I can't recall how many times I've written code like:

void fillInTextBox(Control someControl, string value)
{
    TextBox textBox = someControl as TextBox;  
    if (textBox != null)
    {
        textBox.Text = value;
    }
}

Starting with C# 7 pattern matching, we can combine this into one statement:

void fillInTextBox(Control someControl, string value)
{
    if (someControl is TextBox textBox)
    {
        textBox.Text = value;
    }
}

Now in one line. Much nicer!

Even works nicely with "is not"!

I found a rather unusual implementation of this, which caused me to have to try it myself. You can use this language feature along with an "is not" implementation, as seen here:

void OutputStringBuilder(object input)
{
    if (input is not StringBuilder sb) return;
    Console.WriteLine(sb.ToString());
}

What's odd about this interesting code, is that the sb is scoped to the rest of the function afer the if-statement; which is why the code in line 4 compiles fine. However if you removed the not in line3, line 4 would not compile, because those variables are only scoped within the if-block. Go figure...