Why can't I use the as keyword for a struct?

问题:

I defined the following struct:

public struct Call
{
    public SourceFile caller;
    public SourceFile callee;

    public Call(SourceFile caller, SourceFile callee)
    {
        this.caller = caller;
        this.callee = callee;
    }
}

Later, I assign it to the Tag property of another object:

line.Tag = new Call(sf1, sf2);

But when I try to retrieve the Tag property like so,

Call call = line.Tag as Call;

Visual Studio gives the following compile-time error:

The operator as must be used within a reference type or nullable type

What is the meaning of that? And how can I solve it?

回答1:

A struct is a value type, so it cannot be used with the as operator. The as operator must be able to assign a value of null if the cast fails. This is only possible with a reference type or a nullable value type.

There are a couple ways to solve this, but your best bet is to change your Call type from a struct to a class. This will essentially change your type from a value type to a reference type, which allows the as operator to assign a value of null if the cast fails.

For more information on value types vs. reference types, this is a decent article. Also, have a look on MSDN:

• value types
• reference types
• as-operator
• nullable types.

回答2:

Some of the existing answers aren't quite right. You can't use non-nullable types with as, because the result of as is the null value of the type if the first operand isn't actually of an appropriate type.

However, you can use as with value types... if they're nullable:

int a = 10;
object o = a;

int? x = o as int?; // x is a Nullable<int> with value 10
long? y = o as long?; // y is a Nullable<long> with the null value

So you could use:

Call? call = line.Tag as Call?;

Then you can use it as:

if (call != null)
{
    // Do stuff with call.Value
}

Two caveats though:

• In my experience this is slower than just using is followed by a cast
• You should seriously reconsider your current Call type:
  • It's exposing public fields, which is generally poor encapsulation
  • It's a mutable value type, which is almost certainly a mistake

I would strongly suggest you make it a class instead - at which point this problem goes away anyway.

Another thought: if the tag should always be a Call, then it's better to cast it:

Call call = (Call) line.Tag;

That way, if the data doesn't match your expectation (i.e. there's some bug such that the Tag isn't a Call) then you get to find out about it early, rather than after you've potentially done some other work. Note that this cast will behave differently depending on whether Call is a struct or a class, if Tag is null - you can cast a null value to a variable of a reference type (or a nullable value type), but not to a non-nullable value type.

回答3:

From the C# Spec

§7.10.11 The as operator is used to explicitly convert a value to a given reference type or nullable type. Unlike a cast expression (§7.7.6), the as operator never throws an exception. Instead, if the indicated conversion is not possible, the resulting value is null.

References and nullable types can be null. Stucts are value types so they can't be null.

回答4:

Call? call = line.Tag as Call?;

回答5:

It's a limitation of C#. If the type were a reference type, then if the cast failed it would simply return 'null', but since it's a value type, it doesn't know what to return when the cast fails.

You must replace your use of as with two: 'is' and 'as'

if (line.Tag is Call) {
  call = (Call)line.Tag;
} else {
  // Do whatever you would do if as returned null.
}

回答6:

What is the meaning - As stated, structures are value types.

How can I solve it - Change it to

Call call = line.Tag;