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Reputation: 897

Why does Stream.Read not use an out parameter?

From my understanding it seems logical that the buffer parameter of Stream.Read(byte[] buffer, int offset, int count) should be an out parameter as the value is read into the buffer that passed in, but that is not the case.

What is the reason for this? Is there something I am overlooking?

Upvotes: 2

Views: 341

Answers (3)

Bradley Uffner
Bradley Uffner

Reputation: 16991

out is only used when the instance is assigned from within the method, and passed back out. Stream.Read expects the buffer to have already been created by the programmer before calling the method. The existing instance is just being passed in, where the data within the instance is being modified, not the reference its self.

While out allows you to pass in an existing instance, it guarantees that a different instance will be coming back out. In-fact, even though it allows you to pass in an existing instance, you can't access its value from within the method. It treats the parameter as if it were never assigned. "CS0269 Use of unassigned out parameter 'value'"

You will get a compiler error if you don't assign something to the parameter before the method returns. "CS0177 The out parameter 'value' must be assigned to before control leaves the current method"

Upvotes: 4

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726779

Making buffer an out parameter would make the API less usable, without offering any improvement in return.

Currently, a reference for the buffer is passed to the method (because arrays are reference types), and the output is written directly into the array.

Using out would prevent important use cases, when the buffer is external to the caller of Stream.Read - for example, a read-only property of some of the classes that it uses.

Moreover, there would not be much improvement in usability, because swapping the buffer from under the caller, presumably, to expand it, is not an appropriate course of action when offset and count are passed to you by the caller.

Upvotes: 1

Skintkingle
Skintkingle

Reputation: 1579

the data read is put into an already specified array of bytes. buffer in this situation is already instantiated by the calling code and may have non-zero bytes in the array already. if it was specified as out then the Read method would be creating a buffer, and returning it back. In that situation offset and count would be redundant, as it's made the byte array based on its own findings, it doesn't need to know where to "fit in" its data being read.

Upvotes: 1

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