Indexer binding path with a '.' in the key ([a.b]) is split into two items and binds nothing #24937

Current behavior

An indexer binding path whose key contains a . binds to nothing. Path="[a.b]" is split into two path items, [a and b]. The indexer is then called with the key "a", which returns null, and .b] is looked up on that null, so the bound TextBlock shows an empty string.

The cause is in Uno.UI.DataBinding.BindingPath.Parse (read from Uno.UI 6.7.135, the same code in lib/net10.0 (Skia desktop) and lib/net10.0-android36.0). It walks the path backwards and tracks two flags: one for (...) and one for [...]. The case '.' branch checks only the parenthesis flag:

case '.':
    if (!flag)            // flag = "inside ( )"; the "inside [ ]" flag (flag2) is not consulted
    {
        TryPrependItem(path, num2 + 1, num, ...);
        num = 0;
        continue;
    }
    break;

So a . inside brackets ends a path item. Read through BindingPath.GetPathItems():

Expected behavior

The text between [ and ] is the indexer argument, dots included, so [a.b] is one indexer item that calls this["a.b"].

That is the documented property-path syntax ("the value inside the brackets is the indexer argument"). I have not run the same binding on WinUI / Windows App SDK to confirm what it does there.

How to reproduce it (as minimally and precisely as possible)

public sealed class Row
{
    private readonly Dictionary<string, string> _cells = new() { ["a.b"] = "dotted", ["ab"] = "plain" };
    public string? this[string key] => _cells.TryGetValue(key, out var v) ? v : null;
}

// Page code-behind
var row = new Row();
var dotted = new TextBlock { DataContext = row };
dotted.SetBinding(TextBlock.TextProperty, new Binding { Path = new PropertyPath("[a.b]") });
var plain = new TextBlock { DataContext = row };
plain.SetBinding(TextBlock.TextProperty, new Binding { Path = new PropertyPath("[ab]") });
// plain shows "plain"; dotted shows "" (expected "dotted")

The parse can also be shown without rendering anything. BindingPath is internal, so this uses reflection:

var type = typeof(Microsoft.UI.Xaml.Data.Binding).Assembly.GetType("Uno.UI.DataBinding.BindingPath")!;
var path = (IDisposable)Activator.CreateInstance(type, "[a.b]", null)!;
var items = ((IEnumerable)type.GetMethod("GetPathItems")!.Invoke(path, null)!).Cast<object>()
    .Select(i => (string)i.GetType().GetProperty("PropertyName")!.GetValue(i)!);
// items == ["[a", "b]"]; expected ["[a.b]"]

Workaround

Keep . out of indexer keys in binding paths. We substitute another character in the key and map it back inside the indexer.

Works on UWP/WinUI

I don't know (not tested here).

Environment

Nuget Package: Uno.WinUI (Uno.Sdk 6.7.x)

Package Version(s): 6.7.135

Affected platform(s):

• Android (Skia renderer, measured on an API 35 emulator)
• Skia desktop (Windows; same lib/net10.0 code)
• other platforms not checked

IDE: Visual Studio 2026

Anything else we need to know?

In our app this blanked every cell of a WinUI.TableView grid whose TableViewTextColumn.Binding paths are [TABLE.Field]. The rows, row selection and accessible row names all worked, so the grid looked populated but empty.