[Enhancement]: Support inline comments in the `.testcontainers.properties` file

[HofmeisterAn]

Problem

.NET does not have built-in support for .properties files. Testcontainers for .NET implements very simple parsing capabilities that do not support or follow many (or all) specs. For example, this is the case for inline comments, such as:

docker.host=tcp://127.0.0.1:2375 # Sets the Docker host address

Testcontainers will treat everything after the = as the value and is not able to connect to the Docker host. This is an unusual behavior, and something users usually won't expect.

Solution

Instead of using everything after the = as the value, we can select everything until the comment character # or ! appears. We may need to be careful here. AFAIK, the .properties file spec supports escape sequences too, such as \# and \!.

Benefit

Supporting inline comments in the future will prevent other users from running into an unnecessary issue when configuring Testcontainers.

Alternatives

Instead of using inline comments, users can put comments on a separate line.

Would you like to help contributing this enhancement?

Yes

[HofmeisterAn]

I implemented this feature, but after looking closer at the Java Properties files specification, it appears that inline comments are not supported. Java Properties files can only have single-line # comments.

diff --git a/src/Testcontainers/Configurations/PropertiesFileConfiguration.cs b/src/Testcontainers/Configurations/PropertiesFileConfiguration.cs
index 6d0367c..3e9f716 100644
--- a/src/Testcontainers/Configurations/PropertiesFileConfiguration.cs
+++ b/src/Testcontainers/Configurations/PropertiesFileConfiguration.cs
@@ -4,6 +4,7 @@ namespace DotNet.Testcontainers.Configurations
   using System.IO;
   using System.Linq;
   using System.Text.Json;
+  using System.Text.RegularExpressions;
   using DotNet.Testcontainers.Images;

   /// <summary>
@@ -11,6 +12,8 @@ namespace DotNet.Testcontainers.Configurations
   /// </summary>
   internal class PropertiesFileConfiguration : CustomConfiguration, ICustomConfiguration
   {
+    private static readonly Regex CommentSeparator = new Regex("(?<!\\\\)[!#]", RegexOptions.None, TimeSpan.FromSeconds(1));
+
     static PropertiesFileConfiguration()
     {
     }
@@ -42,11 +45,14 @@ namespace DotNet.Testcontainers.Configurations
       : base(lines
         .Select(line => line.Trim())
         .Where(line => !string.IsNullOrEmpty(line))
-        .Where(line => !line.StartsWith("#", StringComparison.Ordinal))
         .Where(line => !line.StartsWith("!", StringComparison.Ordinal))
+        .Where(line => !line.StartsWith("#", StringComparison.Ordinal))
         .Select(line => line.Split(new[] { '=', ':', ' ' }, 2, StringSplitOptions.RemoveEmptyEntries))
         .Where(property => 2.Equals(property.Length))
-        .ToDictionary(property => property[0], property => property[1]))
+        .ToDictionary(property => property[0], property => CommentSeparator.Split(property[1])[0]
+          .Replace("\\!", "!")
+          .Replace("\\#", "#")
+          .Trim()))
     {
     }

diff --git a/tests/Testcontainers.Tests/Unit/Configurations/CustomConfigurationTest.cs b/tests/Testcontainers.Tests/Unit/Configurations/CustomConfigurationTest.cs
index c30508a..0be5161 100644
--- a/tests/Testcontainers.Tests/Unit/Configurations/CustomConfigurationTest.cs
+++ b/tests/Testcontainers.Tests/Unit/Configurations/CustomConfigurationTest.cs
@@ -331,6 +331,20 @@ namespace DotNet.Testcontainers.Tests.Unit
         ICustomConfiguration customConfiguration = new PropertiesFileConfiguration(new[] { configuration });
         Assert.Equal(expected, customConfiguration.GetHubImageNamePrefix());
       }
+
+      [Theory]
+      [InlineData("", null)]
+      [InlineData("!", null)]
+      [InlineData("#", null)]
+      [InlineData("docker.config=~/.docker/ ! The default configuration directory.", "~/.docker/")]
+      [InlineData("docker.config=~/.docker/ # The default configuration directory.", "~/.docker/")]
+      [InlineData("docker.config=~/.docker/\\!not_a_comment/", "~/.docker/!not_a_comment/")]
+      [InlineData("docker.config=~/.docker/\\#not_a_comment/", "~/.docker/#not_a_comment/")]
+      public void TrimCommentFromPropertiesFileProperty(string configuration, string expected)
+      {
+        ICustomConfiguration customConfiguration = new PropertiesFileConfiguration(new[] { configuration });
+        Assert.Equal(expected, customConfiguration.GetDockerConfig());
+      }
     }
   }
 }