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A GUI tool is the most common kind of DevToys extension. It appears as an entry in the sidebar, renders an interactive view inside the app window, and can optionally receive data routed to it by the Smart Detection engine. The core contract is the IGuiTool interface, which has just two members: a View property that returns the root UIToolView, and an OnDataReceived callback for Smart Detection integration.
1
Create the class library
2
Create a new .NET class library targeting net8.0 and add a reference to the DevToys.Api NuGet package:
3
dotnet new classlib -n MyDevToysExtension -f net8.0
cd MyDevToysExtension
dotnet add package DevToys.Api
4
The DevToys.Api package brings in MEF (System.ComponentModel.Composition) and all the GUI building blocks automatically.
5
Add an IResourceAssemblyIdentifier
6
Every extension assembly needs exactly one exported IResourceAssemblyIdentifier. The app uses it to resolve your .resx strings and to load any bundled fonts. The [Name] value must exactly match the ResourceManagerAssemblyIdentifier property you set on [ToolDisplayInformation].
7
using System.ComponentModel.Composition;
using DevToys.Api;

[Export(typeof(IResourceAssemblyIdentifier))]
[Name(nameof(MyExtensionResourceAssemblyIdentifier))]
public sealed class MyExtensionResourceAssemblyIdentifier : IResourceAssemblyIdentifier
{
    public ValueTask<FontDefinition[]> GetFontDefinitionsAsync()
        => ValueTask.FromResult(Array.Empty<FontDefinition>());
}
8
If you bundle a custom icon font with your extension, return the font definitions here. The font must be a .ttf or .otf file — WOFF and WOFF2 are not supported.
9
Add a resource file and localized strings
10
Add a Strings.resx resource file to your project with at least a ShortDisplayTitle entry. Visual Studio and the dotnet CLI both generate a Strings class from the .resx file automatically.
11
<!-- Strings.resx (simplified) -->
<data name="ShortDisplayTitle">
  <value>Text Upper</value>
</data>
<data name="LongDisplayTitle">
  <value>Text to Uppercase Converter</value>
</data>
<data name="Description">
  <value>Converts text to uppercase.</value>
</data>
12
Implement IGuiTool
13
Create a class that implements IGuiTool and decorate it with the required MEF and metadata attributes.
14
  • [Export(typeof(IGuiTool))] — registers the class with MEF.
  • [Name("...")] — a unique internal identifier used by [Order] and cross-component references.
  • [ToolDisplayInformation(...)] — all display metadata (icon, group, localized strings).
    • 15
    using System.ComponentModel.Composition;
using DevToys.Api;
using static DevToys.Api.GUI;

[Export(typeof(IGuiTool))]
[Name("MyExtension.TextUppercase")]
[ToolDisplayInformation(
    IconFontName = "FluentSystemIcons",
    IconGlyph = '\uF4E3',
    GroupName = PredefinedCommonToolGroupNames.Text,
    ResourceManagerAssemblyIdentifier = nameof(MyExtensionResourceAssemblyIdentifier),
    ResourceManagerBaseName = "MyDevToysExtension.Strings",
    ShortDisplayTitleResourceName = nameof(Strings.ShortDisplayTitle),
    LongDisplayTitleResourceName = nameof(Strings.LongDisplayTitle),
    DescriptionResourceName = nameof(Strings.Description))]
internal sealed class TextUppercaseTool : IGuiTool
{
    private readonly IUIMultiLineTextInput _inputText = MultiLineTextInput("input-text");
    private readonly IUIMultiLineTextInput _outputText = MultiLineTextInput("output-text");

    public UIToolView View
        => new(
            Stack()
                .Vertical()
                .MediumSpacing()
                .WithChildren(
                    _inputText
                        .Title("Input")
                        .OnTextChanged(OnInputTextChanged),
                    _outputText
                        .Title("Output")
                        .ReadOnly()
                        .CanCopyWhenEditable()));

    public void OnDataReceived(string dataTypeName, object? parsedData)
    {
        if (parsedData is string text)
        {
            _inputText.Text(text);
        }
    }

    private void OnInputTextChanged(string text)
    {
        _outputText.Text(text.ToUpperInvariant());
    }
}
    16
    Build the UIToolView
    17
    UIToolView is the root of your tool’s UI. Every element inside it is created through the static GUI.* factory methods (imported via using static DevToys.Api.GUI). All factory methods return fluent builder interfaces — chain method calls to configure each element.
    18
    Common layout containers:
    19
    FactoryDescriptionStack()Arranges children vertically or horizontallyGrid()CSS-style row/column grid with explicit cell placementSplitGrid()Two-pane resizable split layoutWrap()Wraps children onto multiple lines
    20
    Common input/output components:
    21
    FactoryDescriptionMultiLineTextInput(id?)Multi-line code/text editor with syntax highlighting supportSingleLineTextInput(id?)Compact single-line text fieldSwitch(id?)Toggle switch for boolean optionsNumberInput(id?)Numeric input fieldSelectDropDownList(id?)Drop-down selection listButton(id?)Clickable button
    22
    Set isScrollable: false in the UIToolView constructor when your layout manages its own scrolling (for example, a grid with a list that should fill the window height without creating a double-scrollbar). The default is true.

    ​
    Full Working Example

    The following complete example shows a minimal extension with a two-pane text-to-uppercase converter: 
    using System.ComponentModel.Composition;
using DevToys.Api;
using static DevToys.Api.GUI;

// ── Resource assembly identifier ─────────────────────────────────────────────

[Export(typeof(IResourceAssemblyIdentifier))]
[Name(nameof(MyExtensionResourceAssemblyIdentifier))]
public sealed class MyExtensionResourceAssemblyIdentifier : IResourceAssemblyIdentifier
{
    public ValueTask<FontDefinition[]> GetFontDefinitionsAsync()
        => ValueTask.FromResult(Array.Empty<FontDefinition>());
}

// ── GUI tool ──────────────────────────────────────────────────────────────────

[Export(typeof(IGuiTool))]
[Name("MyExtension.TextUppercase")]
[ToolDisplayInformation(
    IconFontName = "FluentSystemIcons",
    IconGlyph = '\uF4E3',
    GroupName = PredefinedCommonToolGroupNames.Text,
    ResourceManagerAssemblyIdentifier = nameof(MyExtensionResourceAssemblyIdentifier),
    ResourceManagerBaseName = "MyDevToysExtension.Strings",
    ShortDisplayTitleResourceName = nameof(Strings.ShortDisplayTitle),
    LongDisplayTitleResourceName = nameof(Strings.LongDisplayTitle),
    DescriptionResourceName = nameof(Strings.Description))]
[AcceptedDataTypeName(PredefinedCommonDataTypeNames.Text)]
internal sealed class TextUppercaseTool : IGuiTool
{
    private readonly IUIMultiLineTextInput _inputText
        = MultiLineTextInput("input-text");

    private readonly IUIMultiLineTextInput _outputText
        = MultiLineTextInput("output-text");

    public UIToolView View
        => new(
            Stack()
                .Vertical()
                .MediumSpacing()
                .WithChildren(
                    _inputText
                        .Title("Input")
                        .OnTextChanged(OnInputTextChanged),
                    _outputText
                        .Title("Output")
                        .ReadOnly()
                        .CanCopyWhenEditable()));

    public void OnDataReceived(string dataTypeName, object? parsedData)
    {
        // Smart Detection passes plain text — pipe it straight to the input.
        if (parsedData is string text)
        {
            _inputText.Text(text);
        }
    }

    private void OnInputTextChanged(string text)
    {
        _outputText.Text(text.ToUpperInvariant());
    }
}

    ​
    ToolDisplayInformationAttribute Reference

    All properties below are set as named arguments on the attribute.
    PropertyTypeDescription
    IconFontNamestringName of the font family used to render IconGlyph. Use "FluentSystemIcons" for the built-in Fluent icon font.
    IconGlyphcharUnicode code point of the icon glyph within the specified font.
    GroupNamestringInternal name of the GuiToolGroup this tool should appear under. Use constants from PredefinedCommonToolGroupNames or the [Name] of your own exported GuiToolGroup.
    ResourceManagerAssemblyIdentifierstringThe [Name] value of your IResourceAssemblyIdentifier implementation.
    ResourceManagerBaseNamestringThe base name passed to ResourceManager — typically "YourNamespace.Strings".
    ShortDisplayTitleResourceNamestringResource key for the short title shown in the sidebar (e.g., "JSON").
    PropertyTypeDescription
    LongDisplayTitleResourceNamestringResource key for the longer title shown in the tool header and search results (e.g., "JSON Formatter"). Defaults to the short title if omitted.
    DescriptionResourceNamestringResource key for a one-sentence tool description shown in search results.
    AccessibleNameResourceNamestringResource key for the name read by screen readers. Falls back to the short title.
    SearchKeywordsResourceNamestringResource key for a comma-separated list of extra search keywords.

    ​
    Optional Behavioural Attributes

    These attributes fine-tune how DevToys treats your tool. All are optional.
    AttributeEffect
    [TargetPlatform(Platform.Windows)]Restricts the tool to the specified platform. Apply multiple times for multiple platforms. Omit entirely to target all platforms (Windows, MacOS, Linux).
    [Order(Before = "OtherToolName")]Places this tool before the named tool in the sidebar list.
    [Order(After = "OtherToolName")]Places this tool after the named tool in the sidebar list.
    [MenuPlacement(MenuPlacement.Footer)]Moves the sidebar entry to the footer area (below the separator). Default is MenuPlacement.Body.
    [NotSearchable]Excludes the tool from the search index.
    [NotFavorable]Prevents users from adding the tool to their favorites.
    [NoCompactOverlaySupport]Disables the Compact Overlay (picture-in-picture) window mode for this tool.

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