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How to Resolve Error "CS0759: No defining declaration found for implementing declaration of partial method" in C#

The Compiler Error CS0759 is a structural error regarding Partial Methods. The message reads: "No defining declaration found for implementing declaration of partial method 'MethodName'".

In C#, a partial method mechanism consists of two halves:

  1. Definition (Signature): partial void MyHook(); (Ends with a semicolon).
  2. Implementation (Body): partial void MyHook() { ... } (Ends with curly braces).

This error occurs when you provide the Implementation (Part 2), but the compiler cannot find the corresponding Definition (Part 1) anywhere in the partial class. Essentially, you are trying to implement a method that nobody asked for.

This guide explains the dependency between the definition and the implementation of partial methods.

Understanding the Dependency​

Partial methods act like "hooks" or "events." One part of the code (often generated) says: "I declare that a method named OnChanged exists here." Your part of the code says: "Here is the body for OnChanged."

If you write the body, but the declaration is missing, the compiler treats it as an orphan. It doesn't know where to link this logic or who is supposed to call it.

Scenario 1: Missing Definition in the Other File​

This happens when you write a partial method implementation manually, assuming the definition exists in another file (like an auto-generated .Designer.cs file), but it actually doesn't.

Example of error​

File 1 (Your Code):

public partial class User
{
    // ⛔️ Error CS0759: No defining declaration found for implementing declaration...
    // You are implementing logic for 'OnSaved', but no other file defined 'OnSaved'.
    partial void OnSaved()
    {
        System.Console.WriteLine("Saved successfully.");
    }
}

File 2 (Missing Code): The definition partial void OnSaved(); is nowhere to be found.

Solution 1: Add the Defining Declaration​

If you intended to create a hook that can be called, you must add the definition (signature) to the class, usually in the file where the event trigger happens.

File 2 (Base/Generated Code):

public partial class User
{
    // ✅ Correct: The definition acts as the anchor.
    partial void OnSaved();

    public void Save()
    {
        // Now valid to call
        OnSaved();
    }
}

Solution 2: Remove the partial Keyword​

If you didn't intend for this to be a split method—if you just wanted a private helper method for your own file—simply remove the partial keyword.

File 1 (Your Code):

public partial class User
{
    // ✅ Correct: Now it is just a standard private method.
    private void OnSaved()
    {
        System.Console.WriteLine("Saved successfully.");
    }
}

Scenario 2: Signature Mismatch (Typos)​

Even if the definition exists, if your implementation does not match it exactly (name, parameters, return type, modifiers), the compiler considers them different methods. Since your implementation doesn't match the existing definition, it is treated as "undefined."

Example of error​

File 1 (Definition):

public partial class DataProcessor
{
    // Defined with an integer parameter
    partial void Process(int count);
}

File 2 (Implementation):

public partial class DataProcessor
{
    // ⛔️ Error CS0759: No defining declaration found.
    // The definition expects 'int', but we provided 'double'.
    // Therefore, this is seen as an orphan method.
    partial void Process(double count)
    {
        // ...
    }
}

Solution: Align the Signatures​

Ensure the implementation matches the definition character-for-character regarding types and names.

File 2 (Fixed Implementation):

public partial class DataProcessor
{
    // ✅ Correct: Matches File 1 exactly.
    partial void Process(int count)
    {
        // ...
    }
}
note

Access Modifiers (C# 9+): If the defining declaration has an access modifier (e.g., private partial void... or public partial...), your implementation must also include that same access modifier. Missing it will cause a signature mismatch.

Conclusion​

CS0759 means you provided an answer without a question.

  1. Check the Other File: Does partial void MethodName(); actually exist in the corresponding partial class file?
  2. Check Spelling: Did you mistype the name in your implementation?
  3. Check Signatures: Do the parameter types and modifiers match exactly?
  4. Decide: If the definition is missing, either add it (to enable the hook) or make your method a standard private void method.

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