c# - 如何使用编译后的 lambda 函数对运行时生成的类型进行分组

Question

我有一个针对接口(interface)运行的 compiledLambda 函数。不幸的是，该接口(interface)只是一个标记接口(interface)，实际类型是在运行时动态生成的，并且具有我想对其进行分组的属性。

下面是一些示例代码:

class Program
{
    static void Main(string[] args)
    {
        // Just an example assignment: In the real life scenario the dynamic generated class is created during runtime. 
        IEnumerable<IDynamicGeneratedModelClass> list = GetDataFromService();

        // get the 'real' type from the list
        LambdaExpression lambdaExpression = DynamicExpression.ParseLambda(list.First().GetType(), typeof(object), "SomeProperty");
        Func<IDynamicGeneratedModelClass, object> compiledLambdaFunction = (Func<IDynamicGeneratedModelClass, object>)lambdaExpression.Compile();

        // Expected result: Group list on "SomeProp"
        var result = list.GroupBy(compiledLambdaFunction);
    }

    private static IList<IDynamicGeneratedModelClass> GetDataFromService()
    {
        return new List<IDynamicGeneratedModelClass> {  
            new DynamicGeneratedModelClass("Class1"),
            new DynamicGeneratedModelClass("Class2")
        };
    }
}

public interface IDynamicGeneratedModelClass
{}

public class DynamicGeneratedModelClass : IDynamicGeneratedModelClass
{
    public DynamicGeneratedModelClass(string someProperty)
    {
        SomeProperty = someProperty;
    }

    public string SomeProperty { get; }
}

当 lambda 表达式被编译时它抛出以下异常:

System.InvalidCastException: 'Unable to cast object of type 'System.Func`2[ConsoleApp12.DynamicGeneratedModelClass,System.Object]' to type 'System.Func`2[ConsoleApp12.IDynamicGeneratedModelClass,System.Object]'.'

你能告诉我我做错了什么以及如何解决吗？

Answer 1

Func<T, TResult> 的第一个通用参数委托(delegate)声明为 contravariant ( in )，这意味着您可以将派生参数较少的委托(delegate)分配给派生参数较多的委托(delegate)，但反之则不行(换句话说，您可以将 Func<IDynamicGeneratedModelClass,Object> 转换为 Func<DynamicGeneratedModelClass,Object>，但不能将 Func<DynamicGeneratedModelClass,Object> 转换为到 Func<IDynamicGeneratedModelClass,Object> )。

为避免此问题，您现在生成的不是 lambda 表达式:

// lambda has "wrong" type Func<DynamicGeneratedModelClass, object>
(DynamicGeneratedModelClass item) => item.SomeProperty

生成与此等效的 lambda:

// lambda now has "correct" type Func<IDynamicGeneratedModelClass, object>
(IDynamicGeneratedModelClass item) => ((DynamicGeneratedModelClass)item).SomeProperty

我不熟悉 DynamicExpression你用来生成 lambda 的库，但这可以很容易地使用 System.Linq.Expression 完成类:

var itemType = list.First().GetType();
var propertyName = "SomeProperty";
var parameterExpr = Expression.Parameter(typeof(IDynamicGeneratedModelClass));
var castExpr = Expression.Convert(parameterExpr, itemType);
var propExpr = Expression.Property(castExpr, propertyName);
var lambdaExpr = Expression.Lambda(propExpr, parameterExpr);

// Compiled lambda is now of type Func<IDynamicGeneratedModelClass, object>
Func<IDynamicGeneratedModelClass, object> compiledLambdaFunction = (Func<IDynamicGeneratedModelClass, object>)lambdaExpr.Compile();

var result = list.GroupBy(compiledLambdaFunction);