如何解决创建用于生成 C# 字典的 lambda 的表达式树
我有数以千计的数据流,我需要将其转换并添加到列表中。转换通过类似于以下的反射发生
_myObservable.Subscribe(d => {
PropertyInfo[] props = d.GetType().GetProperties(BindingFlags.Instance | BindingFlags.Public);
var propValDict = props.ToDictionary(prop => prop.Name,prop => prop.GetValue(d,null));
myList.Add(propValDict);
});
// Datatype of d is determined during runtime and there are only 8 possibilities of the type
但是这种方法会降低性能,我预计使用反射可能是原因。我正在考虑通过其他方式提高性能。
建议似乎指向使用表达式树,创建编译的 lambda (Func<object,Dictionary<string,object>>)
并事先将其存储在查找字典中。
//Foreach possibleType in PossibleTypes,Do below
PropertyInfo[] props = possibleType.GetType().GetProperties(BindingFlags.Instance | BindingFlags.Public);
var rootParam = Expression.Parameter(typeof(object),"d");
var param = Expression.Parameter(typeof(PropertyInfo),"prop");
var propertyFirst = Expression.Property(param,"Name");
var param2 = Expression.Parameter(typeof(PropertyInfo),"prop");
var callMethod = Expression.Call(param2,typeof(PropertyInfo).getmethod(nameof(PropertyInfo.GetValue),new Type[] { typeof(object) }),rootParam);
var pro = Expression.Parameter(typeof(Array),"props");
var toDict = Expression.Invoke(pro,propertyFirst,callMethod);
var lambda = Expression.Lambda<Func<object,object>>>(toDict,rootParam);
var compiled = lambda.Compile();
我无法调用 Enumerable 类的 ToDictionary 我在这种方法中遗漏了一些东西,或者这真的会提高性能吗。
请帮忙...
解决方法
在考虑表达式时,您总是需要弄清楚等效的 C# 代码是什么样的。在这种情况下,等效的 C# 代码不会循环遍历 PropertyInfo
的集合,而是可能看起来像:
public static Func<object,Dictionary<string,object>> CreateConvertToPropertyDict<T>()
{
return input =>
{
var d = (T)input;
return new Dictionary<string,object>())
{
{ "Foo",d.Foo },{ "Bar",d.Bar },};
};
}
myList.Add(propValDict);
在表达领域横向移动,你会得到类似的结果:
public static Func<object,object>> CreatePropertyDict(Type type)
{
// Consider caching these in a static field,since they're constant
var dictType = typeof(Dictionary<string,object>);
var dictCtor = dictType.GetConstructor(new[] { typeof(int) });
var dictAddMethod = dictType.GetMethod("Add");
var properties = type.GetProperties(BindingFlags.Instance | BindingFlags.Public);
var blockExpressions = new List<Expression>();
// 'object input' is our input parameter
var inputParameter = Expression.Parameter(typeof(object),"input");
// MyType d;
var dVariable = Expression.Variable(type,"d");
// d = (MyType)inputObject;
blockExpressions.Add(Expression.Assign(dVariable,Expression.Convert(inputParameter,type)));
// Dictionary<string,object> dict;
var dictVariable = Expression.Variable(dictType,"dict");
// dict = new Dictionary<string,object>(3) (or however many properties there are)
blockExpressions.Add(Expression.Assign(dictVariable,Expression.New(dictCtor,Expression.Constant(properties.Length))));
foreach (var property in properties)
{
var propertyAccess = Expression.Property(dVariable,property);
// dict.Add("Foo",(object)d.Foo)
blockExpressions.Add(Expression.Call(
dictVariable,dictAddMethod,Expression.Constant(property.Name),Expression.Convert(propertyAccess,typeof(object))));
};
// The final statement in a block is the return value
blockExpressions.Add(dictVariable);
var block = Expression.Block(new[] { dVariable,dictVariable },blockExpressions);
return Expression.Lambda<Func<object,object>>>(block,inputParameter).Compile();
}
使用简单的测试用例:
public static void Main()
{
var test = new Test() { Foo = "woop",Bar = 3 };
var expr = CreatePropertyDict(typeof(Test));
expr(test).Dump();
}
这里有几种更高级的Expression用法,我不打算详细介绍每一种。查看 the docs,并尝试一下 C# 编译器为不同的 C# 代码位生成的表达式种类。
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