MahApps/MahApps.Metro · error · MahAppsException

Could not locate any instances of contract {typeof(T)}.

Error message

Could not locate any instances of contract {typeof(T)}.

What it means

MefServiceLocator.GetInstance<T>() calls the CompositionContainer's GetExportedValue<T>(), which throws when there is no matching export or when more than one exists (cardinality mismatch). The catch wraps the original exception in a MahAppsException naming the contract type T. This is a service-locator lookup failure surfaced through the MahApps exception type.

Source

Thrown at src/MahApps.Metro.Samples/MahApps.Metro.Caliburn.Demo/Services/MefServiceLocator.cs:32

    {
        private readonly CompositionContainer compositionContainer;

        [ImportingConstructor]
        public MefServiceLocator(CompositionContainer compositionContainer)
        {
            this.compositionContainer = compositionContainer;
        }

        public T? GetInstance<T>()
            where T : class
        {
            try
            {
                return this.compositionContainer.GetExportedValue<T>();
            }
            catch (Exception exception)
            {
                throw new MahAppsException($"Could not locate any instances of contract {typeof(T)}.", exception);
            }
        }
    }
}

View on GitHub (pinned to 72099e310b)

Solutions

1. Inspect the InnerException of the MahAppsException to see the exact MEF failure (no export vs. multiple exports).
2. Add exactly one [Export(typeof(T))] for the missing contract, or remove duplicate exports to resolve ambiguity.

Example fix

// before: GetInstance<IColorService>() throws — no export
public class ColorService : IColorService { }
// after
[Export(typeof(IColorService))]
public class ColorService : IColorService { }

// Prefer GetExportedValues (plural) to avoid cardinality exceptions, then check count
var values = container.GetExportedValues<T>();
if (values.Count() != 1) {
    // 0 => missing export; >1 => ambiguous; handle explicitly instead of letting MEF throw
}
return values.SingleOrDefault();

try {
    return container.GetExportedValue<T>();
} catch (ImportCardinalityMismatchException ex) {
    throw new InvalidOperationException($"No/multiple exports for {typeof(T)}", ex);
}

When it happens

Trigger: GetInstance<T>() is called for a type T that has no [Export] of that contract, or has multiple exports when exactly one is required. The underlying CompositionContainer throws ImportCardinalityMismatchException (or CompositionException) and it is rewrapped here.

Common situations: A service used by MahApps samples is not exported, or two implementations are exported for the same interface causing ambiguity. The container was built from a catalog missing the exporting assembly. A type was renamed without updating its exports.

Related errors

• Could not locate any instances of contract {contract}.
• Failed to set the window state for {window} from the setting
• Uups, it seems like there is something wrong with the given
• Cannot convert the given input to a valid color
• The inverse dialog theme only works if the window theme abid

AI-assisted analysis of MahApps/MahApps.Metro@72099e310b (2026-08-13). Data as JSON: /api/errors/7a66cfe08b966403. Report an issue: GitHub.