.RESX File Format

Introduction

A .resx file holds the resources of a .NET program in XML: the strings of a form or an application, its pictures and icons, its sizes, colours and positions, and any other value that has a text form or can be serialised. It is the source form: the build turns it into a binary .resources file that is embedded in the assembly, and the program reads it through ResourceManager. Windows Forms designers wrote one for every form (Form1.resx beside Form1.cs), ASP.NET used them for localisation, and a project’s Properties/Resources.resx holds the strings and images the developer adds. .resw is the same shape of file used by UWP and WinUI for the strings of the user interface, where it is named with the culture in a folder and read by the x:Uid of XAML elements. XAML itself does not use .resx for its own text, but a XAML application refers to resources through x:Static and x:Uid.

The file is one <root> element. It starts with an inline XSD schema that describes the rest, then resheader entries that name the reader and writer classes, assembly aliases, and data and metadata entries. A data entry has a name, an optional type and mimetype, a value, and an optional comment.

<?xml version="1.0" encoding="utf-8"?>
<root>
  <!-- xsd:schema … (the standard inline schema) -->
  <resheader name="resmimetype"><value>text/microsoft-resx</value></resheader>
  <resheader name="version"><value>2.0</value></resheader>
  <resheader name="reader"><value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value></resheader>
  <resheader name="writer"><value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value></resheader>
  <data name="Greeting" xml:space="preserve">
    <value>Hello, world</value>
    <comment>Shown on the first page</comment>
  </data>
  <data name="button1.Location" type="System.Drawing.Point, System.Drawing">
    <value>12, 20</value>
  </data>
  <data name="logo" type="System.Resources.ResXFileRef, System.Windows.Forms">
    <value>..\Resources\logo.png;System.Drawing.Bitmap, System.Drawing</value>
  </data>
</root>

The kinds of file

How the viewer reads each file

A file is read as a resource file when its root element is <root> (no prefix) and the start of it holds a resheader, data, metadata or assembly element or the inline schema. Any other .resx or .resw file is shown as plain XML. A file over 24 MiB is read by the XML viewer, in windows. The tabs:

The decoding of a value follows the rules of ResXDataNode: no type is a string; a type with no mimetype is typed text read by its converter (checked here for the common types: Boolean, the integer and floating point types, Point, Size, Rectangle, Padding), System.Byte[] is base64, ResXFileRef is a file reference, ResXNullRef is null; the three mimetypes are decoded as base64, and a BinaryFormatter stream is read record by record (BinaryFormatter streams); nothing is deserialised or run. The problems are: a missing or wrong resheader (the reader refuses such a file), a name used twice, names that differ only by case, an entry with no name or no value, extra values or children, base64 that is not valid, a BinaryFormatter stream that is damaged, a mimetype the reader does not know, a malformed file reference, a typed value that its type cannot read, and an unknown element.

The pages

• File structure and schema – the root, the inline schema, headers, assemblies, data, metadata and comments
• Types, mimetypes and values – how each value is stored and read, base64, file references, images and icons
• BinaryFormatter streams – the record format inside the binary base64 values
• Localisation and the build – cultures, designer files, .resources, .resw and MSBuild
• Tooling – the .NET classes and tools that read and write resource files

Not described

Known, not yet covered

• Reference: the binary .resources file and the satellite assembly format are named, not described.
• Viewer: the ImageListStreamer of Windows Forms image lists (a Data member that starts with MSFt) is shown with its size and the name of its class, not decoded into images. Fonts, colours and other typed text are shown as text: the viewer checks the common types only and does not know the names of colours or enumeration members.
• Viewer: a file reference is shown, not followed: the file it names is not on the machine of the viewer.
• Viewer: the inline XSD schema is recognised, not checked against the file.

Unknown

• Reference: the .NET resource reader was not run here (no .NET SDK is installed); the rules come from the source of ResXResourceReader, ResXDataNode and ResXFileRef in the Windows Forms repository.

Links