c# - 使用表达式将Func <对象，对象>“转换”为Func <T，TEst>

我编写了一个小函数，尝试动态执行以下操作：

        Func<object, object> fa = i => Convert.ChangeType(i, typeof (string));
        Func<int, string> fb = o => (string) fa((int)o);

该功能如下：

    /// <summary>
    ///     Converts <see cref="Func{object, object}" /> to <see cref="Func{T, TResult}" />.
    /// </summary>
    public static Delegate Convert(Func<object, object> func, Type argType, Type resultType)
    {
        Contract.Requires(func != null);
        Contract.Requires(resultType != null);

        var param = Expression.Parameter(argType);

        var converted = Expression.Convert(
            Expression.Call(func.Method, Expression.Convert(param, typeof (object))),
            resultType);

        var delegateType = typeof (Func<,>).MakeGenericType(argType, resultType);
        return Expression.Lambda(delegateType, converted, param).Compile();
    }

现在，在不涉及任何关闭的情况下，此方法可以正常工作-此测试通过：

    [Test]
    public void When_Converting_Without_Closure_Then_Suceeds()
    {
        // Arrange
        Func<object, object> f = i => Convert.ChangeType(i, typeof(string));
        var sut = FuncConversion.Convert(f, typeof(int), typeof(string));

        // Act
        var res = (string) sut.DynamicInvoke(10);

        // Assert
        Assert.AreEqual(typeof(Func<int, string>), sut.GetType());
        Assert.AreEqual("10", res);
    }

但是当涉及到关闭时，此测试将失败：

    [Test]
    public void When_Converting_With_Closure_Then_Succeeds()
    {
        // Arrange
        var typeTo = typeof (string);
        Func<object, object> f = i => Convert.ChangeType(i, typeTo);
        var sut = FuncConversion.Convert(f, typeof(int), typeof(string));

        // Act
        var res = (string)sut.DynamicInvoke(10);

        // Assert
        Assert.AreEqual(typeof(Func<int, string>), sut.GetType());
        Assert.AreEqual("10", res);
    }

System.ArgumentException：静态方法需要空实例，非静态方法需要非空实例。
参数名称：方​​法
   在System.Linq.Expressions.Expression.ValidateStaticOrInstanceMethod（Expression实例，MethodInfo方法）
   在System.Linq.Expressions.Expression.Call（MethodInfo方法，Expressionarg0）

知道有什么问题吗？

最佳答案

确定修复它。问题在于，对于闭包，通常是静态方法的func具有其第一个参数，该参数将是实例方法上的目标实例，用于保持闭包状态。因此，我需要检查该状态是否存在，并使用它进行调用。

瞧！

    /// <summary>
    ///     Converts <see cref="Func{object, object}" /> to <see cref="Func{T, TResult}" />.
    /// </summary>
    public static Delegate Convert(Func<object, object> func, Type argType, Type resultType)
    {
        // If we need more versions of func then consider using params Type as we can abstract some of the
        // conversion then.

        Contract.Requires(func != null);
        Contract.Requires(resultType != null);

        var param = Expression.Parameter(argType);
        var convertedParam = new Expression[] {Expression.Convert(param, typeof (object))};

        // This is gnarly... If a func contains a closure, then even though its static, its first
        // param is used to carry the closure, so its as if it is not a static method, so we need
        // to check for that param and call the func with it if it has one...
        Expression call;
        call = Expression.Convert(
            func.Target == null
            ? Expression.Call(func.Method, convertedParam)
            : Expression.Call(Expression.Constant(func.Target), func.Method, convertedParam), resultType);

        var delegateType = typeof (Func<,>).MakeGenericType(argType, resultType);
        return Expression.Lambda(delegateType, call, param).Compile();
    }