Bounded channel
Learn backpressure and WaitToWriteAsync in bounded channels.
Bounded channel and backpressure
On a bounded channel, WriteAsync waits when full; this is backpressure. It slows the producer to the consumer's pace and prevents memory bloat.
WaitToWriteAsync() waits until writing is possible and returns a ValueTask<bool>. TryWrite returns false immediately without waiting; it is the synchronous way to detect backpressure.
BoundedChannelFullMode
BoundedChannelFullMode chooses the full behavior: Wait (default), DropOldest (drop the oldest), DropNewest (drop the newest), DropWrite (drop the write).
For example new BoundedChannelOptions(3) { FullMode = BoundedChannelFullMode.DropOldest } drops the oldest item when full. It fits log streams where losing old entries is tolerable.
Exercise Try TryWrite on a bounded channel
Create a bounded string channel with capacity 2. Write three items ("a", "b", "c") with channel.Writer.TryWrite and print the bool result of each call with Console.WriteLine. The third call returns False because the channel is full.
Show example solution
var channel = Channel.CreateBounded<string>(2); Then Console.WriteLine(channel.Writer.TryWrite("a")); Console.WriteLine(channel.Writer.TryWrite("b")); Console.WriteLine(channel.Writer.TryWrite("c"));
Example solution:
using System;
using System.Threading.Channels;
class Program
{
static void Main()
{
var channel = Channel.CreateBounded<string>(2);
Console.WriteLine(channel.Writer.TryWrite("a"));
Console.WriteLine(channel.Writer.TryWrite("b"));
Console.WriteLine(channel.Writer.TryWrite("c"));
}
}Test yourself
-
What does WaitToWriteAsync return?
- void
- string
- int
- ValueTask<bool>
Show answer
ValueTask<bool> — Correct. WaitToWriteAsync returns ValueTask<bool>.
-
Match each FullMode behavior
- Wait
- Waits when full
- DropOldest
- Drops the oldest
- DropWrite
- Silently drops the write
-
Complete the WaitToWriteAsync condition
if (await channel.Writer.____) { channel.Writer.TryWrite("x"); }Show answer
if (await channel.Writer.WaitToWriteAsync()) { channel.Writer.TryWrite("x"); }