使用表达式'Cast'object> Func<对象;到Func

本文关键字：Func 对象 TRet gt 表达式 Cast object | 更新日期: 2023-09-27 18:13:50

我写了一个小函数，试图动态地做以下事情:

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

函数如下:

    /// <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，当没有闭包时，这个工作正常-这个测试通过:

    [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);
    }

但是当涉及到闭包时，这个测试失败:

    [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);
    }

系统。静态方法需要空实例，非静态方法需要非空实例。参数名称:method在System.Linq.Expressions.Expression。ValidateStaticOrInstanceMethod(表达式实例，方法)在System.Linq.Expressions.Expression。调用(MethodInfo方法，表达式arg0)

你知道怎么回事吗?

使用表达式'Cast'object> Func<对象;到Func<T, TRet>

修复了。问题是，对于闭包，通常是静态方法的函数有其第一个参数，该参数将是实例方法上的目标实例，用于保存闭包状态。所以我需要检查那个状态是否存在如果存在就调用它

等瞧:

    /// <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();
    }
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