Question

我们正在努力弄清楚如何解决这个InvalidCastException。

"ExceptionType": "System.InvalidCastException",
"StackTrace": "  at ...Objects.Patient.Equals(Object obj) in 
 at System.Collections.Generic.ObjectEqualityComparer`1.Equals(T x, T y)\r\n   at System.Collections.Generic.List`1.Contains(T item)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CheckForCircularReference

事实上，我们在过去发现了类似的帖子，这个帖子也引发了同样的异常。为了重现它，我们将一个简单的WinForm：

放入其中

private void buttonJsonSerializer_Click(object sender, EventArgs e)
{
    var fixture = new Fixture
    {
        Name = "Fixture Name",
        participant = new Participant { Name = "Participant Name"}
    };

    fixture.participant = new Participant();            

    var writer = new StringWriter(new StringBuilder());
    var serializer = new JsonSerializer();
    serializer.Converters.Add(new StringEnumConverter());
    serializer.Serialize(writer, fixture);
    string output = writer.ToString();

}

public class Fixture
{
    public string Name { get; set; }            
    public Participant participant { get; set; }

    public override bool Equals(object obj)
    {
        var fixture = (Fixture)obj; // *** THROWS CAST EXCEPTION ***

        return fixture.Name == Name;
    }

    public override int GetHashCode()
    {
        return Name.GetHashCode();
    }
}

public class Participant
{
    public string Name { get; set; }

    public override bool Equals(object obj)
    {
        var participant = (Participant)obj;

        return participant.Name == Name;
    }

    public override int GetHashCode()
    {
        return Name.GetHashCode();
    }
}

提出的一个问题是：它与Newtonsoft库有什么关系吗？

而且，使用var fixture = obj as Fixture并不是我们在这种情况下要做的事情，因为它只会返回null。

Answer 1

您不能假设Fixture::Equals的论点永远是Fixture：

public override bool Equals(object obj)
{
    if (obj is Fixture fixture)
    {
        return fixture.Name == Name;
    }

    return false;
}

而且，您需要对Participant::Equals执行相同操作：

public override bool Equals(object obj)
{
    if (obj is Participant participant)
    {
        return participant.Name == Name;
    }

    return false;
}

提出的一个问题是：它与Newtonsoft库有什么关系吗？

它确实：

在UserQuery.Fixture.Equals（Object obj）at System.Collections.Generic.ObjectEqualityComparer 1.Equals(T x, T y) at System.Collections.Generic.List 1.包含（T项）at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CheckForCircularReference（JsonWriter writer，Object value，JsonProperty属性，JsonContract契约， JsonContainerContract containerContract，JsonProperty containerProperty）at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CalculatePropertyValues（JsonWriter writer，Object value，JsonContainerContract contract，JsonProperty 成员，JsonProperty财产，JsonContract＆amp; memberContract，Object＆amp; memberValue）at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject（JsonWriter writer，Object value，JsonObjectContract契约，JsonProperty 成员，JsonContainerContract collectionContract，JsonProperty containerProperty）at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize（JsonWriter jsonWriter，Object value，Type objectType）at Newtonsoft.Json.JsonSerializer.SerializeInternal（JsonWriter jsonWriter，Object value，Type objectType）at Newtonsoft.Json.JsonSerializer.Serialize（TextWriter textWriter，Object 在UserQuery.Main（）的at） LINQPad.ExecutionModel.ClrQueryRunner.Run（）at LINQPad.ExecutionModel.Server.RunQuery（QueryRunner runner）at LINQPad.ExecutionModel.Server.StartQuery（QueryRunner runner）at LINQPad.ExecutionModel.Server＆LT;＆GT; c__DisplayClass152_0.b__0（）在LINQPad.ExecutionModel.Server.SingleThreadExecuter.Work（）at System.Threading.ExecutionContext.RunInternal（执行上下文 executionContext，ContextCallback回调，对象状态，布尔值 preserveSyncCtx）at System.Threading.ExecutionContext.Run（执行上下文 executionContext，ContextCallback回调，对象状态，布尔值 preserveSyncCtx）at System.Threading.ExecutionContext.Run（执行上下文 executionContext，ContextCallback回调，对象状态）at System.Threading.ThreadHelper.ThreadStart（）

JsonSerializerInternalWriter::CheckForCircularReference正在调用Equals进行循环引用。

Newtonsoft Serialization抛出InvalidCastException

1 个答案: