Define "defining declaration" and enhance CS0759 compiler error documentation (#47628)

* Initial plan

* Add detailed explanation and examples for CS0759 compiler error

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

* Update target framework to .NET 9 in partial-declarations project

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

* Address review feedback: simplify CS0759 explanation and fix code comment punctuation

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

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: gewarren <[email protected]>

Author: 3 people

docs/csharp/language-reference/compiler-messages/partial-declarations.md

@@ -250,6 +250,20 @@ Certain `partial` method declarations don't require an *implementing declaration
 
 When a partial method includes an implementing declaration, both declarations must be identical. Exactly one implementing declaration can be defined.
 
+**CS0759** occurs when you have an *implementing declaration* (a partial method with a body) but no corresponding *defining declaration* (the method signature without a body). Every partial method with an implementation must have both declarations.
+
+The following example shows code that generates CS0759:
+
+:::code language="csharp" source="./snippets/partial-declarations/CS0759Examples.cs" id="IncorrectExample":::
+
+To fix this error, add the defining declaration:
+
+:::code language="csharp" source="./snippets/partial-declarations/CS0759Examples.cs" id="CorrectExample":::
+
+You can also place both declarations in the same partial class section:
+
+:::code language="csharp" source="./snippets/partial-declarations/CS0759Examples.cs" id="AlternativeCorrect":::
+
 ## Partial properties
 
 The following errors indicate mistakes in your partial property or indexer declarations:

docs/csharp/language-reference/compiler-messages/snippets/partial-declarations/CS0759Examples.cs

@@ -0,0 +1,52 @@
+using System;
+
+namespace CS0759Examples
+{
+    // <IncorrectExample>
+    // This will cause CS0759: No defining declaration found for implementing declaration of partial method.
+    // Uncomment the class below to see the CS0759 error:
+    /*
+    public partial class ExampleClass
+    {
+        // ERROR: This is an implementing declaration without a corresponding defining declaration
+        partial void MyMethod() // CS0759
+        {
+            Console.WriteLine("Implementation without definition");
+        }
+    }
+    */
+    // </IncorrectExample>
+
+    // <CorrectExample>
+    // Correct way: Provide both the defining declaration and implementing declaration
+    public partial class CorrectExampleClass
+    {
+        // Defining declaration (signature without body)
+        partial void MyMethod();
+    }
+
+    public partial class CorrectExampleClass
+    {
+        // Implementing declaration (signature with body)
+        partial void MyMethod()
+        {
+            Console.WriteLine("This works correctly");
+        }
+    }
+    // </CorrectExample>
+
+    // <AlternativeCorrect>
+    // Alternative correct approach: defining and implementing in same partial class
+    public partial class AlternativeExampleClass
+    {
+        // Defining declaration
+        partial void MyMethod();
+        
+        // Implementing declaration in same partial class section
+        partial void MyMethod()
+        {
+            Console.WriteLine("This also works correctly");
+        }
+    }
+    // </AlternativeCorrect>
+}

docs/csharp/language-reference/compiler-messages/snippets/partial-declarations/partial-declarations.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>