Add DateOnly example and documentation to indexers article

Co-authored-by: BillWagner <[email protected]>

Author: Copilot

docs/csharp/programming-guide/indexers/index.md

@@ -55,6 +55,14 @@ Another common scenario is when you need to model a dictionary or a map. This sc
 
 In this example, the `ArgsAction` collection maps closely to the underlying collection. The `get` determines if a given option is configured. If so, it returns the <xref:System.Action> associated with that option. If not, it returns an <xref:System.Action> that does nothing. The public accessor doesn't include a `set` accessor. Rather, the design is using a public method for setting options.
 
+### Date-based indexers
+
+When working with date-based data, you can use either <xref:System.DateTime> or <xref:System.DateOnly> as indexer keys. Use <xref:System.DateOnly> when you only need the date part and want to avoid time-related complications. The following example shows a temperature tracking system that uses `DateOnly` as the primary indexer key:
+
+:::code language="csharp" source="./snippets/indexers/DateOnlyIndexer.cs":::
+
+This example demonstrates both `DateOnly` and `DateTime` indexers. While the `DateOnly` indexer is the primary interface, the `DateTime` overload provides convenience by extracting only the date portion. This approach ensures that all temperature data for a given day is treated consistently, regardless of the time component.
+
 ### Multi-Dimensional Maps
 
 You can create indexers that use multiple arguments. In addition, those arguments aren't constrained to be the same type.

docs/csharp/programming-guide/indexers/snippets/indexers/DateOnlyIndexer.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Collections.Generic;
+
+namespace Indexers;
+
+public class DailyTemperatureData
+{
+    private readonly Dictionary<DateOnly, (double High, double Low)> _temperatureData = new();
+
+    // Indexer using DateOnly for date-only scenarios
+    public (double High, double Low) this[DateOnly date]
+    {
+        get
+        {
+            if (_temperatureData.TryGetValue(date, out var temp))
+            {
+                return temp;
+            }
+            throw new KeyNotFoundException($"No temperature data available for {date:yyyy-MM-dd}");
+        }
+        set
+        {
+            _temperatureData[date] = value;
+        }
+    }
+
+    // Overload using DateTime for convenience, but only uses the date part
+    public (double High, double Low) this[DateTime dateTime]
+    {
+        get => this[DateOnly.FromDateTime(dateTime)];
+        set => this[DateOnly.FromDateTime(dateTime)] = value;
+    }
+
+    public bool HasDataFor(DateOnly date) => _temperatureData.ContainsKey(date);
+
+    public IEnumerable<DateOnly> AvailableDates => _temperatureData.Keys;
+}

docs/csharp/programming-guide/indexers/snippets/indexers/DateOnlyProgram.cs

@@ -0,0 +1,32 @@
+using System;
+using Indexers;
+
+namespace DateOnlyExample;
+
+class Program
+{
+    static void Main()
+    {
+        var temperatureData = new DailyTemperatureData();
+
+        // Store temperature data using DateOnly
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        temperatureData[today] = (75.2, 62.1);
+        temperatureData[today.AddDays(1)] = (78.5, 64.3);
+        temperatureData[today.AddDays(-1)] = (72.8, 59.7);
+
+        // Retrieve data using DateOnly
+        Console.WriteLine($"Today's temperature: High {temperatureData[today].High}°F, Low {temperatureData[today].Low}°F");
+
+        // Can also use DateTime - only the date part is used
+        var tomorrow = DateTime.Today.AddDays(1);
+        Console.WriteLine($"Tomorrow's temperature: High {temperatureData[tomorrow].High}°F, Low {temperatureData[tomorrow].Low}°F");
+
+        // Show available dates
+        Console.WriteLine("Available temperature data for:");
+        foreach (var date in temperatureData.AvailableDates)
+        {
+            Console.WriteLine($"  {date:yyyy-MM-dd}");
+        }
+    }
+}

docs/csharp/programming-guide/indexers/snippets/indexers/Indexers.csproj

@@ -2,10 +2,9 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net9.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
-    <LangVersion>preview</LangVersion>
   </PropertyGroup>
 
 </Project>

docs/csharp/programming-guide/indexers/snippets/indexers/indexer-2.cs

@@ -1,9 +1,14 @@
 namespace Indexers;
 
-public class ReadOnlySampleCollection<T>(params IEnumerable<T> items)
+public class ReadOnlySampleCollection<T>
 {
    // Declare an array to store the data elements.
-   private T[] arr = [.. items];
+   private T[] arr;
+
+   public ReadOnlySampleCollection(params T[] items)
+   {
+       arr = items;
+   }
 
    public T this[int i] => arr[i];