[英]Cannot convert from Func<T,T,T> to Func<T,T,T>
I'm quite confused by this error: 我对这个错误很困惑:
Cannot implicitly convert type 'System.Func<T,T,T> [c:\\Program Files (x86)\\Reference Assemblies\\Microsoft\\Framework\\.NETFramework\\v4.0\\mscorlib.dll]' to 'System.Func<T,T,T> [c:\\Program Files (x86)\\Reference Assemblies\\Microsoft\\Framework\\.NETFramework\\v4.0\\mscorlib.dll]' path\\to\\my\\project\\Operators.cs
The types are identical, why is it even trying to do a cast? 类型是相同的,为什么它甚至试图进行演员表演? Here's the code: 这是代码:
public static class Operators<T>
{
private static Func<T,T,T> _add = null;
public static T Add<T>(T a, T b)
{
if (_add == null) {
var param1Expr = Expression.Parameter(typeof (T));
var param2Expr = Expression.Parameter(typeof (T));
var addExpr = Expression.Add(param1Expr, param2Expr);
var expr = Expression.Lambda<Func<T, T, T>>(addExpr, param1Expr, param2Expr);
_add = expr.Compile(); // <--- error occurs here
}
return _add.Invoke(a, b);
}
}
The problem is that your method is generic, introducing a new type parameter T
. 问题是你的方法是通用的,引入了一个新的类型参数T
So the T
outside the method isn't the same as the T
inside the method. 所以T
方法以外是不一样的T
的方法内。
Just change your method to not be generic: 只需将您的方法更改为不通用:
public static T Add(T a, T b)
... and it should be fine. ......它应该没问题。
To be clearer, your code is currently equivalent to this: 为了更清楚,您的代码目前等效于此:
public static class Operators<TC>
{
private static Func<TC, TC, TC> _add = null;
public static TM Add<TM>(TM a, TM b)
{
if (_add == null) {
var param1Expr = Expression.Parameter(typeof(TM));
var param2Expr = Expression.Parameter(typeof(TM));
var addExpr = Expression.Add(param1Expr, param2Expr);
var expr = Expression.Lambda<Func<TM, TM, TM>>
(addExpr, param1Expr, param2Expr);
_add = expr.Compile();
}
return _add.Invoke(a, b);
}
}
Note how I've renamed the T
introduced by the class to TC
, and the T
introduced by the method to TM
. 请注意我是如何改名T
由全班同学介绍了TC
,而T
的方法来介绍TM
。 The error message now looks more reasonable: 现在,错误消息看起来更合理:
Test.cs(19,20): error CS0029: Cannot implicitly convert type
'System.Func<TM,TM,TM>' to 'System.Func<TC,TC,TC>'
The T
for your Operators<T>
class and the T
type parameter for Add
are different types, so there's no guarantee that the types are compatible. 该T
你的Operators<T>
类和T
的类型参数Add
有不同的类型,所以没有保证的类型兼容。
For example you could do: 例如,您可以这样做:
Operators<string>.Add<int>(1, 2);
The compiler emits a warning to this effect: 编译器会发出此效果的警告:
Type parameter 'T' has the same name as the type parameter from outer type 'Operators' 类型参数'T'与外部类型'Operators'中的type参数同名
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