Unintuitive behaviour with struct initialization and default arguments

Question

public struct Test 
{
    public double Val;
    public Test(double val = double.NaN) { Val = val; }
    public bool IsValid { get { return !double.IsNaN(Val); } }
}

Te         


        

Answer 1

Because a struct can't have a user-defined parameterless constructor.

Test(double val = double.NaN) looks like one, but it's actually compiled as Test(double val) with some metadata about the default value.