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How to Resolve Error "CS1055: An add or remove accessor expected" in C#

The Compiler Error CS1055 is a syntax error related to Event Declarations. The message reads: "An add or remove accessor expected".

In C#, events can be declared in two ways:

  1. Field-like events: public event EventHandler MyEvent; (The compiler generates storage and logic automatically).
  2. Custom events: You provide { ... } braces and explicitly define the add and remove accessors to handle subscription logic manually.

This error occurs when you open the curly braces { for an event but fail to provide the required add or remove blocks inside. Unlike properties (which use get and set), events strictly require add and remove.

Understanding Event Syntax​

When defining an event with a body, you are responsible for managing the underlying delegate storage.

Valid Syntax:

public event EventHandler MyEvent
{
    add { /* Add subscriber */ }
    remove { /* Remove subscriber */ }
}

If the compiler sees the opening brace {, it stops treating the event as a "field-like event" and expects the custom accessor syntax immediately.

Scenario 1: Using Property Keywords (get/set)​

This is a common mistake for developers used to writing properties. You might instinctively type get and set inside an event definition.

Example of error​

public class Button
{
    // ⛔️ Error CS1055: An add or remove accessor expected.
    // Events do not have 'get' or 'set' accessors.
    public event EventHandler Click
    {
        get; 
        set;
    }
}

Solution: Use add and remove​

You must use the correct keywords for events. Note that you also need to implement the storage (a private backing field) yourself when using this syntax.

public class Button
{
    private EventHandler _click; // Backing field

    public event EventHandler Click
    {
        // ✅ Correct: 'add' handles subscription (+=)
        add 
        { 
            _click += value; 
        }
        
        // ✅ Correct: 'remove' handles unsubscription (-=)
        remove 
        { 
            _click -= value; 
        }
    }
}

Scenario 2: Empty Event Bodies​

Sometimes a developer might open the braces intending to implement logic later, or by accident, leaving the body empty.

Example of error​

public class DataStream
{
    // ⛔️ Error CS1055: The braces indicates a custom event, 
    // but no accessors are defined inside.
    public event EventHandler DataReceived { } 
}

Solution: Remove Braces or Implement​

If you don't need custom logic, simply remove the braces and end the line with a semicolon. This creates a standard Field-like Event.

public class DataStream
{
    // ✅ Correct: The compiler will now auto-generate the add/remove logic.
    public event EventHandler DataReceived;
}

If you do need the braces (e.g., for CS0069 explicit interface implementation logic), you must write the add and remove blocks as shown in Scenario 1.

Conclusion​

CS1055 is the compiler reminding you of the event contract.

  1. Check Keywords: Events use add and remove, not get and set.
  2. Check Braces: If you don't need custom logic, delete the { } and use a ;.
  3. Implement Logic: If you keep the braces, you must provide the implementation for adding and removing subscribers.

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