Use local variable to evaluate property value only once in JsonSourceGenerator (#87725)

* Use temporary variable to evaluate property value only once

* Add test

Author: etemi

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs

@@ -776,7 +776,18 @@ private void GenerateFastPathFuncForObject(SourceWriter writer, ContextGeneratio
                         ? $"(({propertyGenSpec.DeclaringType.FullyQualifiedName}){ValueVarName})"
                         : ValueVarName;
 
-                    string propValueExpr = $"{objectExpr}.{propertyGenSpec.NameSpecifiedInSourceCode}";
+                    string propValueExpr;
+                    if (defaultCheckType != DefaultCheckType.None)
+                    {
+                        // Use temporary variable to evaluate property value only once
+                        string localVariableName =  $"__value_{propertyGenSpec.NameSpecifiedInSourceCode}";
+                        writer.WriteLine($"{propertyGenSpec.PropertyType.FullyQualifiedName} {localVariableName} = {objectExpr}.{propertyGenSpec.NameSpecifiedInSourceCode};");
+                        propValueExpr = localVariableName;
+                    }
+                    else
+                    {
+                        propValueExpr = $"{objectExpr}.{propertyGenSpec.NameSpecifiedInSourceCode}";
+                    }
 
                     switch (defaultCheckType)
                     {

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/JsonSerializerContextTests.cs

@@ -535,6 +535,7 @@ public class CustomWrappingResolver<T> : IJsonTypeInfoResolver
 
         [JsonSourceGenerationOptions(GenerationMode = JsonSourceGenerationMode.Serialization)]
         [JsonSerializable(typeof(JsonMessage))]
+        [JsonSerializable(typeof(AllocatingOnPropertyAccess))]
         public partial class FastPathSerializationContext : JsonSerializerContext
         { }
 
@@ -816,5 +817,22 @@ public TestResolver(Func<Type, JsonSerializerOptions, JsonTypeInfo?> getTypeInfo
 
             public JsonTypeInfo? GetTypeInfo(Type type, JsonSerializerOptions options) => _getTypeInfo(type, options);
         }
+
+        [Fact]
+        public static void FastPathSerialization_EvaluatePropertyOnlyOnceWhenIgnoreNullOrDefaultIsSpecified()
+        {
+            JsonSerializerOptions options = FastPathSerializationContext.Default.Options;
+            JsonTypeInfo<AllocatingOnPropertyAccess> allocatingOnPropertyAccessInfo = (JsonTypeInfo<AllocatingOnPropertyAccess>)options.GetTypeInfo(typeof(AllocatingOnPropertyAccess));
+            Assert.NotNull(allocatingOnPropertyAccessInfo.SerializeHandler);
+
+            var value = new AllocatingOnPropertyAccess();
+            Assert.Equal(0, value.WhenWritingNullAccessCounter);
+            Assert.Equal(0, value.WhenWritingDefaultAccessCounter);
+
+            string expectedJson = """{"SomeAllocatingProperty":"Current Value: 1","SomeAllocatingProperty2":"Current Value: 1"}""";
+            Assert.Equal(expectedJson, JsonSerializer.Serialize(value, options));
+            Assert.Equal(1, value.WhenWritingNullAccessCounter);
+            Assert.Equal(1, value.WhenWritingDefaultAccessCounter);
+        }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/TestClasses.cs

@@ -154,6 +154,18 @@ public class JsonMessage
         public int Length => Message?.Length ?? 0; // Read-only property
     }
 
+    public class AllocatingOnPropertyAccess 
+    {
+        public int WhenWritingNullAccessCounter = 0;
+        public int WhenWritingDefaultAccessCounter = 0;
+
+        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+        public string SomeAllocatingProperty => $"Current Value: {++WhenWritingNullAccessCounter}";
+
+        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
+        public string SomeAllocatingProperty2 => $"Current Value: {++WhenWritingDefaultAccessCounter}";
+    }
+
     internal struct MyStruct { }
 
     public struct PersonStruct