Clarify usage of IUnionMembers interface

docs/csharp/language-reference/builtin-types/snippets/unions/MemberProvider.cs

@@ -1,6 +1,6 @@
 // <MemberProvider>
 [System.Runtime.CompilerServices.Union]
-public record class Outcome<T> : Outcome<T>.IUnionMembers
+public struct Outcome<T> : Outcome<T>.IUnionMembers
 {
     private readonly object? _value;
 
@@ -11,9 +11,35 @@ public interface IUnionMembers
         static Outcome<T> Create(T? value) => new(value);
         static Outcome<T> Create(Exception? value) => new(value);
         object? Value { get; }
+
+        // only when needed
+        bool TryGetValue(out T value);
+        bool TryGetValue(out Exception value);
     }
 
     object? IUnionMembers.Value => _value;
+
+    public bool TryGetValue(out T value)
+    {
+        if (_value is T t)
+        {
+            value = t;
+            return true;
+        }
+        value = default!;
+        return false;
+    }
+
+    public bool TryGetValue(out Exception value)
+    {
+        if (_value is Exception e)
+        {
+            value = e;
+            return true;
+        }
+        value = default!;
+        return false;
+    }
 }
 // </MemberProvider>
 

docs/csharp/language-reference/builtin-types/union.md

@@ -171,7 +171,7 @@ The compiler prefers `TryGetValue` over the `Value` property when implementing p
 
 ### Union member providers
 
-A union type can delegate its union members to a nested `IUnionMembers` interface. When this interface is present, the compiler looks for `Create` factory methods instead of constructors:
+A union type can delegate its union members to a nested `IUnionMembers` interface. When this interface is present, the `union` type behaves as a *union member provider*. The compiler generates code to call the `IUnionMembers` interface. It won't generate calls to members declared on the union type that aren't members of the nested `IUnionMembers` interface. As the following example shows, that means you must add the necessary factory methods and the appropriate `TryGetValue` methods for all case types:
 
 :::code language="csharp" source="snippets/unions/MemberProvider.cs" id="MemberProvider":::