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); } }
}

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Answer 1

I suspect this is because a default constructor with no parameters is not allowed in c# so when you call the constructor Test without parameters it just initialises them as usual. Check this post for more details (not quite a duplicate): Why can't I define a default constructor for a struct in .NET?