Fix JsonException message property type info in class parameterized constructor by XeronOwO · Pull Request #126575 · dotnet/runtime

XeronOwO · 2026-04-06T08:20:17Z

This pull request fixes an incorrect type hint in the thrown JsonException when deserialization fails for classes with parameterized constructors.

Reproduction

using System.Text.Json;

Test<NormalClass>();
Test<ParameterizedNormalClass>();
Test<RecordClass>();
Test<NormalStruct>();
Test<ParameterizedNormalStruct>();
Test<RecordStruct>();

static void Test<T>()
{
	try
	{
		JsonSerializer.Deserialize<T>("{\"Text\":{}}"); // create json convert exception manually
	}
	catch (Exception e)
	{
		Console.WriteLine(e.Message);
	}
}

class NormalClass
{
	public string Text { get; set; } = null!;
}

class ParameterizedNormalClass(string text)
{
	public string Text { get; set; } = text;
}

record RecordClass(string Text);

struct NormalStruct()
{
	public string Text { get; set; } = null!;
}

struct ParameterizedNormalStruct(string text)
{
	public string Text { get; set; } = text;
}

record struct RecordStruct(string Text);

Expected Output

The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.

Actual Output

The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to ParameterizedNormalClass. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to RecordClass. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.
The JSON value could not be converted to System.String. Path: $.Text | LineNumber: 0 | BytePositionInLine: 9.

Root Cause

For classes with parameterized constructors, state.Current.CtorArgumentState.JsonParameterInfo was used instead of state.Current.JsonPropertyInfo.
(All structs, including parameterized structs and record structs, work correctly without this issue.)

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Object/ObjectWithParameterizedConstructorConverter.cs

Lines 636 to 642 in 3f14c31

    
           jsonParameterInfo = jsonPropertyInfo.AssociatedParameter; 
        
           if (jsonParameterInfo != null) 
        
           { 
        
               state.Current.JsonPropertyInfo = null; 
        
               state.Current.CtorArgumentState!.JsonParameterInfo = jsonParameterInfo; 
        
               state.Current.NumberHandling = jsonParameterInfo.NumberHandling;

As a result, the exception message fell back to state.Current.JsonTypeInfo.Type for affected classes.

runtime/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs

Lines 542 to 544 in 3f14c31

    
           // Use a default message. 
        
           Type propertyType = state.Current.JsonPropertyInfo?.PropertyType ?? state.Current.JsonTypeInfo.Type; 
        
           message = SR.Format(SR.DeserializeUnableToConvertValue, propertyType);

Fix

Added logic to retrieve type information from state.Current.CtorArgumentState.JsonParameterInfo.ParameterType, ensuring correct type resolution for classes with parameterized constructors.

danmoseley · 2026-04-11T23:08:56Z

Does this need a test?

XeronOwO · 2026-04-12T02:55:15Z

@dotnet-policy-service agree

XeronOwO · 2026-04-12T02:58:20Z

@danmoseley Yes, this needs a test. I've added a regression test to ConstructorTests.Exceptions.cs that verifies the JsonException message contains the correct property type (System.String) rather than the declaring class type (ParameterizedNormalClass / RecordClass) when deserialization fails for classes with parameterized constructors.

Copilot

Pull request overview

Fixes JsonException default message type reporting during deserialization failures for object types using parameterized constructors by resolving the failing member type from the active constructor parameter (when JsonPropertyInfo is unavailable).

Changes:

Update ThrowHelper.AddJsonExceptionInformation to prefer state.Current.CtorArgumentState?.JsonParameterInfo?.ParameterType when JsonPropertyInfo is null.
Add a regression test ensuring parameterized class/record constructor failures report the property/parameter type (System.String) rather than the declaring type.

Show a summary per file

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs	Adjusts default exception type resolution to use active constructor parameter type when applicable.
src/libraries/System.Text.Json/tests/Common/ConstructorTests/ConstructorTests.Exceptions.cs	Adds coverage validating the corrected exception message type for parameterized constructors.

Copilot's findings

Files reviewed: 2/2 changed files
Comments generated: 0

…onstructor

…tructors This test verifies that when deserialization fails for classes with parameterized constructors, the JsonException message reports the correct property type (e.g., System.String) rather than the declaring class type. Covers both parameterized classes and records to ensure the fix in ThrowHelper.Serialization.cs works correctly across different scenarios.

…essage test Make test classes public and register them in source generation contexts to fix compilation errors in System.Text.Json.SourceGeneration.Tests. The test classes need to be public (not private) so they can be referenced by the JsonSerializable attributes in the source generation test contexts.

github-actions bot added the area-System.Text.Json label Apr 6, 2026

dotnet-policy-service bot added the community-contribution Indicates that the PR has been added by a community member label Apr 6, 2026

XeronOwO force-pushed the fix_json_exception_type branch from be4ce8b to c7c0874 Compare April 11, 2026 12:09

This was referenced Apr 11, 2026

Unable to pull image from mcr.microsoft.com #117164

Open

System.Runtime.InteropServices.Tests.DestroyStructureTests.DestroyStructure_Blittable_Success failing in CI #126788

Closed

Copilot AI mentioned this pull request Apr 11, 2026

Fix NullReferenceException in DestroyStructure for blittable types #126795

Merged

danmoseley requested review from Copilot and eiriktsarpalis April 12, 2026 03:45

Copilot AI reviewed Apr 12, 2026

View reviewed changes

XeronOwO added 3 commits April 12, 2026 14:05

Fix JsonException message property type info in class parameterized c…

2a7302e

…onstructor

XeronOwO force-pushed the fix_json_exception_type branch from 13c27a4 to a14616c Compare April 12, 2026 06:05

build-analysis bot mentioned this pull request Apr 12, 2026

Android arm32 device not found (armeabi-v7a architecture unavailable) #125440

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Fix JsonException message property type info in class parameterized constructor#126575

Fix JsonException message property type info in class parameterized constructor#126575
XeronOwO wants to merge 3 commits intodotnet:mainfrom
XeronOwO:fix_json_exception_type

XeronOwO commented Apr 6, 2026 •

edited

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	jsonParameterInfo = jsonPropertyInfo.AssociatedParameter;
	if (jsonParameterInfo != null)
	{
	state.Current.JsonPropertyInfo = null;
	state.Current.CtorArgumentState!.JsonParameterInfo = jsonParameterInfo;
	state.Current.NumberHandling = jsonParameterInfo.NumberHandling;

	// Use a default message.
	Type propertyType = state.Current.JsonPropertyInfo?.PropertyType ?? state.Current.JsonTypeInfo.Type;
	message = SR.Format(SR.DeserializeUnableToConvertValue, propertyType);

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XeronOwO commented Apr 6, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Reproduction

Expected Output

Actual Output

Root Cause

Fix

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danmoseley commented Apr 11, 2026

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XeronOwO commented Apr 12, 2026

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XeronOwO commented Apr 12, 2026

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XeronOwO commented Apr 6, 2026 •

edited

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