Define "defining declaration" and enhance CS0759 compiler error documentation

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

@@ -250,6 +250,23 @@ 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 signature without a body). Every partial method with an implementation must have both:
+
+1. A *defining declaration*: the method signature without a body, as mentioned in the [Partial members](#partial-members) section.
+2. An *implementing declaration*: the method signature with a body.
+
+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>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>