前言
众所周知，C#中的mongodb驱动默认是不会序列化和反序列只读属性的。所以当我们存储Protobuf的类型时，如果有属性是RepeatedField和MapField类型，那么该属性并不会被存储到mongodb数据库中。如果要正常存储，则需要自己调用RegisterClassMap方法注册该Protobuf类型的映射。类似下面的代码:
class Test
{public RepeatedField<int> RepeatedField { get; }
}static Test CreateTest(RepeatedField<int> repeatedField)
{var res = new Test();res.RepeatedField.AddRange(repeatedField);return res;
}
static void RegisterTestClassMap()
{BsonClassMap.RegisterClassMap<Test>(cm =>{cm.AutoMap();// SetDefaultValue是为了防止Protobuf新加了属性,但是数据库的老数据没有该属性,导致反序列化失败的问题cm.MapProperty(x => x.RepeatedField).SetDefaultValue(new RepeatedField<int>());cm.MapCreator(p => CreateTest(p.RepeatedField));});
}我们需要为每一个类型都注册类似的映射，非常的麻烦。而且如果protobuf的字段有新增，还需要改动代码，如果修改不及时还会导致线上的数据丢失。麻烦且不安全。
自动映射
我们自定义一个IClassMapConvention,自动生成并注册包含只读属性的类的构造函数的委托。
核心是利用表达式树，自动生成对应类的构造函数的委托。
生成的委托只支持protobuf里的RepeatedField和MapField类型。需要支持其他的需要自己修改。
/// <summary>/// Protobuf的默认映射(映射只读属性及其默认值和构造函数)/// </summary>public class ProtobufDefaultMapConvention : ConventionBase, IClassMapConvention{private readonly BindingFlags _bindingFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.DeclaredOnly;public ProtobufDefaultMapConvention() { }private string RepeatedFieldName = typeof(RepeatedField<>).Name;private string MapFieldName = typeof(MapField<,>).Name;/// <summary>/// 应用/// </summary>/// <param name="classMap"></param>public void Apply(BsonClassMap classMap){var readOnlyProperties = classMap.ClassType.GetTypeInfo().GetProperties(_bindingFlags).Where(p => IsReadOnlyProperty(classMap, p)).ToList();foreach (var property in readOnlyProperties){//<c cref="RepeatedField{T}">和<c cref="MapField{TKey, TValue}">和值类型所有对象共用同一个默认实例。要自己映射请覆盖默认值。// 必须设置默认值。否则会无法匹配构造函数if (property.PropertyType.Name == RepeatedFieldName || property.PropertyType.Name == MapFieldName || property.PropertyType.IsValueType){var defaultVal = Activator.CreateInstance(property.PropertyType);classMap.MapMember(property).SetDefaultValue(defaultVal);}else{classMap.MapMember(property).SetDefaultValue(() => Activator.CreateInstance(property.PropertyType));LogUtil.Warn($"ProtobufDefaultMapConvention->PB发现异常只读类型:{property.PropertyType.Name}");}}var res = GetProtobufCreatorFunc(classMap.ClassType, readOnlyProperties);classMap.MapCreator(res.Item1, res.Item2);}/// <summary>/// 获取protobuf的构造函数的委托(赋值只读属性)/// </summary>/// <param name="targetType"></param>/// <param name="readOnlyPropList"></param>/// <returns></returns>public (Delegate, string[]) GetProtobufCreatorFunc(Type targetType, List<PropertyInfo> readOnlyPropList){// 返回值var result = Expression.Variable(targetType, "result");// 初始化要创建的对象var initializeResult = Expression.Assign(result, Expression.New(targetType));var ctorParamList = new List<ParameterExpression>();// 构造函数func的方法体语句var ctorBlockExperssionList = new List<Expression>() { initializeResult };// 构造函数func的泛型类型列表var ctorFuncTypeList = new List<Type>(readOnlyPropList.Count + 1);// 构造函数func的参数名称列表var paramNames = new List<string>(readOnlyPropList.Count);foreach (var item in readOnlyPropList){ctorFuncTypeList.Add(item.PropertyType);paramNames.Add(item.Name);// 将需要赋值的只读属性设置为参数var ctorParam = Expression.Parameter(item.PropertyType, item.Name);ctorParamList.Add(ctorParam);// 取要赋值的只读属性var readOnlyMember = Expression.Property(result, item.Name);// 赋值语句Expression methodCall = null;if (item.PropertyType.Name == RepeatedFieldName){// 如果传入的参数不为null的话，则调用RepeatedField的AddRange方法//methodCall = Expression.IfThen(Expression.NotEqual(ctorParam, Expression.Constant(null)), Expression.Call(readOnlyMember, nameof(RepeatedField<int>.AddRange), null, ctorParam));// 不判null,如果要自己设置默认值请勿设置为nullmethodCall = Expression.Call(readOnlyMember, nameof(RepeatedField<int>.AddRange), null, ctorParam);}else if (item.PropertyType.Name == MapFieldName){// 如果传入的参数不为null的话，则调用MapField的Add方法//methodCall = Expression.IfThen(Expression.NotEqual(ctorParam, Expression.Constant(null)), Expression.Call(readOnlyMember, nameof(MapField<string, int>.Add), null, ctorParam));// 不判null,如果要自己设置默认值请勿设置为nullmethodCall = Expression.Call(readOnlyMember, nameof(MapField<string, int>.Add), null, ctorParam);}if (methodCall != null){ctorBlockExperssionList.Add(methodCall);}}ctorFuncTypeList.Add(targetType);ctorBlockExperssionList.Add(result);// 构造函数func的方法块var ctorFuncBody = Expression.Block(new[] { result }, ctorBlockExperssionList);// 构造函数func的Typevar ctorFuncType = Expression.GetFuncType(ctorFuncTypeList.ToArray());// 构造函数func的lambda表达式var ctorlambda = Expression.Lambda(ctorFuncType, ctorFuncBody, $"create{targetType.Name}", true, ctorParamList).Compile();return (ctorlambda, paramNames.ToArray());}/// <summary>/// 只读属性/// </summary>/// <param name="classMap"></param>/// <param name="propertyInfo"></param>/// <returns></returns>private static bool IsReadOnlyProperty(BsonClassMap classMap, PropertyInfo propertyInfo){if (!propertyInfo.CanRead) return false;if (propertyInfo.CanWrite) return false; // already handled by default conventionif (propertyInfo.GetIndexParameters().Length != 0) return false; // skip indexersvar getMethodInfo = propertyInfo.GetMethod;if (getMethodInfo == null){return false;}// skip overridden properties (they are already included by the base class)if (getMethodInfo.IsVirtual && getMethodInfo.GetBaseDefinition().DeclaringType != classMap.ClassType) return false;return true;}}
最后注册该Convention
ConventionRegistry.Register("PBConventionsPack", new ConventionPack { new ProtobufDefaultMapConvention() }, t => t.IsAssignableTo(typeof(IMessage)) && t is { IsAbstract: false });