C# local functions as explanatory variables

TL;DR; Among other use cases, local functions can be explanatory variables for short routines.

For instance this anonymous function ...

private void SomeMethod() 
{
    // anonymous function
    var characters = dNormal.Where(c => 
        CharUnicodeInfo.GetUnicodeCategory(c) !=
        UnicodeCategory.NonSpacingMark);
}

...might become this local func .

private void SomeMethod() 
{
    // local func as explanatory variable
    Func<char, bool> isDiacritic = c => 
        CharUnicodeInfo.GetUnicodeCategory(c) = 
        UnicodeCategory.NonSpacingMark;

    var characters = dNormal.Where(x => !isDiacritic(x));
}

Though a local function might be better, because we aren't going to hot swap the routine with another implementation.

private void SomeMethod() 
{
    // local function as explanatory variable
    bool isDiacritic(char c) => 
        CharUnicodeInfo.GetUnicodeCategory(c) = 
        UnicodeCategory.NonSpacingMark;

    var characters = dNormal.Where(x => !isDiacritic(x));
}

Another option is to make a private static method .

// static method as explanatory variable
private static bool IsDiacritic(char c) => 
    CharUnicodeInfo.GetUnicodeCategory(c) = 
    UnicodeCategory.NonSpacingMark;

private void SomeMethod() 
{
    var characters = dNormal.Where(x => !IsDiacritic(x));
}

Possible usage guidelines for naming routines.

anonymous function when the function's implementation explains itself.
Func/Action when we might hot swap the implementation; scope these to the method, to the class, or to the object as appropriate.
local function when the routine only makes sense in the context of its parent method.
private static method when the routine makes sense in the context of its class and/or when the routine's implementation becomes large and/or complex.