Best way to do a discriminated union struct

public readonly struct Value
{
    public string? StringValue { get; } = null;
    public double? NumberValue { get; } = null;

    public ValueKind Kind { get; } 

    public bool IsUndefined => Kind == ValueKind.Undefined;
    [MemberNotNullWhen(true, nameof(StringValue))] 
    public bool IsString => Kind == ValueKind.String;
    [MemberNotNulWhen(true, nameof(NumberValue))] 
    public bool IsNumber => Kind == ValueKind.Number;

    public Value()
    {
        Kind = ValueKind.Undefined;
    } 
    public Value(string value) 
    {
        StringValue = value;
        Kind = ValueKind.String;
    } 
    public Value(double value)
    {
        NumberValue = value;
        Kind = ValueKind.Number;
    } 
}

public enum ValueKind
{
    Undefined, 
    String, 
    Number 
}