AvaloniaUI/Avalonia · error · InvalidCastException

Could not convert list index '{i}' of type '{argument}' to '

Error message

Could not convert list index '{i}' of type '{argument}' to '{type}'.

What it means

ReflectionIndexerNode.ConvertIndexes throws InvalidCastException when TypeUtilities.TryConvert fails to convert a binding indexer argument to the indexer's declared parameter type. The node has already found a matching indexer (parameter count is correct) but cannot coerce the supplied string/object argument into the expected index type.

Source

Thrown at src/Avalonia.Base/Data/Core/ExpressionNodes/Reflection/ReflectionIndexerNode.cs:116

        if (_getter is not null && _indexes is not null)
            SetValue(_getter.Invoke(source, _indexes));
        else
            ClearValue();
    }

    private static object?[] ConvertIndexes(ParameterInfo[] indexParameters, IList arguments)
    {
        var result = new List<object?>();

        for (var i = 0; i < indexParameters.Length; i++)
        {
            var type = indexParameters[i].ParameterType;
            var argument = arguments[i];

            if (TypeUtilities.TryConvert(type, argument, CultureInfo.InvariantCulture, out var value))
                result.Add(value);
            else
                throw new InvalidCastException(
                    $"Could not convert list index '{i}' of type '{argument}' to '{type}'.");
        }

        return result.ToArray();
    }

    private static bool GetIndexer(Type? type, [NotNullWhen(true)] out MethodInfo? getter, out MethodInfo? setter)
    {
        getter = setter = null;

        if (type is null)
            return false;

        if (type.IsArray)
        {
            getter = type.GetMethod("Get");
            setter = type.GetMethod("Set");
            return getter is not null;

View on GitHub (pinned to 11c5427268)

Solutions

1. Use an indexer argument whose type is convertible to the indexer parameter type (e.g. an integer for a list indexer).
2. If indexing a dictionary, ensure the key matches the dictionary's key type (and the string token parses to it).
3. Expose a helper property or method on the source that performs the lookup with the correct types instead of relying on the indexer.

Example fix

// before - IList<int> indexed with a non-int token
{Binding Items[abc]}
// after
{Binding Items[0]}

// Before binding to an indexer, check the argument converts to the parameter type.
static bool IndexConverts(Type containerType, object arg)
{
    var indexer = containerType.GetProperties()
        .FirstOrDefault(p => p.GetIndexParameters().Length > 0);
    if (indexer is null) return false;
    var paramType = indexer.GetIndexParameters()[0].ParameterType;
    return Avalonia.Utilities.TypeUtilities.TryConvert(paramType, arg,
        CultureInfo.InvariantCulture, out _);
}
if (!IndexConverts(source.GetType(), indexArgument))
    throw new InvalidCastException($"Index '{indexArgument}' not convertible to indexer type.");

static bool CanIndexWith(Type containerType, object arg) =>
    IndexConverts(containerType, arg);

try { /* evaluate indexer binding */ }
catch (InvalidCastException ex) when (ex.Message.Contains("Could not convert list index"))
{ /* supply a correctly-typed index argument */ }

When it happens

Trigger: Binding to an indexer with an argument whose type is not convertible to the indexer parameter type, e.g. indexing a List<T> with a string key when the indexer expects int, or supplying a non-numeric string where an int index is required.

Common situations: Binding like {Binding Items[abc]} where Items is an IList (int indexer); a dictionary indexer where the key type does not match the supplied argument; culture/parse issues converting a string index token.

Related errors

• Property {Name} doesn't have a getter
• Property {Name} doesn't have a setter
• Expected indexer argument.
• Expected '{delimiter}'.
• Expected '{close}'.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/f8adbf1a97a57f4f. Report an issue: GitHub.