Because of the way async/await methods are rewritten by the compiler, any exceptions thrown during the parameters check will happen
only when the task is observed. That could happen far away from the source of the buggy code or never happen for fire-and-forget tasks.
Therefore it is recommended to split the method into two: an outer method handling the parameter checks (without being async/await)
and an inner method to handle the iterator block with the async/await pattern.
This rule raises an issue when an async method throws any exception derived from ArgumentException and contains
await keyword.
Noncompliant code example
public static async Task SkipLinesAsync(this TextReader reader, int linesToSkip) // Noncompliant
{
if (reader == null) { throw new ArgumentNullException(nameof(reader)); }
if (linesToSkip < 0) { throw new ArgumentOutOfRangeException(nameof(linesToSkip)); }
for (var i = 0; i < linesToSkip; ++i)
{
var line = await reader.ReadLineAsync().ConfigureAwait(false);
if (line == null) { break; }
}
}
Compliant solution
public static Task SkipLinesAsync(this TextReader reader, int linesToSkip)
{
if (reader == null) { throw new ArgumentNullException(nameof(reader)); }
if (linesToSkip < 0) { throw new ArgumentOutOfRangeException(nameof(linesToSkip)); }
return reader.SkipLinesInternalAsync(linesToSkip);
}
private static async Task SkipLinesInternalAsync(this TextReader reader, int linesToSkip)
{
for (var i = 0; i < linesToSkip; ++i)
{
var line = await reader.ReadLineAsync().ConfigureAwait(false);
if (line == null) { break; }
}
}
