C# method parameter syntax - what does this mean? [duplicate]

Question

Once I know what this means, there's probably a better way of expressing this question, but I don't know where to start.

Here's the Read method for a StreamReader:

public override int Read([In, Out] char[] buffer, int index, int count) { ... }

That "In, Out" bit - what is it for, what does it do, what's it called?

Answer 1

That "In, Out" bit - what is it for, what does it do, what's it called?

They are parameter attributes.

In this case System.Runtime.InteropServices.InAttribute and System.Runtime.InteropServices.OutAttribute which are used for interop with code outside the .NET runtime.