add PropertyObject & PropertyValue, move to Pilz, make everything .net 8 only

Pilz/Data/PropertyObject.cs

@@ -0,0 +1,47 @@
+namespace Pilz.Data;
+
+#pragma warning disable CA2231
+
+[Serializable]
+public struct PropertyObject<T>(T value) where T : class
+{
+    private readonly bool hasValue = true; // Do not rename (binary serialization)
+    internal T? value = value; // Do not rename (binary serialization) or make readonly (can be mutated in ToString, etc.)
+
+    public static PropertyObject<T> Empty => default;
+
+    public readonly bool HasValue
+    {
+        get => hasValue;
+    }
+
+    public readonly T? Value
+    {
+        get
+        {
+            if (!hasValue)
+                throw new InvalidOperationException("No value");
+            return value;
+        }
+    }
+
+    public readonly T? GetValueOrDefault() => value;
+
+    public readonly T? GetValueOrDefault(T defaultValue) =>
+        hasValue ? value : defaultValue;
+
+    public override readonly bool Equals(object? other)
+    {
+        if (!hasValue || value is null) return other == null;
+        if (other == null) return false;
+        return value.Equals(other);
+    }
+
+    public override readonly int GetHashCode() => hasValue && value is not null ? value.GetHashCode() : 0;
+
+    public override readonly string? ToString() => hasValue && value is not null ? value.ToString() : string.Empty;
+
+    public static implicit operator PropertyObject<T>(T value) => new(value);
+
+    public static explicit operator T?(PropertyObject<T> value) => value.Value;
+}