Re-evaluate ValueGetters if the current value is null during binding evaluation

BinarySerializer.Test/Issues/Issue216/Issue216Test.cs

@@ -0,0 +1,27 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace BinarySerialization.Test.Issues.Issue216
+{
+    [TestClass]
+    public class Issue216Tests : TestBase
+    {
+        [TestMethod]
+        public void TestIssue216()
+        {
+            var expected = new Preview
+            {
+                ResolutionX = 2,
+                ResolutionY = 3,
+                Data = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 },
+                Empty = 0,
+            };
+
+            var actual = Roundtrip(expected, new byte[] { 2, 0, 0, 0, 3, 0, 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 0, 0, 0, 0 });
+            Assert.AreEqual(expected.ResolutionX, actual.ResolutionX);
+            Assert.AreEqual(expected.ResolutionY, actual.ResolutionY);
+            Assert.AreEqual(expected.Data.Length, actual.Data.Length);
+            for (int i = 0; i < expected.Data.Length; i++)
+                Assert.AreEqual(expected.Data[i], actual.Data[i]);
+        }
+    }
+}

BinarySerializer.Test/Issues/Issue216/PreviewClass.cs

@@ -0,0 +1,27 @@
+namespace BinarySerialization.Test.Issues.Issue216
+{
+    public class Preview
+    {
+        /// <summary>
+        /// Gets the image width, in pixels.
+        /// </summary>
+        [FieldOrder(0)]
+        public uint ResolutionX { get; set; }
+
+        /// <summary>
+        /// Gets the image height, in pixels.
+        /// </summary>
+        [FieldOrder(2)]
+        public uint ResolutionY { get; set; }
+
+        [Ignore]
+        public uint DataSize => ResolutionX * ResolutionY * 2;
+
+        [FieldOrder(3)]
+        [FieldCount(nameof(DataSize), BindingMode = BindingMode.OneWay)]
+        public byte[] Data { get; set; }
+
+        [FieldOrder(4)]
+        public uint Empty;
+    }
+}

BinarySerializer/Graph/Binding.cs

@@ -77,6 +77,12 @@ public object GetValue(ValueNode target)
 
             CheckSource(source);
 
+            // When using a calculated Ignore field, the evaluation of the value is not
+            // deferred until required non-Ignore fields are calculated.
+            // This is a hack, but if the value is null, we will re-evaluate the getter
+            if (source.Value == null && source.TypeNode.ValueGetter != null)
+                source.Value = source.TypeNode.ValueGetter.Invoke(source.Parent.Value);
+
             return ValueConverter == null
                 ? source.Value
                 : Convert(source.Value, target.CreateLazySerializationContext());