AvaloniaUI/Avalonia · error · InvalidOperationException

Unknown binding path element type {element.GetType().FullNam

Error message

Unknown binding path element type {element.GetType().FullName}

What it means

Thrown as InvalidOperationException during CompiledBindingPath.Build when an ICompiledBindingPathElement in the path does not match any of the known element-type cases in the switch. Each path element must be a recognized type (PropertyElement, MethodAsDelegateElement, ArrayElementPathElement, stream/typecast/ancestor/name/self/templated-parent elements). An unknown element type indicates a version mismatch or an unsupported custom element injected into the path.

Source

Thrown at src/Avalonia.Base/Data/CompiledBindingPath.cs:84

                        node = null;
                        isRooted = true;
                        break;
                    case ElementNameElement name:
                        node = new NamedElementNode(name.NameScope, name.Name);
                        isRooted = true;
                        break;
                    case IStronglyTypedStreamElement stream:
                        node = new StreamNode(stream.CreatePlugin());
                        break;
                    case ITypeCastElement typeCast:
                        node = new FuncTransformNode(typeCast.Cast);
                        break;
                    case TemplatedParentPathElement:
                        node = new TemplatedParentNode();
                        isRooted = true;
                        break;
                    default:
                        throw new InvalidOperationException($"Unknown binding path element type {element.GetType().FullName}");
                }

                if (node is not null)
                    result.Add(node);
            }


            for (var i = 0; i < negated; ++i)
                result.Add(new LogicalNotNode());
        }

        internal IEnumerable<ICompiledBindingPathElement> Elements => _elements;

        /// <inheritdoc />
        public override string ToString()
            => string.Concat((IEnumerable<ICompiledBindingPathElement>)_elements);
    }

View on GitHub (pinned to 11c5427268)

Solutions

1. Only use the public CompiledBindingPathBuilder methods (Property, Method, Command, Stream, etc.) to construct paths so all elements are recognized.
2. If on a custom/older Avalonia build, update to a version whose Build() switch handles your element type.
3. Do not inject custom ICompiledBindingPathElement implementations; extend the binding pipeline through supported extension points instead.

Example fix

// before — custom element injected via reflection
builder._elements.Add(new MyCustomElement());
path.Build(); // throws: unknown element type
// after — use a supported builder method
builder.Property(propInfo, accessorFactory);

// Only use public CompiledBindingPathBuilder methods; do not inject custom elements.
// If extending, ensure Build()'s switch handles your element type.

static bool IsKnownElement(ICompiledBindingPathElement e) =>
    e is PropertyElement or MethodAsDelegateElement or MethodAsCommandElement
    or ArrayElementPathElement or VisualAncestorPathElement or AncestorPathElement
    or SelfPathElement or ElementNameElement or IStronglyTypedStreamElement
    or ITypeCastElement or TemplatedParentPathElement;

try { path.Build(); }
catch (InvalidOperationException ex) when (ex.Message.Contains("Unknown binding path element"))
{ /* remove unsupported custom element or upgrade Avalonia */ }

When it happens

Trigger: A CompiledBindingPathBuilder accumulates an ICompiledBindingPathElement subclass that Build() does not handle. In normal use the builder only adds known element types, so this fires when custom/unsupported elements are injected, or when a newer path element type is fed to an older Avalonia build.

Common situations: Using reflection to add a custom ICompiledBindingPathElement to a CompiledBindingPathBuilder. Version mismatch: code compiled against a newer Avalonia adds an element type the runtime build cannot handle. Internal API misuse by a source generator or extension.

Related errors

• Unable to parse '{expression}': expected an instance of '{_h
• Bad format specifier.
• 'errorType' may not be None
• BindingErrorType may not be None
• BindingValue has no value.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/6cf5cb57c063bc7b. Report an issue: GitHub.