c# - "where T : somevalue"是什么意思？

Question

where T : somevalue 是什么意思？我刚刚看到一些代码说 where T : Attribute。我认为这与泛型有关，但我不确定这意味着什么或它在做什么。

有人知道吗？

Answer 1

这是一个constraint on a type parameter ，表示类型 T赋予泛型类或方法的必须继承自类 Attribute

例如:

public class Foo<T> : 
   where T : Attribute
{
    public string GetTypeId(T attr) { return attr.TypeId.ToString(); }
 // ..
}

Foo<DescriptionAttribute> bar; // OK, DescriptionAttribute inherits Attribute
Foo<int> baz; // Compiler error, int does not inherit Attribute

这很有用，因为它允许泛型类对 T 类型的对象进行操作知道任何东西都是 T也必须是 Attribute .

在上面的例子中，GetTypeId 没问题查询 TypeId的 attr因为TypeId是 Attribute 的属性，因为 attr是 T它必须是继承自 Attribute 的类型.

约束也可以用在泛型方法上，效果一样:

public static void GetTypeId<T>(T attr) where T : Attribute
{
   return attr.TypeId.ToString();
}

您还可以对类型施加其他约束；来自 MSDN :

where T: struct

The type argument must be a value type. Any value type except Nullable can be specified.

where T : class

The type argument must be a reference type; this applies also to any class, interface, delegate, or array type.

where T : new()

The type argument must have a public parameterless constructor. When used together with other constraints, the new() constraint must be specified last.

where T : <base class name>

The type argument must be or derive from the specified base class.

where T : <interface name>

The type argument must be or implement the specified interface. Multiple interface constraints can be specified. The constraining interface can also be generic.

where T : U

The type argument supplied for T must be or derive from the argument supplied for U. This is called a naked type constraint.