c# - C# using 语句需要赋值吗？

Question

起初我有一些这样的代码:

WindowsIdentity otherIdentity = // got that from somewhere else
WindowsImpersonationContext context = otherIdentity.Impersonate();
Ldap.DoStuff();
context.Undo();
context.Dispose();

知道WindowsImpersonationContext实现IDisposable和Dispose()还可以调用Undo() ，我想我应该使用 using相反:

using (var context = otherIdentity.Impersonate())
{
    // Run as other user
    Ldap.DoStuff();
}

现在，ReSharper 正确地注意到我没有使用 context并建议删除该分配。这也行得通吗？它在编译时扩展为什么代码？

using (otherIdentity.Impersonate())
{
    Ldap.DoStuff();
}

Answer 1

是的，您可以在using语句中省略该变量，编译器会自动引入一个隐藏变量，就像您编写的一样

using (var temp = otherIdentity.Impersonate())
{
    Ldap.DoStuff();
}

除了您无法在 using 语句主体中访问 temp 之外。

奇怪的是，此语法似乎没有记录在 MSDN Library 中。相反，请参阅 C# 规范:

A using statement of the form

using (ResourceType resource = expression) statement

corresponds to one of three possible expansions.

[...]

A using statement of the form

using (expression) statement

has the same three possible expansions. In this case ResourceType is implicitly the compile-time type of the expression, if it has one. Otherwise the interface IDisposable itself is used as the ResourceType. The resource variable is inaccessible in, and invisible to, the embedded statement.