protobuf-net 是否支持可为空的类型？

Question

是否可以在 protobuf-net 中生成可为空的成员？

message ProtoBuf1 {
    optional Int32? databit = 1;
    optional Nullable<bool> databool = 2;
}

Answer 1

是的，但如果您从 .proto 进行代码生成，默认情况下它不会生成它们。

如果这只是 C#，当然，您不需要 .proto - 只需：

[ProtoContract]
public class ProgoBuf1
{
    [ProtoMember(1)]
    public int? Foo {get;set;}

    [ProtoMember(2)]
    public float? Bar {get;set;}
}

如果您正在使用 .proto，您可以考虑复制和编辑 csharp.xslt 以适应您的首选布局。

Answer 2

这是我在使用需要 Protocol Buffers 版本 3 的 Google 的 Protobuf .NET API 时针对可空类型的解决方案。（请注意，这不是使用 Marc Gravell 的 protobuf-net，因此这不是对所提问题的确切答案。）

在NullableInt32.proto:

syntax = "proto3";

message NullableInt32 {
  int32 value = 1;
}

在NullableInt32Extensions.cs:

public static class NullableInt32Extensions
{
  public static bool HasValue(this NullableInt32 source)
  {
    return source != null;
  }
}

public partial class NullableInt32
{
  public static implicit operator int? (NullableInt32 other)
  {
    return other == null ? (int?)null : other.Value;
  }

  public static implicit operator NullableInt32(int? other)
  {
    return other == null ? null : new NullableInt32 { Value = other.Value };
  }
}

此模式可用于任何 Protobuf 非长度分隔标量值 —double、float、int32、int64、uint32、uint64、sint32、@987654332 @、fixed32、fixed64、sfixed32、sfixed64 和 bool。

---

这就是所有这些的工作原理。假设您有一个 Record 消息，其中包含一个 NullableInt32 字段，在这个人为的示例中，它是唯一的字段。

在Record.proto:

syntax = "proto3";

import "NullableInt32.proto";

message Record {
  NullableInt32 id = 1;
}

使用 Google 的 protoc.exe 将其编译为 C# 后，您可以将 Id 属性与 Nullable<int> 几乎完全相同。

var r = new Record();

// r.Id is null by default, but we can still call HasValue()
// because extension methods work on null references.
r.Id.HasValue(); // => false

// We can explicitly set Id to null.
r.Id = null;

// We can set Id to a primitive numeric value directly
// thanks to our implicit conversion operators.
r.Id = 1;

// We can also use NullableInt32 in any context that expects a
// Nullable<int>. The signature of the following method is
// bool Equals(int?, int?).
Nullable.Equals<int>(r.Id, 1); // => true

// We can explicitly set Id to a NullableInt32.
r.Id = new NullableInt32 { Value = 1 };

// Just like Nullable<int>, we can get or set the Value of a
// NullableInt32 directly, but only if it's not null. Otherwise,
// we'll get a NullReferenceException. Use HasValue() to avoid this.
if(r.Id.HasValue())
  r.Id.Value.ToString(); // => "1"

// Setting Id to 0 is the same as setting Id to a new
// NullableInt32 since the default value of int32 is 0.
// The following expressions are equivalent.
r.Id = 0;
r.Id = new NullableInt32();
r.Id = new NullableInt32 { Value = 0 };
r.Id.Value = 0; // as long as Id is not null

---

最后，让我们看看我们的Record 消息将如何通过Id 的不同值通过网络传输。

var r = new Record();

// When Id is null, Record is empty since it has no other fields.
// Explicitly setting Id to null will have the same effect as
// never setting it at all.
r.Id = null;
r.ToByteArray(); // => byte[0]

// Since NullableInt32 is a Protobuf message, it's encoded as a
// length delimited type. Setting Id to 1 will yield four bytes.
// The first two indicate the type and length of the NullableInt32
// message, and the last two indicate the type and value held within.
r.Id = 1;
r.ToByteArray(); // => byte[] {
                 //      0x0a, // field  = 1, type = 2 (length delimited)
                 //      0x02, // length = 2
                 //      0x08, // field  = 1, type = 0 (varint)
                 //      0x01, // value  = 1
                 //    }

// When Id is set to the default int32 value of 0, only two bytes
// are needed since default values are not sent over the wire.
// These two bytes just indicate that an empty NullableInt32 exists.
r.Id = 0;
r.ToByteArray(); // => byte[] {
                 //      0x0a, // field  = 1, type = 2 (length delimited)
                 //      0x00, // length = 0
                 //    }

Answer 3

原型 3

导入“wrappers.proto”支持可空值：

• 字符串（字符串值），
• int(Int32Value),
• bool(BoolValue)
• 等

支持类型的完整列表 - https://docs.microsoft.com/en-us/dotnet/architecture/grpc-for-wcf-developers/protobuf-data-types#nullable-types

例子：

syntax = "proto3";
import "google/protobuf/wrappers.proto";

message ProtoPerson {
    google.protobuf.StringValue firstName = 1;
    google.protobuf.StringValue lastName = 2;
    google.protobuf.StringValue address1 = 3;
    google.protobuf.Int32Value age = 4;
}