dotnet/wpf · error · InvalidOperationException

SR.Format(SR.PropertyIsImmutable, "ResourceAssembly"…

Error message

SR.Format(SR.PropertyIsImmutable, "ResourceAssembly", "Application")

What it means

Application.ResourceAssembly can only be set once. After the resource assembly has been established (either explicitly or implicitly when the Application is constructed), any attempt to assign a different value throws InvalidOperationException stating the property is immutable. Re-assigning the same value is allowed.

Solutions

Set ResourceAssembly once, at startup, before any resource lookups occur
Check Application.ResourceAssembly != value before assigning
If resources live in another assembly, reference them via pack URIs with the assembly name (pack://application:,,,/MyLib;component/x.png) instead of changing ResourceAssembly
Restart the application/process rather than reassigning the assembly

Example fix

// before
Application.ResourceAssembly = newAssembly; // throws if already set
// after
if (Application.ResourceAssembly != newAssembly)
    throw new InvalidOperationException("ResourceAssembly already fixed; use pack URIs for other assemblies");

Defensive patterns

Strategy: validation

Validate before calling

bool canSetAssembly(Assembly a) => Application.ResourceAssembly == null || Application.ResourceAssembly == a;

Try / catch

try { Application.ResourceAssembly = asm; } catch (InvalidOperationException) { /* already fixed; use pack URIs with assembly names */ }

Prevention

Set ResourceAssembly exactly once at startup
Use pack URIs with ;component/ for cross-assembly resources
Never swap assemblies at runtime
Guard assignments with an equality check

When it happens

Trigger: Setting Application.ResourceAssembly a second time with a different Assembly, e.g. in multi-assembly plugin scenarios or after Application.Current already initialized it; unit tests reconfiguring the resource assembly per test.

Common situations: Resource resolution failures prompting developers to 'fix' the assembly at runtime; satellite/localization assemblies swapped dynamically; test frameworks running multiple apps in one process.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/45b1a645ada45be6. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Application.cs:1095

                    }
                }

                return _resourceAssembly;
            }
            set
            {
                lock (_globalLock)
                {
                    if (_resourceAssembly != value)
                    {
                        if ((_resourceAssembly == null) && (Assembly.GetEntryAssembly() == null))
                        {
                            _resourceAssembly = value;
                            BaseUriHelper.ResourceAssembly = value;
                        }
                        else
                        {
                            throw new InvalidOperationException(SR.Format(SR.PropertyIsImmutable, "ResourceAssembly", "Application"));
                        }
                    }
                }
            }
        }

        #endregion Public Properties

        //------------------------------------------------------
        //
        //  Public Events
        //
        //------------------------------------------------------

        #region Public Events
        /// <summary>
        ///     The Startup event is fired when an application is starting.
        ///     This event is raised by the OnStartup method.

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