zero property patterns. What that leaves you with is effectively a pattern that tests if the expression is of the inferred type, which is always true unless the expression is null.

TL;DR: It's a weird pattern form of null-check.

[333fred]

There is no bug here. The expression does evaluate to a bool, and the var pattern always matches the input value. The pattern you want is s.IndexOf("a") is > 0.