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How to Resolve Error "CS0756: A partial method may not have multiple defining declarations" in C#

The Compiler Error CS0756 is a structural definition error regarding Partial Methods. The message reads: "A partial method may not have multiple defining declarations".

In C#, a partial method consists of two potential parts:

  1. Defining Declaration: The signature (e.g., partial void OnEvent();). There can be only one of these.
  2. Implementing Declaration: The body (e.g., partial void OnEvent() { ... }). There can be at most one of these.

This error occurs when the compiler finds two or more "Defining Declarations" (signatures ending in a semicolon) for the same method. It essentially asks: "You already told me what this method looks like. Why are you telling me again?"

This guide explains how to distinguish between defining and implementing partial methods.

Understanding the One-Definition Rule​

Partial methods are used to allow one part of a class (often auto-generated code) to declare a hook:

partial void OnCreated();

And allow another part of the class (your manual code) to implement it:

partial void OnCreated() { Console.WriteLine("Hi"); }

If you write partial void OnCreated(); in File A and also write partial void OnCreated(); in File B, you have defined the contract twice. This is invalid.

Scenario 1: Duplicate Definitions Across Files​

This often happens when a developer mistakenly thinks they need to "declare" the method in their file before "implementing" it later, or when copying code from a generated file into a user file.

Example of error​

File 1 (Auto-Generated):

public partial class User
{
    // The original definition
    partial void OnChanged(); 
}

File 2 (User Code):

public partial class User
{
    // ⛔️ Error CS0756: A partial method may not have multiple defining declarations.
    // We confusingly re-declared the signature here with a semicolon ';'.
    // The compiler sees two definitions and zero implementations.
    partial void OnChanged(); 
}

Solution 1: Convert One to an Implementation​

If your intention was to provide the logic for the method defined in File 1, you must provide a body (curly braces), not a semicolon.

File 2 (User Code):

public partial class User
{
    // ✅ Correct: Replaced ';' with '{ ... }'.
    // This is now the Implementing Declaration.
    partial void OnChanged()
    {
        System.Console.WriteLine("User changed");
    }
}

Solution 2: Delete the Duplicate​

If you accidentally copy-pasted the definition into the same file or a sibling file without intending to implement it, simply delete the extra line.

File 2 (User Code):

public partial class User
{
    // ✅ Correct: Removed the duplicate definition.
    // If you don't implement the method, the compiler will just 
    // remove the call from File 1 automatically.
}

Conclusion​

CS0756 ensures that the contract for a partial method is defined exactly once.

  1. Check your files: Search your solution for the method name.
  2. Check the syntax:
    • Definition: Ends with ;. (Only one allowed).
    • Implementation: Ends with { ... }. (Only one allowed).
  3. The Fix: If you have two definitions (;), change one of them to an implementation ({}) or delete it.

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