Develop Reference

Protobuf-net enum backwards compatibility

I was trying to add a new enum value for a certain protobuf-serialized class in a new app version, and while testing, noticed that the previous version will throw an exception, given this new file format:
An unhandled exception of type 'ProtoBuf.ProtoException' occurred in protobuf-net.dll
Additional information: No {enum-type-name} enum is mapped to the wire-value 3
It is fairly obvious that it's telling me that there is no enum value for the int value of 3, but I always had the idea that Protocol Buffers defaulted to the zero-valued ("default") enum value (if such exists), in case that an actual enum value couldn't be mapped to.
To clarify, this can be reproduced using the following example (I am intentionally doing the deserialization step into a different class to mimic old app trying to load the new format):
// --- version 1 ---
public enum EnumV1
{
Default = 0,
One = 1,
Two = 2
}
[ProtoContract]
public class ClassV1
{
[ProtoMember(1)]
public EnumV1 Value { get; set; }
}
// --- version 2 ---
public enum EnumV2
{
Default = 0,
One = 1,
Two = 2,
Three = 3 // <- newly added
}
[ProtoContract]
public class ClassV2
{
[ProtoMember(1)]
public EnumV2 Value { get; set; }
}
And the following code will fail:
// serialize v2 using the new app
var v2 = new ClassV2() { Value = EnumV2.Three };
var v2data = Serialize(v2);
// try to deserialize this inside the old app to v1
var v1roundtrip = Deserialize<ClassV1>(v2data);
Since v1 is out in the open, is there some metadata I can use when serializing in v2 to avoid this issue? I can, of course, get myself out of this trouble by rewriting v2 to use a separate property and leave the enum values unmodified, but I'd like to make enums backwards compatible if possible.

Adding [ProtoContract(EnumPassthru=true)] to your enums will allow protobuf-net to deserialize unknown values.
Unfortunately, there is no way to retroactively fix your v1. You'll have to use a different property.

Since v1 is out in the open, is there some metadata I can use when serializing in v2 to avoid this issue? I can, of course, get myself out of this trouble by rewriting v2 to use a separate property and leave the enum values unmodified, but I'd like to make enums backwards compatible if possible.
What you are experiencing is a protobuf-net bug described here protobuf-net - issue #422: Invalid behaviour while deserializing unknown enum value.
It seems that it's not fixed yet according to here protobuf-net faulty enum exception (issue 422) need a good workaround (and of course your post).
Unfortunately you need to either fix the protobuf-net source code or use the workarounds mentioned.
UPDATE: I've checked the code in the GitHub repository and confirming that the issue is still not fixed. Here is the problematic code inside the EnumSerializer.cs (the ISSUE #422 comment is mine):
public object Read(object value, ProtoReader source)
{
Helpers.DebugAssert(value == null); // since replaces
int wireValue = source.ReadInt32();
if(map == null) {
return WireToEnum(wireValue);
}
for(int i = 0 ; i < map.Length ; i++) {
if(map[i].WireValue == wireValue) {
return map[i].TypedValue;
}
}
// ISSUE #422
source.ThrowEnumException(ExpectedType, wireValue);
return null; // to make compiler happy
}

Your ClassV1 lacks forward compatiblity.
I would have implemented the Proto contract in such a way that it serializes/deserializes the string representation of the enum value. This way you can handle the fallback to the default value by yourself. The Value property would not be serialized/deserialized.
public enum EnumV1
{
Default = 0,
One = 1,
Two = 2
}
public enum EnumV2
{
Default = 0,
One = 1,
Two = 2,
Three = 3 // <- newly added
}
[ProtoContract]
public class ClassV1
{
[ProtoMember(1)]
public string ValueAsString
{
get { return Value.ToString(); }
set
{
try
{
Value = (EnumV1) Enum.Parse(typeof (EnumV1), value);
}
catch (Exception)
{
Value = EnumV1.Default;
}
}
}
public EnumV1 Value { get; set; }
}
[ProtoContract]
public class ClassV2
{
[ProtoMember(1)]
public string ValueAsString
{
get { return Value.ToString(); }
set
{
try
{
Value = (EnumV2)Enum.Parse(typeof(EnumV2), value);
}
catch (Exception)
{
Value = EnumV2.Default;
}
}
}
public EnumV2 Value { get; set; }
}
Still it does not solve the problem of having a non-forward-compabtible class in production.

You could add the DefaultValue attribute to your proto member property.
[ProtoContract]
public class ClassV1
{
[ProtoMember(1), DefaultValue(EnumV1.Default)]
public EnumV1 Value { get; set; }
}
To make clear how the property should be initialized for the default case.

unable to create two property with same name in C#

I have created two properties with same name but the data type is different.
I am getting this error:
The type 'Fields' already contains a definition for 'Subject'
Is it possible to overcome this issue?
public String Subject
{
get { return this.subject; }
set { this.subject = value; }
}
public AppSettings Subject
{
get {
return this.subjectObj; }
set {
this.subjectObj = value; }
}

No, it's not possible. You would not be able to tell these properties apart when using them. How would you know which property is set in this case?
someInstance.Subject = null;
The official documentation on class members states:
The name of a constant, field, property, event, or type must differ
from the names of all other members declared in the same class.
Possible solution:
Your naming suggests that you may want to create another class Subject with at least two properties. (Without more context, I can't say if this is appropriate.)
public class Subject
{
public string Name { get; set; }
public AppSettings Settings { get; set; }
}
This new class could be used in the original class:
public OriginalClass
{
public Subject subject { get; set; }
}

No, you can't overcome that. Imagine you're doing this line:
object myValue = MyClassInstance.Subject;
How would it know how to tell them apart? It's better to give them a more appropriate name:
public String Subject
{
get { return this.subject; }
set { this.subject = value; }
}
public AppSettings Settings
{
get { return this.subjectObj; }
set { this.subjectObj = value; }
}

No you cannot create that. The C# 4 spec, section 10.2:and the standards does not allow you do that
The names of constants, fields, properties, events, or types must
differ from the names of all other members declared in the same class.
On a side note you can refer this thread: Is there a way to use a property with same name but is of different type in derived class?

Most of the answers seem to put forth the reasoning that polymorphism is not allowed in Properties due to something like:
How would you resolve this:
MyObject.MyProperty = null;
Well that's essentially the same as:
MyObject.set_Property(null);
Where set_Property is a method like:
void set_Property(String name) { ... }
void set_Property(AppSettings settings) { ... }
This is legal in C#
So MyObject.MyProperty = null; can be resolved as:
MyObject.set_MyPropert(null);
The compiler will report an Error in the line where the call is made, since it could use both overloads.
But MyObject.set_MyProperty(string.Empty);
is legal all together.
The reason why C# disallows polymorphism on Properties is the same as why it disallows different return types of functions with the same name.
Illigeal:
String get_Property() { return ...; }
AppSettings get_Property() { return ...; }
Now if you do var value = get_Property(); the compiler can not decide which overload to use.
So in principle set only properties could be overloaded. The problem arises when you introduce a getter.
I guess for convenience and consistency reasons the standard disallows polymorphism on properties entirely.
Also because in the language context Properties are not really considered as methods.

How to make a property required in c#?

I have requirement in a custom class where I want to make one of my properties required.
How can I make the following property required?
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}

If you mean "the user must specify a value", then force it via the constructor:
public YourType(string documentType) {
DocumentType = documentType; // TODO validation; can it be null? blank?
}
public string DocumentType {get;private set;}
Now you can't create an instance without specifying the document type, and it can't be removed after that time. You could also allow the set but validate:
public YourType(string documentType) {
DocumentType = documentType;
}
private string documentType;
public string DocumentType {
get { return documentType; }
set {
// TODO: validate
documentType = value;
}
}

.NET 7 or newer
Syntax
public class MyClass
{
public required string Name { get; init; }
}
new MyClass(); // illegal
new MyClass { Name = "Me" }; // works fine
Remarks
The required properties must declare a setter (either init or set).
Access modifiers on properties or setters cannot be less visible than their containing type, as they would make impossible to initialize the class in some cases.
public class MyClass
{
internal required string Name { get; set; } // illegal
}
Documentation
Official documentation here
Feature demo here
.NET 6 or older
See this answer

If you mean you want it always to have been given a value by the client code, then your best bet is to require it as a parameter in the constructor:
class SomeClass
{
private string _documentType;
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}
public SomeClass(string documentType)
{
DocumentType = documentType;
}
}
You can do your validation – if you need it – either in the property's set accessor body or in the constructor.

With the release of .NET 7 and C# 11 in November 2022 you can now use the required modifier this way:
public class Person
{
public Person() { }
[SetsRequiredMembers]
public Person(string firstName) => FirstName = firstName;
public required string FirstName { get; init; }
public int Age { get; set; }
}
And when you don't have the required properties it will throw an error when you try to initialize an object.
For more information refer to:
https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-11#required-members
https://learn.microsoft.com/en-us/dotnet/csharp/properties#init-only

Add a required attribute to the property
Required(ErrorMessage = "DocumentTypeis required.")]
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}
For custom attribute detail Click Here

I used an other solution, not exactly what you want, but worked for me fine because I declare the object first and based on specific situation I have different values. I didnt want to use the constructor because I then had to use dummy data.
My solution was to create Private Sets on the class (public get) and you can only set the values on the object by methods. For example:
public void SetObject(string mandatory, string mandatory2, string optional = "", string optional2 = "")

This one liner works in C# 9:
public record Document(string DocumentType);
new Document(); // compiler error
new Document("csv"); // correct way to construct with required parameter
This explains how it works. In the above code, Document is the name of the class or "record". That first line of code actually defines an entire class. In addition to this solution essentially making a required DocumentType property (required by an auto implemented constructor), because it uses records, there are additional implications. So this may not always be an appropriate solution, and the C# 11 required keyword will still come in handy at times. Just using record types doesn't automatically make properties required. The above code is a special syntax way of using records that essentially has this effect as well as making the property init only and causes a deconstructor to be automatically implemented.
A better example would be using an int property instead of a string since a string could still be empty. Unfortunately I don't know of any good way to do extra validation within the record to make sure the string is not empty or an int is in range, etc. You would have to go deeper down the TOP (type driven development) rabbit hole, which may not be a bad thing. You could create your own type that doesn't allow empty strings or integers outside your accepted range. Unfortunately such an approach would lead to runtime discovery of invalid input instead of compile time. There might be a better way using static analysis and metadata, but I've been away from C# for too long to know anything about that.

C# XmlSerializer: keep the value, override the element label

I am currently using a LINQ query to read an XML file e.g.
<MyObjects>
<MyObject>
<MyElement>some_text</MyElement>
<MyOtherElement>some_more_text</MyOtherElement>
</MyObject>
</MyObjects>
into a list of custom objects containing custom HistoryString properties. HistoryString contains 2 strings, a currentValue and a previousValue.
This all works great except when using XmlSerializer to write the custom objects back to an XML file, the output fairly obviously contains additional tags i.e.
<MyObjects>
<MyObject>
<MyElement>
<currentValue>some_text</currentValue>
<previousValue>some_text</previousValue>
</MyElement>
<MyOtherElement>
<currentValue>some_more_text</currentValue>
<previousValue>some_more_text</previousValue>
</MyOtherElement>
</MyObject>
</MyObjects>
Q: What would be the neatest and/or most efficient way of reading and writing XML in the same format, based on this fundamental difference?
Some initial ideas:
1) Mark the previousValue property with [System.Xml.Serialization.XmlIgnore] then sweep through the XML string that is to be written removing all traces of <currentValue> and </currentValue>
2) Open the existing file and manually make any updates/deletes/additions - this is surely more long winded.
3) Any way of having a HistoryString automatically resolve to its currentValue rather than serialize each of its properties, similar to how ToString() works?
I have done some research into this, including the useful MSDN articles here and here but I can't see any other attributes that would solve this problem, I am still unsure whether this is possible. Any ideas?

Here is another idea. If you define your class like so:
[Serializable]
public class MyObject
{
[XmlElement(ElementName = "MyElement")]
public string CurrentValueElement
{
get
{
return Element.CurrentValue;
}
set
{
Element = new MyElement
{
CurrentValue = value, PreviousValue = value
};
}
}
[XmlElement(ElementName = "MyOtherElement")]
public string CurrentValueOtherElement
{
get
{
return OtherElement.CurrentValue;
}
set {}
}
[XmlIgnore]
public MyElement Element { get; set; }
[XmlIgnore]
public MyElement OtherElement { get; set; }
}
Then, when the object is serialized, the output XML will look exactly like your example.
Also, if you extend the CurrentValueElement/CurrentValueOtherElement setter like this:
[XmlElement(ElementName = "MyElement")]
public string CurrentValueElement
{
get
{
return Element.CurrentValue;
}
set
{
Element = new MyElement
{
CurrentValue = value, PreviousValue = value
};
}
}
Then you'll be able to use the XmlSerializer to deserialize your objects directly without needing to resorting to LINQ.

Well why not serialize back using original schema and feeding into it the list of transformed objects from history using only current value?
e.g.
from h in HistoryEntryList
select new OriginalEntry{ field = h.field.current_value, ... };

Serializing an arraylist in C#

I have a class that contains a number of standard fields and an arraylist.
Is there any way to serialize the class using an XmlSerializer?
Attempts so far result in an error message saying:
Unhandled Exception: System.InvalidOperationException: There was an error
generating the XML document. ---> System.InvalidOperationException: The type
XMLSerialization.DataPoints was not expected. Use the XmlInclude or
SoapInclude attribute to specify types that are not known statically.
Some cut-down representations of the classes are shown below:
public class StationData
{
private DateTime _CollectionDate;
private string _StationID;
private ArrayList _PolledData;
public StationData()
{
}
public DateTime CollectionDate
{
get { return _CollectionDate; }
set { _CollectionDate = value; }
}
public string StationID
{
get { return _StationID; }
set { _StationID = value; }
}
[XmlInclude(typeof(DataPoints))]
public ArrayList PolledData
{
get { return _PolledData; }
set { _PolledData = value; }
}
}
public class DataPoints
{
private string _SubStationID;
private int _PointValue;
public DataPoints
{
}
public string SubStationID
{
get { return _SubStationID; }
set { _SubStationID = value; }
}
public int PointValue
{
get { return _PointValue; }
set { _PointValue = value; }
}
}

I have had success with the following:
[XmlArray("HasTypeSpecialisations")]
[XmlArrayItem("TypeObject", typeof(TypeObject), IsNullable = false)]
public List<TypeObject> TypeSpecialisations
This results in:
<HasTypeSpecialisations>
<TypeObject />
<TypeObject />
</HasTypeSpecialisations>
In your situation I would try something like:
[XmlArrayItem(typeof(DataPoints))]
public ArrayList PolledData
Based on this link http://msdn.microsoft.com/en-us/library/2baksw0z(VS.85).aspx you should also be able to use this
[XmlElement(Type = typeof(DataPoints))]
public ArrayList PolledData

The XmlSerializer generates some code at runtime to serialize your class. It is necessary for this class to know all types that can occur.
The ArrayList does not give this information, but you can give it by using a XmlInclude attribute on the property that returns the ArrayList.
[XmlInclude(typeof(DataPoints))]
public ArrayList Points {
...
}
You could also use the generic List<> class.

I think you could get around this by using a generic list (List<>) instead of an ArrayList, however, I'm going to assume you can't use generics for one reason or another.
You need to tell the compiler what type is contained in the ArrayList so it can serialize it since all it knows it that it contains objects.
Add this above your property and it should clear it up.
[System.Xml.Serialization.XmlInclude(typeof(XMLSerialization.DataPoints))]
Of course, replace XMLSerialization.DataPoints with whatever class is contained in the ArrayList.

Take a look at this article which describes the basic problem you are having and a solution around it (Like using the XmlInclude attribute). Basically what that says is that the serializer encountered a type it doesn't know how to serialize. If you post some code it would also help greatly.

The method I always used to serialize lists was to convert them to Arrays (which has the necessary type information). I admit this is a bit dirty, but if you can't get a proper list to serialize, this will work.