Why does IConfigurationBuilder.Build() fail for config files with names starting with a dot?

The problem is not with the file’s content or even its existence—it’s with how the INI file provider (and underlying file provider) interprets the file name. When you pass an absolute path like "C:\temp.ext", the provider splits the path into a directory, file name, and extension. Since there’s no "name" (only an extension), the provider doesn’t recognize it as a valid file name and reports that the file is missing.

There isn’t a built‐in way to “fix” this behavior from the configuration API side. The recommended workarounds are:

• Use a file name that includes a name portion.
  Rename the file (for example, to "config.ext" or "my.ext") so the provider can correctly locate the file.

• Use a custom file provider.
  You may write a custom implementation or wrap a PhysicalFileProvider that directly maps the full file path without trying to split out a name. Then pass that to the configuration builder. For example:

  CSHARPCopied
  using Microsoft.Extensions.Configuration;
  using Microsoft.Extensions.FileProviders;
  using System.IO;
  
  var filePath = @"C:\temp\.ext";
  var directory = Path.GetDirectoryName(filePath);
  var fileName = Path.GetFileName(filePath);
  
  // Create a provider pointing to the directory
  var fileProvider = new PhysicalFileProvider(directory);
  
  // Use the provider explicitly in AddIniFile
  var config = new ConfigurationBuilder()
      .SetFileProvider(fileProvider)
      .AddIniFile(fileName, optional: false)
      .Build();

In this example, by creating a PhysicalFileProvider for the directory, you can specify the exact file name (".ext") without the provider trying to resolve a “normal” file name.

Ultimately, if renaming isn’t an option, your best approach is to supply a custom file provider (or adjust your configuration to work on the directory level) so that the file name with an empty base portion is handled properly.