When adding new overloads such as params Span<T> alongside existing params T[] methods, the compiler might choose an unexpected overload or report ambiguity errors. Library authors had no way to guide the compiler's choice without breaking existing callers.

C# 13 introduces OverloadResolutionPriorityAttribute to let library authors specify which overload should be preferred when multiple are applicable. Higher priority values are preferred.

Code

Now

class Logger
{
    // New overload, preferred by the compiler
    [OverloadResolutionPriority(1)]
    public void Log(params ReadOnlySpan<string> messages) { }

    // Original overload, still available for binary compatibility
    public void Log(params string[] messages) { }
}

var logger = new Logger();
logger.Log("hello", "world"); // Calls the Span overload

Before

class Logger
{
    // Adding a Span overload alongside an array overload
    // would cause ambiguity errors at call sites
    public void Log(params ReadOnlySpan<string> messages) { }
    public void Log(params string[] messages) { }
}

var logger = new Logger();
logger.Log("hello", "world"); // Error: ambiguous call

Notes

• The default priority is 0; higher values are preferred
• Only affects overload resolution when multiple candidates are equally applicable
• Primarily designed for library authors evolving APIs without breaking existing callers

More information

• What's new in C# 13.0
• Specification