Allow validation to be looked up by name

Toolkit.Foundation/IReadOnlyIndexDictionary.cs

@@ -0,0 +1,10 @@
+
+namespace Toolkit.Foundation
+{
+    public interface IReadOnlyIndexDictionary<TKey, TValue> : 
+        IReadOnlyDictionary<TKey, TValue>
+        where TKey : notnull
+    {
+        KeyValuePair<TKey, TValue> this[int index] { get; }
+    }
+}

Toolkit.Foundation/IValidation.cs

@@ -6,7 +6,7 @@ namespace Toolkit.Foundation;
 public interface IValidation :
     INotifyPropertyChanged
 {
-    IReadOnlyDictionary<string, string> Errors { get; }
+    IReadOnlyIndexDictionary<string, string> Errors { get; }
 
     bool HasErrors { get; }
 

Toolkit.Foundation/ReadOnlyIndexDictionary.cs

@@ -0,0 +1,56 @@
+using System.Collections;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Toolkit.Foundation;
+
+public class ReadOnlyIndexDictionary<TKey, TValue> :
+    IReadOnlyDictionary<TKey, TValue>, 
+    IReadOnlyIndexDictionary<TKey, TValue> 
+    where TKey : notnull
+{
+    private readonly Dictionary<TKey, TValue> dictionary;
+    private readonly List<KeyValuePair<TKey, TValue>> indexedItems;
+
+    public ReadOnlyIndexDictionary(IDictionary<TKey, TValue> items)
+    {
+        dictionary = new Dictionary<TKey, TValue>(items);
+        indexedItems = [.. dictionary];
+    }
+
+    public TValue this[TKey key] =>
+        dictionary[key];
+
+    public KeyValuePair<TKey, TValue> this[int index] =>
+        indexedItems[index];
+
+    public IEnumerable<TKey> Keys =>
+        dictionary.Keys;
+
+    public IEnumerable<TValue> Values =>
+        dictionary.Values;
+
+    public int Count => dictionary.Count;
+
+    public bool ContainsKey(TKey key) =>
+        dictionary.ContainsKey(key);
+
+    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator() =>
+        dictionary.GetEnumerator();
+
+    IEnumerator IEnumerable.GetEnumerator() => dictionary.GetEnumerator();
+
+    public bool TryGetValue(TKey key, [MaybeNullWhen(false)] out TValue value)
+    {
+        if (dictionary.TryGetValue(key, out TValue? localValue))
+        {
+            if (localValue is not null)
+            {
+                value = localValue;
+                return true;
+            }
+        }
+
+        value = default;
+        return false;
+    }
+}

Toolkit.Foundation/Validation.cs

@@ -10,8 +10,8 @@ public class Validation(IValidatorCollection validators) :
 
     public event PropertyChangedEventHandler? PropertyChanged;
 
-    public IReadOnlyDictionary<string, string> Errors =>
+    public IReadOnlyIndexDictionary<string, string> Errors => 
-        errors.AsReadOnly();
+        new ReadOnlyIndexDictionary<string, string>(errors);
 
     public bool HasErrors =>
         Errors.Count > 0;
@@ -116,6 +116,7 @@ public class Validation(IValidatorCollection validators) :
             OnPropertyChanged(nameof(Errors), null, null);
         }
     }
+
     private string GetPropertyName<T>(Expression<Func<T>> expression)
     {
         return expression.Body switch