如何创建具有动态类型的 lambda 表达式

Question

我的程序将在其运行时间内获取服务名称和方法名称，并且为了动态执行该方法，我正在创建一个基于 lambda 表达式的函数。

public static Func<object,object,object> CreateLambdaExpression2(string tService, string methodName)
{
    var inputServiceType = Type.GetType(tService);
    var methodInfo = inputServiceType.GetMethod(methodName);
    var inputType = methodInfo.GetParameters().First().ParameterType;
    var outputType = methodInfo.ReturnParameter.ParameterType;

    var instance = Expression.Parameter(inputServiceType, "serviceInstance");
    var input = Expression.Parameter(inputType, "inputData");
    var call = Expression.Call(instance, methodInfo, input);

    var lambdaFunc = Expression.Lambda<Func<object,object, object>>(call, instance, input).Compile(); //<= this line throws the error.
    return lambdaFunc;
}

但它不会，它会在运行时间抛出错误

var compiledMethod = ServiceMapper.CreateLambdaExpression2(tService,"Get");

var serviceInstance = new TestDemoService();
var inputData = new TestDemoPersonRequest()
{
    Id = 555
};
var result = compiledMethod(serviceInstance, inputData);

System.ArgumentException: 'ParameterExpression of type 'UnitTests.ITestDemoService' cannot be used for delegate parameter of type 'System.Object''

有没有办法指定 Expression.Lambda 的类型？

Expression.Lambda<Func<object,object, object>>

到

Expression.Lambda<Func<inputServiceType ,inputType , outputType >>

Answer 1

您的表达式缺少类型转换。要使其编译，您需要明确地将 object 转换为 inputServiceType 等等。试试这个代码：

var objType = typeof(object);
var instance = Expression.Parameter(objType, "serviceInstance");
var input = Expression.Parameter(objType, "inputData");
var call = Expression.Call(
    Expression.Convert(instance, inputServiceType), // convert first arg 
    methodInfo,
    Expression.Convert(input, inputType)); // and second
var body = Expression.Convert(call, objType); // and even return type

var lambdaFunc = Expression.Lambda<Func<object, object, object>>(body, instance, input).Compile();
return lambdaFunc;

试试看here

编辑你可以让它更安全：

public static Func<TService, TInput, TReturn>
    CreateTypedLambdaExpression<TService, TInput, TReturn>(
        string methodName)
{
    var inputServiceType = typeof(TService);
    var methodInfo = inputServiceType.GetMethod(methodName);
    var inputType = typeof(TInput);

    // now you need to check if TInput is equal to methodInfo.GetParameters().First().ParameterType
    // same check for return type

    var instance = Expression.Parameter(inputServiceType, "serviceInstance");
    var input = Expression.Parameter(inputType, "inputData");
    var call = Expression.Call(instance, methodInfo, input);

    var lambdaFunc = Expression.Lambda<Func<TService, TInput, TReturn>>(call, instance, input);
    return lambdaFunc.Compile();
}

用法：

var func = CreateTypedLambdaExpression<Program, bool, int>("TestMethod");
var result = func(service, false);

如何创建具有动态类型的 lambda 表达式

How to create a lambda expression with dynamic types

c#

generics

reflection

lambda

dynamic-typing