Local functions were introduced in C# 7.0 but could not have attributes applied to them. This meant you could not use compiler-recognized attributes like [Conditional], [NotNullWhen], or [DoesNotReturn] on local functions.

C# 9.0 allows attributes on local functions and their parameters, bringing them to parity with regular methods.

Code

Now

static void Validate(string name)
{
    [DoesNotReturn]
    static void ThrowHelperError(string paramName)
    {
        throw new ArgumentNullException(paramName);
    }

    if (name is null)
        ThrowHelperError(nameof(name));
}

static bool TryParse(string input, [NotNullWhen(true)] out MyType? result)
{
    [return: NotNullIfNotNull(nameof(input))]
    static MyType? ParseCore(string s) => new MyType(s);

    result = input is not null ? ParseCore(input) : null;
    return result is not null;
}

Before

static void Validate(string name)
{
    // Could not mark local functions with [DoesNotReturn]
    // so they had to be promoted to regular private methods
    if (name is null)
        ThrowHelperError(nameof(name));
}

[DoesNotReturn]
private static void ThrowHelperError(string paramName)
{
    throw new ArgumentNullException(paramName);
}

More information

• Specification