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Throw JsonException for duplicate JsonObject properties #132766

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Merged
PranavSenthilnathan merged 1 commit into from
Aug 28, 2026
Merged

Throw JsonException for duplicate JsonObject properties #132766

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6 changes: 4 additions & 2 deletions ...ystem.Text.Json/src/System/Text/Json/Serialization/Converters/Node/JsonObjectConverter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -103,8 +103,10 @@ internal static JsonObject ReadAsJsonNode(ref Utf8JsonReader reader, JsonNodeOpt
reader.Read(); // Move to the value token.
JsonNode? value = JsonNodeConverter.ReadAsJsonNode(ref reader, options);

// To have parity with the lazy JsonObject, we throw on duplicates.
jObject.Add(propertyName, value);
if (!jObject.TryAdd(propertyName, value))
{
ThrowHelper.ThrowJsonException_DuplicatePropertyNotAllowed(propertyName);
}
}

// JSON is invalid so reader would have already thrown.
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20 changes: 10 additions & 10 deletions src/libraries/System.Text.Json/tests/System.Text.Json.Tests/JsonNode/JsonObjectTests.cs
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Expand Up @@ -1736,9 +1736,9 @@ private static void AssertDuplicatePropertyThrows<T>(string jsonPayload, bool is
}
else
{
AssertExtensions.ThrowsContains<ArgumentException>(
AssertExtensions.ThrowsContains<JsonException>(
() => JsonSerializer.Deserialize<T>(jsonPayload, JsonTestSerializerOptions.DisallowDuplicateProperties),
"An item with the same key has already been added.");
"Duplicate property");

// Default options don't throw on deserialize but will throw when accessed
T node = JsonSerializer.Deserialize<T>(jsonPayload);
Expand All @@ -1756,14 +1756,14 @@ public static void JsonObject_DuplicatePropertyCaseInsensitiveThrows()
string jsonPayload = """{"a":1,"A":2}""";

_ = JsonSerializer.Deserialize<JsonObject>(jsonPayload); // Assert no throw
AssertExtensions.ThrowsContains<ArgumentException>(
AssertExtensions.ThrowsContains<JsonException>(
() => JsonSerializer.Deserialize<JsonObject>(jsonPayload, options),
"An item with the same key has already been added.");
"Duplicate property");

_ = JsonSerializer.Deserialize<JsonNode>(jsonPayload); // Assert no throw
AssertExtensions.ThrowsContains<ArgumentException>(
AssertExtensions.ThrowsContains<JsonException>(
() => JsonSerializer.Deserialize<JsonNode>(jsonPayload, options),
"An item with the same key has already been added.");
"Duplicate property");
}

[Fact]
Expand All @@ -1774,14 +1774,14 @@ public static void JsonObject_NestedDuplicatePropertyCaseInsensitiveThrows()
string jsonPayload = """[{"a":1,"A":2}]""";

_ = JsonSerializer.Deserialize<JsonArray>(jsonPayload); // Assert no throw
AssertExtensions.ThrowsContains<ArgumentException>(
AssertExtensions.ThrowsContains<JsonException>(
() => JsonSerializer.Deserialize<JsonArray>(jsonPayload, options),
"An item with the same key has already been added.");
"Duplicate property");

_ = JsonSerializer.Deserialize<JsonNode>(jsonPayload); // Assert no throw
AssertExtensions.ThrowsContains<ArgumentException>(
AssertExtensions.ThrowsContains<JsonException>(
() => JsonSerializer.Deserialize<JsonNode>(jsonPayload, options),
"An item with the same key has already been added.");
"Duplicate property");
}

[Theory]
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