如果您编写Expression< Func< dynamic,dynamic>> expr1 = x => 2 * x;编译器会抱怨,你不会得到任何地方.然而,似乎如果您通过一个方法创建一个这样的表达式,那么编译器似乎对它很满意,并且生成的应用程序的工作原理.这没有意义,所以我想知道窗帘后面会发生什么.

我想,在引擎盖下,ConvertExpression返回的表达式可能是Expression< Func< object,object>类型,它是一个有效的类型,但是它使我无法使用Expression&Func< dynamic,动态>>键入一个声明,但我可以使用它作为方法的返回类型.见下面的例子. 非常感谢！

public class ExpressionExample
{
    public void Main()
    {
        // Doesn't compile:
        //Expression<Func<dynamic,dynamic>> expr1 = x => 2 * x;

        // Compiles and works - OK
        Expression<Func<double,double>> expr2 = x => 2 * x;
        Func<double,double> func2 = (Func<double,double>)expr2.Compile();
        Console.WriteLine(func2(5.0).ToString()); // Outputs 10

        // Compiles and works - ??? This is the confusing block...
        Expression<Func<dynamic,dynamic>> expr3 = ConvertExpression(expr2);
        Func<dynamic,dynamic> func3 = (Func<dynamic,dynamic>)expr3.Compile();
        Console.WriteLine(func3(5.0).ToString()); // Outputs 10

        // Side note: compiles and works:
        Expression<Func<object,object>> expr4 = x => double.Parse(2.ToString()) * double.Parse(x.ToString());
        Func<object,object> func4 = (Func<object,object>)expr4.Compile();
        Console.WriteLine(func4(5.0).ToString()); // Outputs 10
    }

    private Expression<Func<dynamic,dynamic>> ConvertExpression<TInput,TOutput>(Expression<Func<TInput,TOutput>> expression)
    {
        Expression<Func<object,TInput>> convertToInput = value => (TInput)value;
        // The following doesn't compile: var input = Expression.Parameter(typeof(dynamic),"input");

        var input = Expression.Parameter(typeof(object),"input");        

        Expression<Func<TOutput,dynamic>> convertToOutput = value => (dynamic)value;

        var body = Expression.Invoke(convertToOutput,Expression.Invoke(expression,Expression.Invoke(convertToInput,input)));
        var lambda = Expression.Lambda<Func<dynamic,dynamic>>(body,input);

        return lambda;
    }
}