[英]C# generic method with integer constraint refuses to cast from integer to the generic type
If I have a generic method that is constrained to be type 'int' then surely I should be able to cast an integer to the generic T type. 如果我有一个被限制为类型'int'的泛型方法,那么我当然应该能够将一个整数转换为泛型T类型。 For example...
例如...
public T ExampleMethod<T>(int unchanged) where T : int
{
return (T)unchanged;
}
...the compiler complains that Cannot convert type 'int' to 'T' but I have a constraint indicating that the target is as integer. ...编译器抱怨无法将类型'int'转换为'T',但我有一个约束,表明目标是整数。 So surely it should work?
那肯定应该有用吗?
Update: 更新:
The actual scenario is that I want to a helper method that returns an enum value. 实际情况是我想要一个返回枚举值的辅助方法。 So my ideal helper method would be more like this....
所以我理想的助手方法会更像这样....
public T GetAttributeAsEnum<T>(XmlReader reader, string name) where T : enum
{
string s = reader.GetAttribute(name);
int i = int.Parse(s);
return (T)i;
}
...and use it like this... ......并像这样使用它......
StateEnum x = GetAttributeAsEnum<StateEnum>(xmlReader, "State");
CategoryEnum y = GetAttributeAsEnum<CategoryEnum>(xmlReader, "Category");
OtherEnum z = GetAttributeAsEnum<OtherEnum>(xmlReader, "Other");
...but you cannot constrain by enum. ......但你无法通过枚举来约束。
" Only class or interface could be specified as constraint. " (c) ReSharper “ 只能将类或接口指定为约束。 ”(c)ReSharper
int (Int32) is just a struct. int(Int32)只是一个结构。 You can constrain that T is a struct .
您可以约束T是结构 。 but you can't use any struct as constraint.
但你不能使用任何结构作为约束。
the whole list of possible constraints you can find here - http://msdn.microsoft.com/en-us/library/d5x73970.aspx 你可以在这里找到可能的约束的完整列表 - http://msdn.microsoft.com/en-us/library/d5x73970.aspx
UPD UPD
and for Enum constraint see this question - Is there a workaround for generic type constraint of "special class" Enum in C# 3.0? 对于Enum约束,请参阅此问题 - C#3.0中是否存在“特殊类”Enum的泛型类型约束的解决方法?
int (and all other numeric types, and enums) cannot be used as a generic constraint. int (以及所有其他数字类型和枚举)不能用作通用约束。
See 看到
Generic C# Code and the Plus Operator 通用C#代码和Plus运算符
for further details and options. 了解更多详情和选项。
For a discussion with Anders Hejlsberg, the creator of C#, about generics and type constraints see 有关C#的创建者Anders Hejlsberg的讨论,请参阅泛型和类型约束
http://www.artima.com/intv/generics.html http://www.artima.com/intv/generics.html
One can place a type constraint of struct like this: 可以像这样放置struct的类型约束:
public class Generic<T> where T : struct { }
Generic<int> gen = new Generic<int>();
Are you sure its compiling? 你确定它的编译?
Here, it gives following error: 在这里,它给出以下错误:
error CS0701: 'int' is not a valid constraint.
错误CS0701:'int'不是有效约束。 A type used as a constraint must be an interface, a non-sealed class or a type parameter.
用作约束的类型必须是接口,非密封类或类型参数。
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