Why can’t the operator ‘==’ be applied to a struct and default(struct)?

For classes, the == operator uses reference equality. Of course, structs are value types, so they can’t be compared by reference. There is no default implementation of == for structs because memberwise comparison isn’t always a valid comparison, depending on the type.

You can instead use the Object.Equals method, which does compare memberwise:

Console.WriteLine(user.Equals(default(User)) ? "not found" : "found");

Or you could just implement == to call Object.Equals:

public static bool operator ==(User lhs, User rhs)
{
    return lhs.Equals(rhs);
}

However, the default implementation of Equals for structs uses reflection, and so is very slow. It would be better to implement Equals yourself, along with == and != (and possibly GetHashCode too):

public override bool Equals(Object obj)
{
    return obj is User && Equals((User)obj);
}

public bool Equals(User other)
{
    return UserGuid == other.UserGuid && Username == other.Username;
}

public static bool operator ==(User lhs, User rhs)
{
    return lhs.Equals(rhs);
}

public static bool operator !=(User lhs, User rhs)
{
    return !lhs.Equals(rhs);
}