我有一个反射代码,用于创建 List<>
的实例(类型参数在运行时已知),并调用Add
方法向其添加一些值。我的代码片段是这样的:
// here is my type parameter
var genericType = typeof(MyRunTimeType);
// here is a list of my values
MyRunTimeType[] values = MyRunTimeValuesOfTypeMyRunTimeType();
// creating instance of List<>
var listType = typeof(List<>);
var listGenericType = listType.MakeGenericType(genericType);
var listInstance = Activator.CreateInstance(listGenericType);
// getting Add method and call it
var addMethod = listGenericType.GetMethod("Add", genericType);
foreach (var value invalues)
addMethod.Invoke(listInstance, new[] { value });
那么,您建议如何将此反射片段转换为表达式树?
更新:
嗯,我写了这个片段,它似乎无法工作:
public static Func<IEnumerable<object>, object> GetAndFillListMethod(Type genericType) {
var listType = typeof(List<>);
var listGenericType = listType.MakeGenericType(genericType);
var values = Expression.Parameter(typeof(IEnumerable<>).MakeGenericType(genericType), "values");
var ctor = listGenericType.GetConstructor(BindingFlags.Instance | BindingFlags.Public, null, new Type[0], null);
var instance = Expression.Parameter(listGenericType, "list");
var assign = Expression.Assign(instance, Expression.New(ctor));
var addMethod = listGenericType.GetMethod("AddRange", new[] { typeof(IEnumerable<>).MakeGenericType(genericType) });
var addCall = Expression.Call(instance, addMethod, new Expression[] { values });
var block = Expression.Block(
new[] { instance },
assign,
addCall,
Expression.Convert(instance, typeof(object))
);
return (Func<IEnumerable<object>, object>)Expression.Lambda(block, values).Compile();
}
但是,我收到此错误:
Unable to cast object of type
'System.Func`2[System.Collections.Generic.IEnumerable`1[System.String],System.Object]'
to type
'System.Func`2[System.Collections.Generic.IEnumerable`1[System.Object],System.Object]'.
请问有什么建议吗?
最佳答案
工作:
public static Func<IEnumerable<object>, object> GetAndFillListMethod(Type genericType)
{
var listType = typeof(List<>);
var listGenericType = listType.MakeGenericType(genericType);
var values = Expression.Parameter(typeof(IEnumerable<object>), "values");
var ctor = listGenericType.GetConstructor(BindingFlags.Instance | BindingFlags.Public, null, new Type[0], null);
// I prefer using Expression.Variable to Expression.Parameter
// for internal variables
var instance = Expression.Variable(listGenericType, "list");
var assign = Expression.Assign(instance, Expression.New(ctor));
var addMethod = listGenericType.GetMethod("AddRange", new[] { typeof(IEnumerable<>).MakeGenericType(genericType) });
// Enumerable.Cast<T>
var castMethod = typeof(Enumerable).GetMethod("Cast", new[] { typeof(IEnumerable) }).MakeGenericMethod(genericType);
// For the parameters there is a params Expression[], so no explicit array necessary
var castCall = Expression.Call(castMethod, values);
var addCall = Expression.Call(instance, addMethod, castCall);
var block = Expression.Block(
new[] { instance },
assign,
addCall,
Expression.Convert(instance, typeof(object))
);
return (Func<IEnumerable<object>, object>)Expression.Lambda(block, values).Compile();
}
您的问题在于您试图返回 Func<IEnumerable<object>, object>
但你的功能实际上是 Func<IEnumerable<T>, object>
。解决办法就是将参数设为 IEnumerable<object>
然后使用 Enumerable.Cast<T>
在传递到 AddRange
之前
我已经更改了Expression.Parameter
用于instance
到 Expression.Variable
...但这只是为了更清楚地表明它是一个变量,而不是一个参数。 Expression.Variable
生成的表达式树和 Expression.Parameter
是相同的(因为两个函数具有相同的代码)。使用它的上下文定义了它是参数还是变量。我又做了一个小改动:Expression.Call
不需要参数的显式数组初始化。
啊...请注意 Block
的最后一行可能是:
addCall,
instance
而不是
addCall,
Expression.Convert(instance, typeof(object))
因为任何引用类型都可以隐式转换为 object
.
关于c# - 从反射转向表达式树,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/35913495/