设置任何 Func<> 的参数

Question

我有一个 func<>、Func<,>、func<,> 中任何一种类型的对象，我想用常量值替换其中一个输入参数。

例如：

object SetParameter<T>(object function, int index, T value){
    //I don't know how to code this.
}

Func<int, String, String> function = (a, b) => a.ToString() + b;
object objectFunction = function;
object newFunction = SetParameter<int>(objectFunction, 0, 5);
// Here the new function should be a Func<String, String> which value "(b) => function(5, b)"

我现在已经知道如何获取结果函数的类型，但这并不能真正帮助我实现所需的行为：

private Type GetNewFunctionType<T>(object originalFunction, int index, T value)
{
    Type genericType = originalFunction.GetType();

    if (genericType.IsGenericType)
    {
        var types = genericType.GetGenericArguments().ToList();
        types.RemoveAt(index);
        Type genericTypeDefinition = genericType.GetGenericTypeDefinition();
        return genericTypeDefinition.MakeGenericType(types.ToArray());
    }

    throw new InvalidOperationException($"{nameof(originalFunction)} must be a generic type");
}

Answer 1

不太清楚你转换的目的是什么，但是避免所有反射不是更容易吗？例如：

Func<int, string, string> func3 = (a, b) => a.ToString() + b;

Func<string, string> func3withConst = (b) => func3(10, b);

由于您谈论的范围非常有限（仅支持 Func<TReturn>、Func<T1, TReturn> 和 Func<T1, T2, TReturn>），因此通过反射执行此操作更容易出错且更难阅读。

Answer 2

以防万一你需要 use expression tree to build the function:

object SetParameter<T>(object function, int index, T value)
{
    var parameterTypes = function.GetType().GetGenericArguments();

    // Skip where i == index
    var newFuncParameterTypes = parameterTypes.SkipWhile((_, i) => i == index).ToArray();

    // Let's assume function is Fun<,,> to make this example simple :)
    var newFuncType = typeof(Func<,>).MakeGenericType(newFuncParameterTypes);

    // Now build a new function using expression tree.
    var methodCallParameterTypes = parameterTypes.Reverse().Skip(1).Reverse().ToArray();
    var methodCallParameters = methodCallParameterTypes.Select(
        (t, i) => i == index
            ? (Expression)Expression.Constant(value, typeof(T))
            : Expression.Parameter(t, "b")
        ).ToArray();

    // func.Invoke(5, b)
    var callFunction = Expression.Invoke(
        Expression.Constant(function),
        methodCallParameters);

    // b => func.Invoke(5, b)
    var newFunc = Expression.Lambda(
        newFuncType,
        callFunction,
        methodCallParameters.OfType<ParameterExpression>()
    ).Compile();

    return newFunc;
}

要使用这个：

Func<int, string, string> func = (a, b) => a.ToString() + b;

var newFunc = (Func<string, string>)SetParameter<int>(func, 0, 5);

// Output: 5b
Console.WriteLine(newFunc("b"));

设置任何 Func<> 的参数

Set parameter of any Func<>

c#

reflection

system.reflection