铸造字符串泛型类型为字符串 [英] Casting string to generic type that is a string
问题描述
我在写做一个聪明的类型转换的方法 - 使用的ToString()如果类型参数恰好是一个字符串,否则铸造但是返回null如果投不工作。基本上得到了的尽可能多的信息v
它可以在不抛出异常。
我检查 T
的确是一个字符串
之前,我尝试中投,但编译器仍然不是一个球迷:
无法转换类型'字符串'到'T'
这是我的方法:
公共T'转换< T>(宾语)
{
如果(typeof运算(T)== typeof运算(字符串)){
回报率(T)v.ToString(); //不能键入'串'转换为'T'
}其他{试
返回(T)V;
}赶上(InvalidCastException的){
返回NULL;
}
}
也让我知道这是某种不可原谅罪。我用它来处理,可以有混合型的一些数据结构。
您基本上需要通过<去code>对象强制转换为泛型类型时:
收益率(T)(对象) v.ToString()
和
收益率(T)(对象)诉;
我会使用是
,而不是抓一个 InvalidCastException的
虽然。
请参阅的Eric利珀特最近的博客文章对于为什么这是必要的更多细节。
$ b
由于编译器知道这种转换可能可能成功的唯一途径是,如果<:
$ b在特定的code> U 是布尔,但
U
可以是任何东西!编译器假定大部分时间U
不会与布尔
构造,因此,这种代码几乎是肯定是一个错误,编译器把这个事实你注意。
块引用>
(替换
T
为U
和字符串
为布尔
。 ..)I'm writing a method to do a intelligent type conversion - using ToString() if the type parameter happens to be a string, otherwise casting but returning null if the cast doesn't work. Basically gets as much information out of
v
it can without throwing an exception.I check that
T
is indeed astring
before I attempt the cast, but the compiler is still not a fan:Cannot convert type 'string' to 'T'
And here's my method:
public T? Convert<T>(object v) { if (typeof(T) == typeof(string)) { return (T)v.ToString(); // Cannot convert type 'string' to 'T' } else try { return (T)v; } catch (InvalidCastException) { return null; } }
Also let me know if this is some sort of unforgivable sin. I'm using it to deal with some data structures that could have mixed types.
解决方案You basically need to go via
object
when casting to a generic type:return (T)(object) v.ToString()
and
return (T)(object) v;
I would use
is
rather than catching anInvalidCastException
though.See Eric Lippert's recent blog post for more details of why this is necessary.
In particular:
Because the compiler knows that the only way this conversion could possibly succeed is if
U
is bool, butU
can be anything! The compiler assumes that most of the timeU
is not going to be constructed withbool
, and therefore this code is almost certainly an error, and the compiler is bringing that fact to your attention.(Substitute
T
forU
andstring
forbool
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