C# interpolated string with conditional-operator

You need to put the string in parentheses within {}, so: {(1 == 1 ? "yes" : "no")}.

$"test {(foo ? "foo is true" : "foo is false")}";   

The code inside the parentheses returns a variable, and that's the only thing allowed inside the curly brackets. The colon ':' is a special character in string interpolation, hence it needs to be parenthesised.