Inline variable declaration doesn't compile when using '== false' instead of negation operator

Question

Consider the following snippets:

void Foo(object sender, EventArgs e)
{
    if (!(sender is ComboBox comboBox)) return;
    comboBox.DropDownWidth = 100;
}

compared to

void Bar(object sender, EventArgs e)
{
    if ((sender is ComboBox comboBox) == false) return;
    comboBox.DropDownWidth = 100;
}

Code including Foo() successfully compiles in .Net 4.6.1, while code including Bar() results in Use of unassigned local variable 'comboBox'.

Without getting into a debate over the reasons behind using == false instead of the negation operator, can someone explain why one compiles and the other does not?

Answer 1

Updated Answer Thanks to Julien opening a GitHub issue.

See Neal Gafter's response (copied here from here):

However, the error you're seeing is not about scope. It is about definite assignment. A pattern variable is definitely assigned when the pattern-matching expression is true. The unary ! operator reverses assigned-when-true and assigned-when-false. However, the boolean equality operator == throws away the distinction between assigned-when-true and assigned-when-false.

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I believe the comboBox variable will only be created if the pattern matches.