AvaloniaUI/Avalonia · error · FileNotFoundException

The resource {uri} could not be found.

Error message

The resource {uri} could not be found.

What it means

StandardAssetLoader.OpenAndGetAssembly(uri) attempts TryGetAsset and, if no asset descriptor is produced for the (uri, baseUri) pair, throws FileNotFoundException. This is the top-level asset loader entry point: every higher-level 'open this resource URI' call funnels here, so this is the canonical 'Avalonia could not resolve your asset URI' error.

Source

Thrown at src/Avalonia.Base/Platform/StandardAssetLoader.cs:92

    /// </summary>
    /// <param name="uri">The URI.</param>
    /// <param name="baseUri">
    /// A base URI to use if <paramref name="uri"/> is relative.
    /// </param>
    /// <returns>
    /// The stream containing the resource contents together with the assembly.
    /// </returns>
    /// <exception cref="FileNotFoundException">
    /// The asset could not be found.
    /// </exception>
    public (Stream stream, Assembly assembly) OpenAndGetAssembly(Uri uri, Uri? baseUri = null)
    {
        if (TryGetAsset(uri, baseUri, out var assetDescriptor))
        {
            return (assetDescriptor.GetStream(), assetDescriptor.Assembly);
        }

        throw new FileNotFoundException($"The resource {uri} could not be found.");
    }

    public Assembly? GetAssembly(Uri uri, Uri? baseUri)
    {
        if (!uri.IsAbsoluteUri && baseUri != null)
        {
            uri = new Uri(baseUri, uri);
        }

        if (TryGetAssembly(uri, out var assemblyDescriptor))
        {
            return assemblyDescriptor.Assembly;
        }

        return null;
    }

    /// <summary>

View on GitHub (pinned to 11c5427268)

Solutions

1. Use an absolute avares:// URI with the correct assembly name and forward-slash path: new Uri("avares://MyAssembly/Assets/icon.png").
2. Confirm the asset's build action is AvaloniaResource and the owning assembly is registered with the loader (AssetLoader.SetDefaultAssembly / RegisterResamplerPng etc., or WithAssemblyDescriptor on the loader).
3. When using relative URIs, always pass a valid baseUri so the loader can resolve the absolute path; null baseUri with a relative uri cannot be resolved.
4. Check spelling and casing of every path segment; on case-sensitive platforms a wrong-case path resolves at design time but fails here.

Example fix

// before: relative uri, no base, asset not found
using var s = AssetLoader.Open(new Uri("Assets/icon.png"));

// after: absolute avares URI with assembly + correct build action
using var s = AssetLoader.Open(new Uri("avares://MyAssembly/Assets/icon.png"));

var uri = new Uri("avares://MyAssembly/Assets/icon.png");
if (!AssetLoader.Exists(uri)) return;

bool AssetResolvable(Uri uri, Uri? baseUri = null) => Avalonia.Platform.AssetLoader.GetAsset(uri, baseUri) is not null; // or use the public Exists API

try { using var s = AssetLoader.Open(uri); }
catch (FileNotFoundException ex) { /* log missing asset, fall back to bundled default */ }

When it happens

Trigger: Calling OpenAndGetAssembly / Open / GetAsset with a URI whose scheme, path, or owning assembly do not match any registered asset. TryGetAsset returns false when no resolver matches 'resm:'/'avares:' or the resolved path is absent from the assembly's resource index.

Common situations: Missing or wrong URI prefix (using a relative path instead of 'avares://AssemblyName/...'), typo in assembly name or asset path, asset assembly not registered with the asset loader (WithAssemblyDescriptor not added), asset built into the wrong assembly, or case-sensitivity mismatch on case-sensitive filesystems/builds.

Related errors

• Could not find manifest resource stream '{_name}',
• Could not find manifest resource stream '{Constants.Avalonia
• File path is expected to be an absolute link with "file" or
• Folder path is expected to be an absolute link with "file" o
• Duration value cannot be negative.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/44e4ee5ee05e818b. Report an issue: GitHub.