What are partial methods in C#?

Partial methods are a feature of C# that allows defining a method in one part of a class, struct, or interface with the partial keyword, and implementing it in another part.

Features:

• Declared with the partial keyword.
• Have no access modifiers (implicitly private).
• Cannot return a value other than void (before C# 7.0). In C# 7.0 and above, they can return any type but must have an implementation.
• Can have ref parameters.
• Cannot have out parameters.
• Declaration and implementation can be in different source code files, but within the same project and assembly.
• If a declared partial method has no implementation, the compiler removes both the declaration and all calls to it.

Usage:

• Code generation: To separate code generated by automated tools (e.g., Entity Framework, Windows Forms Designer) from user logic. The generated part declares the partial method, and the developer implements it in another file.
• Extensibility: Allows creating classes that can be extended by adding implementations of partial methods without modifying the generated or main code.

Example:

// File: MyClass.Generated.cs
public partial class MyClass
{
    // Declaration of a partial method - no implementation
    partial void ProcessData(string data);

    public void DoWork(string inputData)
    {
        // Possibly, a call in generated code
        ProcessData(inputData);
    }
}

// File: MyClass.Custom.cs
public partial class MyClass
{
    // Implementation of the partial method
    partial void ProcessData(string data)
    {
        // User-defined data processing logic
        Console.WriteLine($"Processing data: {data}");
    }
}