Use reference equality to compare model instances in EditContext (#18172)

* Use reference equality to compare model instances in EditContext

Fixes https://github.com/aspnet/AspNetCore/issues/18069

src/Components/Forms/src/FieldIdentifier.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Linq.Expressions;
+using System.Runtime.CompilerServices;
 
 namespace Microsoft.AspNetCore.Components.Forms
 {
@@ -35,7 +36,7 @@ namespace Microsoft.AspNetCore.Components.Forms
         /// <param name="fieldName">The name of the editable field.</param>
         public FieldIdentifier(object model, string fieldName)
         {
-            if (model == null)
+            if (model is null)
             {
                 throw new ArgumentNullException(nameof(model));
             }
@@ -64,7 +65,16 @@ namespace Microsoft.AspNetCore.Components.Forms
 
         /// <inheritdoc />
         public override int GetHashCode()
-            => (Model, FieldName).GetHashCode();
+        {
+            // We want to compare Model instances by reference. RuntimeHelpers.GetHashCode returns identical hashes for equal object references (ignoring any `Equals`/`GetHashCode` overrides) which is what we want.
+            var modelHash = RuntimeHelpers.GetHashCode(Model);
+            var fieldHash = StringComparer.Ordinal.GetHashCode(FieldName);
+            return (
+                modelHash,
+                fieldHash
+            )
+            .GetHashCode();
+        }
 
         /// <inheritdoc />
         public override bool Equals(object obj)
@@ -75,7 +85,7 @@ namespace Microsoft.AspNetCore.Components.Forms
         public bool Equals(FieldIdentifier otherIdentifier)
         {
             return
-                otherIdentifier.Model == Model &&
+                ReferenceEquals(otherIdentifier.Model, Model) &&
                 string.Equals(otherIdentifier.FieldName, FieldName, StringComparison.Ordinal);
         }
 

src/Components/Forms/test/EditContextTest.cs

@@ -236,5 +236,35 @@ namespace Microsoft.AspNetCore.Components.Forms
             Assert.False(isValid);
             Assert.Equal(new[] { "Some message" }, editContext.GetValidationMessages());
         }
+
+        [Fact]
+        public void LookingUpModel_ThatOverridesGetHashCodeAndEquals_Works()
+        {
+            // Test for https://github.com/aspnet/AspNetCore/issues/18069
+            // Arrange
+            var model = new EquatableModel();
+            var editContext = new EditContext(model);
+
+            editContext.NotifyFieldChanged(editContext.Field(nameof(EquatableModel.Property)));
+
+            model.Property = "new value";
+
+            Assert.True(editContext.IsModified(editContext.Field(nameof(EquatableModel.Property))));
+        }
+
+        class EquatableModel : IEquatable<EquatableModel>
+        {
+            public string Property { get; set; } = "";
+
+            public bool Equals(EquatableModel other)
+            {
+                return string.Equals(Property, other?.Property, StringComparison.Ordinal);
+            }
+
+            public override int GetHashCode()
+            {
+                return StringComparer.Ordinal.GetHashCode(Property);
+            }
+        }
     }
 }

src/Components/Forms/test/FieldIdentifierTest.cs

@@ -76,6 +76,19 @@ namespace Microsoft.AspNetCore.Components.Forms
             Assert.False(fieldIdentifier1.Equals(fieldIdentifier2));
         }
 
+        [Fact]
+        public void FieldIdentifier_ForModelWithoutField_ProduceSameHashCodesAndEquality()
+        {
+            // Arrange
+            var model = new object();
+            var fieldIdentifier1 = new FieldIdentifier(model, fieldName: string.Empty);
+            var fieldIdentifier2 = new FieldIdentifier(model, fieldName: string.Empty);
+
+            // Act/Assert
+            Assert.Equal(fieldIdentifier1.GetHashCode(), fieldIdentifier2.GetHashCode());
+            Assert.True(fieldIdentifier1.Equals(fieldIdentifier2));
+        }
+
         [Fact]
         public void SameContentsProduceSameHashCodesAndEquality()
         {
@@ -89,6 +102,20 @@ namespace Microsoft.AspNetCore.Components.Forms
             Assert.True(fieldIdentifier1.Equals(fieldIdentifier2));
         }
 
+        [Fact]
+        public void SameContents_WithOverridenEqualsAndGetHashCode_ProduceSameHashCodesAndEquality()
+        {
+            // Arrange
+            var model = new EquatableModel();
+            var fieldIdentifier1 = new FieldIdentifier(model, nameof(EquatableModel.Property));
+            model.Property = "changed value"; // To show it makes no difference if the overridden `GetHashCode` result changes
+            var fieldIdentifier2 = new FieldIdentifier(model, nameof(EquatableModel.Property));
+
+            // Act/Assert
+            Assert.Equal(fieldIdentifier1.GetHashCode(), fieldIdentifier2.GetHashCode());
+            Assert.True(fieldIdentifier1.Equals(fieldIdentifier2));
+        }
+
         [Fact]
         public void FieldNamesAreCaseSensitive()
         {
@@ -194,5 +221,20 @@ namespace Microsoft.AspNetCore.Components.Forms
         {
             public TestModel Child { get; set; }
         }
+
+        class EquatableModel : IEquatable<EquatableModel>
+        {
+            public string Property { get; set; } = "";
+
+            public bool Equals(EquatableModel other)
+            {
+                return string.Equals(Property, other?.Property, StringComparison.Ordinal);
+            }
+
+            public override int GetHashCode()
+            {
+                return StringComparer.Ordinal.GetHashCode(Property);
+            }
+        }
     }
 }