dotnet/wpf · error · IOException

SR.EntryAssemblyIsNull

Error message

SR.EntryAssemblyIsNull

What it means

ResourceContainer.GetResourceManagerWrapper builds a ResourceManager wrapper for resource parts in the application package. When Application.ResourceAssembly (the assembly backing application resources) is null — typically because no EntryAssembly is available — it throws IOException with SR.EntryAssemblyIsNull. Resources cannot be located without a resource assembly.

Solutions

  1. Set Application.ResourceAssembly to the assembly containing your resources before loading resource URIs.
  2. Ensure the host process has a valid entry assembly (avoid loading WPF resources from assemblies invoked without an entry point context).
  3. Defer resource loading until after Application startup completes.
  4. Catch IOException around resource resolution and fall back to explicit assembly resource loading (GetManifestResourceStream).

Example fix

// before
var part = package.GetPart(new Uri("images/logo.png", UriKind.Relative));
// after
if (Application.ResourceAssembly == null)
    Application.ResourceAssembly = typeof(App).Assembly;
var part = package.GetPart(new Uri("images/logo.png", UriKind.Relative));
Defensive patterns

Strategy: validation

Validate before calling

if (Application.ResourceAssembly == null)
    Application.ResourceAssembly = typeof(App).Assembly;
// then resolve the resource part

Type guard

static bool CanResolveResources() => Application.ResourceAssembly != null;

Try / catch

try
{
    part = package.GetPart(resourceUri);
}
catch (IOException ex) when (ex.Message.Contains(SR.EntryAssemblyIsNull.ToString()))
{
    part = null; // fall back to manifest resource stream
}

Prevention

When it happens

Trigger: Accessing a resource part through a ResourceContainer-backed Package when Application.ResourceAssembly is null, e.g. resource pack URIs (pack://application:...) resolved before Application is initialized or in a host without an entry assembly.

Common situations: Unit tests or designers creating Application-derived contexts without setting ResourceAssembly; plugin/M Franework hosts where Assembly.GetEntryAssembly() returns null; loading BAML/resource parts too early in startup.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/1254599a0dec525c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/AppModel/ResourceContainer.cs:305

                }
            }

            if (rmwResult == ApplicationResourceManagerWrapper)
            {
                if (rmwResult != null)
                {
                    // If this is not a resource from a component then it might be
                    // a content file and not an application resource.
                    if (ContentFileHelper.IsContentFile(partName))
                    {
                        isContentFile = true;
                        rmwResult = null;
                    }
                }
                else
                {
                    // Throw when Application.ResourceAssembly is null. 
                    throw new IOException(SR.EntryAssemblyIsNull);
                }
            }

            return rmwResult;
        }

        #endregion

        //------------------------------------------------------
        //
        //  Private Fields
        //
        //------------------------------------------------------

        #region Private Members

        private static readonly Dictionary<string, ResourceManagerWrapper> s_registeredResourceManagers = new(StringComparer.Ordinal);
        private static readonly Dictionary<string, ResourceManagerWrapper>.AlternateLookup<ReadOnlySpan<char>> s_registeredResourceManagersLookup = s_registeredResourceManagers.GetAlternateLookup<ReadOnlySpan<char>>();

View on GitHub (pinned to 81131a70a4)