Don't consider it an error if there is nothing to export (#1349)

DiscordChatExporter.Cli.Tests/Specs/DateRangeSpecs.cs

@@ -146,4 +146,33 @@ public class DateRangeSpecs
                         .WhenTypeIs<DateTimeOffset>()
             );
     }
+
+    [Fact]
+    public async Task Export_file_is_created_even_when_nothing_to_export()
+    {
+        var long_in_the_past = new DateTimeOffset(1921, 08, 01, 0, 0, 0, TimeSpan.Zero);
+
+        // Arrange
+        var before = long_in_the_past;
+        using var file = TempFile.Create();
+
+        // Act
+        await new ExportChannelsCommand
+        {
+            Token = Secrets.DiscordToken,
+            ChannelIds = [ChannelIds.DateRangeTestCases],
+            ExportFormat = ExportFormat.Json,
+            OutputPath = file.Path,
+            Before = Snowflake.FromDate(before),
+        }.ExecuteAsync(new FakeConsole());
+
+        // Assert
+        var timestamps = Json.Parse(await File.ReadAllTextAsync(file.Path))
+            .GetProperty("messages")
+            .EnumerateArray()
+            .Select(j => j.GetProperty("timestamp").GetDateTimeOffset())
+            .ToArray();
+
+        timestamps.Should().BeEmpty();
+    }
 }

DiscordChatExporter.Cli/Commands/Base/ExportCommandBase.cs

@@ -179,6 +179,7 @@ public abstract class ExportCommandBase : DiscordCommandBase
         // Export
         var cancellationToken = console.RegisterCancellationHandler();
         var errorsByChannel = new ConcurrentDictionary<Channel, string>();
+        var warningsByChannel = new ConcurrentDictionary<Channel, string>();
 
         await console.Output.WriteLineAsync($"Exporting {channels.Count} channel(s)...");
         await console
@@ -236,6 +237,10 @@ public abstract class ExportCommandBase : DiscordCommandBase
                                 }
                             );
                         }
+                        catch (ChannelEmptyException ex)
+                        {
+                            warningsByChannel[channel] = ex.Message;
+                        }
                         catch (DiscordChatExporterException ex) when (!ex.IsFatal)
                         {
                             errorsByChannel[channel] = ex.Message;
@@ -252,6 +257,28 @@ public abstract class ExportCommandBase : DiscordCommandBase
             );
         }
 
+        // Print warnings
+        if (warningsByChannel.Any())
+        {
+            await console.Output.WriteLineAsync();
+
+            using (console.WithForegroundColor(ConsoleColor.Yellow))
+            {
+                await console.Error.WriteLineAsync(
+                    $"Warnings reported for the following channel(s):"
+                );
+            }
+
+            foreach (var (channel, message) in warningsByChannel)
+            {
+                await console.Error.WriteAsync($"{channel.GetHierarchicalName()}: ");
+                using (console.WithForegroundColor(ConsoleColor.Yellow))
+                    await console.Error.WriteLineAsync(message);
+            }
+
+            await console.Error.WriteLineAsync();
+        }
+
         // Print errors
         if (errorsByChannel.Any())
         {
@@ -259,16 +286,14 @@ public abstract class ExportCommandBase : DiscordCommandBase
 
             using (console.WithForegroundColor(ConsoleColor.Red))
             {
-                await console.Error.WriteLineAsync(
+                await console.Error.WriteLineAsync($"Failed to export the following channel(s):");
-                    $"Failed to export {errorsByChannel.Count} the following channel(s):"
-                );
             }
 
-            foreach (var (channel, error) in errorsByChannel)
+            foreach (var (channel, message) in errorsByChannel)
             {
                 await console.Error.WriteAsync($"{channel.GetHierarchicalName()}: ");
                 using (console.WithForegroundColor(ConsoleColor.Red))
-                    await console.Error.WriteLineAsync(error);
+                    await console.Error.WriteLineAsync(message);
             }
 
             await console.Error.WriteLineAsync();

DiscordChatExporter.Core/Exceptions/ChannelEmptyException.cs

@@ -0,0 +1,8 @@
+using System;
+
+namespace DiscordChatExporter.Core.Exceptions;
+
+// Thrown when there is circumstancially no message to export with given parameters,
+// though it should not be treated as a runtime error; simply warn instead
+public class ChannelEmptyException(string message)
+    : DiscordChatExporterException(message, false, null) { }

DiscordChatExporter.Core/Exporting/ChannelExporter.cs

@@ -27,45 +27,42 @@ public class ChannelExporter(DiscordClient discord)
             );
         }
 
-        // Check if the channel is empty
-        if (request.Channel.IsEmpty)
-        {
-            throw new DiscordChatExporterException(
-                $"Channel '{request.Channel.Name}' "
-                    + $"of guild '{request.Guild.Name}' "
-                    + $"does not contain any messages."
-            );
-        }
-
-        // Check if the 'after' boundary is valid
-        if (request.After is not null && !request.Channel.MayHaveMessagesAfter(request.After.Value))
-        {
-            throw new DiscordChatExporterException(
-                $"Channel '{request.Channel.Name}' "
-                    + $"of guild '{request.Guild.Name}' "
-                    + $"does not contain any messages within the specified period."
-            );
-        }
-
-        // Check if the 'before' boundary is valid
-        if (
-            request.Before is not null
-            && !request.Channel.MayHaveMessagesBefore(request.Before.Value)
-        )
-        {
-            throw new DiscordChatExporterException(
-                $"Channel '{request.Channel.Name}' "
-                    + $"of guild '{request.Guild.Name}' "
-                    + $"does not contain any messages within the specified period."
-            );
-        }
-
         // Build context
         var context = new ExportContext(discord, request);
         await context.PopulateChannelsAndRolesAsync(cancellationToken);
 
         // Export messages
         await using var messageExporter = new MessageExporter(context);
+
+        // Check if the channel is empty
+        if (request.Channel.IsEmpty)
+        {
+            throw new ChannelEmptyException(
+                $"Channel '{request.Channel.Name}' "
+                    + $"of guild '{request.Guild.Name}' "
+                    + $"does not contain any messages; an empty file will be created."
+            );
+        }
+
+        // Check if the 'before' and 'after' boundaries are valid
+        if (
+            (
+                request.Before is not null
+                && !request.Channel.MayHaveMessagesBefore(request.Before.Value)
+            )
+            || (
+                request.After is not null
+                && !request.Channel.MayHaveMessagesAfter(request.After.Value)
+            )
+        )
+        {
+            throw new ChannelEmptyException(
+                $"Channel '{request.Channel.Name}' "
+                    + $"of guild '{request.Guild.Name}' "
+                    + $"does not contain any messages within the specified period; an empty file will be created."
+            );
+        }
+
         await foreach (
             var message in discord.GetMessagesAsync(
                 request.Channel.Id,
@@ -98,15 +95,5 @@ public class ChannelExporter(DiscordClient discord)
                 );
             }
         }
-
-        // Throw if no messages were exported
-        if (messageExporter.MessagesExported <= 0)
-        {
-            throw new DiscordChatExporterException(
-                $"Channel '{request.Channel.Name}' (#{request.Channel.Id}) "
-                    + $"of guild '{request.Guild.Name}' (#{request.Guild.Id}) "
-                    + $"does not contain any matching messages within the specified period."
-            );
-        }
     }
 }

DiscordChatExporter.Core/Exporting/MessageExporter.cs

@@ -70,7 +70,12 @@ internal partial class MessageExporter(ExportContext context) : IAsyncDisposable
         MessagesExported++;
     }
 
-    public async ValueTask DisposeAsync() => await ResetWriterAsync();
+    public async ValueTask DisposeAsync()
+    {
+        // causes the file to be created whether there were messages written or not
+        await GetWriterAsync();
+        await ResetWriterAsync();
+    }
 }
 
 internal partial class MessageExporter

DiscordChatExporter.Gui/ViewModels/Components/DashboardViewModel.cs

@@ -283,6 +283,13 @@ public partial class DashboardViewModel : ViewModelBase
 
                         Interlocked.Increment(ref successfulExportCount);
                     }
+                    catch (ChannelEmptyException ex)
+                    {
+                        _snackbarManager.Notify(ex.Message.TrimEnd('.'));
+
+                        // FIXME: not exactly successful, but not a failure either. Not ideal to duplicate the line
+                        Interlocked.Increment(ref successfulExportCount);
+                    }
                     catch (DiscordChatExporterException ex) when (!ex.IsFatal)
                     {
                         _snackbarManager.Notify(ex.Message.TrimEnd('.'));