Fix GetLink method to throw exception when link does not exist

csharp/Platform.Data.Tests/ILinksExtensionsTests.cs

@@ -0,0 +1,76 @@
+using System;
+using System.Collections.Generic;
+using System.Numerics;
+using Xunit;
+using Platform.Data.Exceptions;
+using Platform.Delegates;
+
+namespace Platform.Data.Tests
+{
+    /// <summary>
+    /// Tests for ILinksExtensions methods
+    /// </summary>
+    public static class ILinksExtensionsTests
+    {
+        private class MockLinks : ILinks<ulong, LinksConstants<ulong>>
+        {
+            private readonly LinksConstants<ulong> _constants;
+
+            public MockLinks()
+            {
+                _constants = new LinksConstants<ulong>(enableExternalReferencesSupport: true);
+            }
+
+            public LinksConstants<ulong> Constants => _constants;
+
+            public ulong Count(IList<ulong>? restriction) => 0;
+
+            public ulong Each(IList<ulong>? restriction, ReadHandler<ulong>? handler)
+            {
+                // Return Constants.Break to simulate no matching links found
+                return _constants.Break;
+            }
+
+            public ulong Create(IList<ulong>? substitution, WriteHandler<ulong>? handler) => 0;
+
+            public ulong Update(IList<ulong>? restriction, IList<ulong>? substitution, WriteHandler<ulong>? handler) => 0;
+
+            public ulong Delete(IList<ulong>? restriction, WriteHandler<ulong>? handler) => 0;
+        }
+
+        /// <summary>
+        /// Tests that GetLink throws ArgumentLinkDoesNotExistsException when link doesn't exist
+        /// </summary>
+        [Fact]
+        public static void GetLinkThrowsExceptionWhenLinkDoesNotExist()
+        {
+            var links = new MockLinks();
+            var nonExistentLinkId = 999UL;
+
+            // The mock implementation returns Constants.Break from Each, which means no link was found
+            // and the Setter.Result remains null, so GetLink should throw ArgumentLinkDoesNotExistsException
+            var exception = Assert.Throws<ArgumentLinkDoesNotExistsException<ulong>>(() => 
+                links.GetLink(nonExistentLinkId));
+
+            // Verify the exception contains the correct link id (checking if it's in the message)
+            Assert.Contains(nonExistentLinkId.ToString(), exception.Message);
+        }
+
+        /// <summary>
+        /// Tests that GetLink works correctly for external references
+        /// </summary>
+        [Fact]
+        public static void GetLinkWorksForExternalReferences()
+        {
+            var links = new MockLinks();
+            var externalReference = new Hybrid<ulong>(0, true); // External reference
+
+            // For external references, GetLink should return a Point without calling Each
+            var result = links.GetLink(externalReference);
+
+            Assert.NotNull(result);
+            // The Point should contain the external reference value
+            Assert.Equal(externalReference.Value, result[0]);
+        }
+    }
+}

csharp/Platform.Data/ILinksExtensions.cs

@@ -148,7 +148,12 @@ public static TLinkAddress Each<TLinkAddress, TConstants>(this ILinks<TLinkAddre
             }
             var linkPartsSetter = new Setter<IList<TLinkAddress>?, TLinkAddress>(constants.Continue, constants.Break);
             links.Each(linkPartsSetter.SetAndReturnTrue, link);
-            return linkPartsSetter.Result;
+            var result = linkPartsSetter.Result;
+            if (result == null)
+            {
+                throw new ArgumentLinkDoesNotExistsException<TLinkAddress>(link);
+            }
+            return result;
         }
 
         #region Points