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[英]Why can I not set a Func to return a parameter type to an integer value?
[英]Set parameter of any Func<>
我有一個 object 的任何 func 類型 func<>, Func<,>, func<,,>... 我想用一個常量值替換其中一個輸入參數。
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)"
我現在已經如何獲取生成的 function 的類型,但這並不能真正幫助我實現所需的行為:
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");
}
目前尚不清楚轉換的目的是什么,但避免所有反射不是更容易。 例如:
Func<int, string, string> func3 = (a, b) => a.ToString() + b;
Func<string, string> func3withConst = (b) => func3(10, b);
由於您談論的是非常有限的 scope (僅支持Func<TReturn>
, Func<T1, TReturn>
和Func<T1, T2, TReturn>
)通過反射執行此操作更容易出錯且更難閱讀。
以防萬一您需要使用表達式樹來構建 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;
}
要使用這個:
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"));
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